NUTRITION AND MANANGEMENT OF FEEDLOT HEIFERS

NUTRITION AND MANANGEMENT OF FEEDLOT HEIFERS

INTRODUCTION

Jolene Kelzer Department of Animal Science

University of Minnesota

Ultimate goals of cattle producers in the feedlot industry include raising healthy animals in efficient production settings and supplying high quality retail products that exceed consumer acceptability. While technology, science, and education have greatly improved over the last two decades, raising cattle is still complicated and requires careful management from birth to harvest. Most beef calves in the United States are spring-born and are therefore weaned during the fall months. As of July 1, 2009, the total number of cattle and calves in the United States was 101.8 million head, which was down 1 percent from 103.3 million head in 2008. The calf crop for 2009 is estimated at 35.6 million head, with 25.8 million calves being born during the first six months of the year. Total number of cattle on feed as of July 1, 2009 was 11.6 million, which is five percent lower than the 12.2 million head on feed in 2008. Heifers (500 pounds BW or greater) not specified for replacement purposes totaled 7.7 million head for 2009, and as of July 1, 2009, there were 3.72 million head of heifers and heifer calves compared to 5.99 million steers and steers calves in the feedlot (NASS, USDA 2009).

Although finishing heifers in feedlots typically requires more management than finishing steers, the numbers indicate that approximately 32% of total cattle on feed are heifers and heifer calves. Therefore, heifer nutrition and management in the feedlot should not be overlooked. For the purposes of this paper, the major management practices involved in backgrounding and finishing heifers in the feedlot will be addressed. Furthermore, an experiment assessing the performance and value of spayed heifers will be discussed as a valid alternative to traditional practices of finishing intact heifers in the feedlot.

BACKGROUNDING PHASE In most beef cow/calf production settings, calves have never been separated from their dams, held in confinement, eaten from a bunk, or drank water from an automatic watering system or water tank until weaning. Therefore, weaning is highly stressful on calves and increases their susceptibility to pulmonary-related diseases and reduced performance. Producers are encouraged to precondition naYve calves to acclimate them to their new environment prior to backgrounding. Preconditioning involves training calves to consume dry forages and feedstuffs from bunks and drinking water from waterers or troughs. Other practices, such as vaccinating, deworming, dehorning, implanting, and castrating male calves can be completed during preconditioning. Calves should be vaccinated at weaning or upon arrival to the feedyard against IBR, PB, BRSV, and 7 clostridial strains. If BYD is prevalent to the area, calves also should be vaccinated against this disease. Perhaps more importantly, calves must receive booster vaccinations in a timely manner according to label directions. If these preventative health management practices are completed correctly, calves will be less susceptible to diseases as they recover from weaning stress and are more likely to perform to their genetic potential as they begin the backgrounding phase.

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Backgrounding is generally known as a management system that uses mainly pastures, crop residues, or harvested forages with little or no grain to efficiently develop muscle mass and frame size on calves during the period between weaning and feedlot entry. The decision whether to place calves into feedlots as calf-feds or as yearlings, and therefore whether or not to background calves, is based on several factors that include sex, weight, breed, frame size, and breed characteristics (such as early or late maturity) of calves, availability and location of forages, cost of energy and protein feedstuffs, market price outlooks, traditional marketing practices, and appeal for retained ownership of calves for more favorable future markets. Commonly, backgrounding systems are 60-90 days in length, and cattle should gain 100-400 pounds depending on forage availability, diets fed, and number of days on feed in the system. Typically, energy intake will be restricted during this time by limit-feeding calves to reduce ruminal passage rates and increase rumen retention time for improved nutrient utilization and enhanced feed conversion ratios (Galyean, 1999).

Because the two factors are not linearly related, maximum feed efficiency does not occur at maximum feed intake, so there are economic, as well as performance, benefits of controlling energy intake at various growth phases (Ferrell and Jenkins, 1998). Coffey et al. (2002) conducted an experiment that compared growth performance of stocker calves backgrounded on winter annuals or supplemented with 6 lb/d of a grain sorghum-based supplement containing 15.3% CP and 1.3 Meal NEg while having ad libitum access to Bermudagrass hay. Calves grazing the annual forage pastures had greater daily gains (2.2 pounds vs. 1.4 pounds) compared to calves consuming hay and supplement after a 112-d backgrounding period.

Not only does backgrounding add weight and frame size to calves in an efficient manner, it also helps to maintain a steady supply of cattle in feedlots. Similar to steers, heifers can be managed in one of two common programs that determine age at time of feedlot entrance. The two major programs include calf-feds and yearlings, and according to Klopfenstein et al. (2000), approximately 30% of calves born in the US enter feedlots as calf-feds. The goal of the producer, the type of cattle, availability of forages and feedstuffs, and market outlooks will dictate which program is used. In the calf-fed system, calves are placed directly into the feedlot and fed high-energy diets approximately 30 to 40 days after weaning. Calves placed in yearling programs are usually nutritionally restricted to varying degrees for different lengths of time to induce compensatory gain during the finishing phase (Klopfenstein et al., 2000). Calves, especially heifers or British breeds, placed in yearling programs are generally grazed on pastures, crop-residues, or other low-input, forage-based systems or fed low-energy, forage-based rations to increase frame size and BW prior to feedlot entrance. The differences in total days on feed, nutritional backgrounds, and feedlot entrance weights according to system may influence feedlot performance and carcass characteristics of steers and heifers (Klopfenstein et al., 2000).

Depending on age, breed, weight, and nutritional status, heifers may reach puberty between 9 and 16 months of age. Estrus activity during the backgrounding phase or in the feedlot increases risk for bodily injury from riding activity. Additionally, one heifer exhibiting estrus in a pen can severely disrupt the entire pen and reduce overall intake and performance. Undetected pregnant heifers in the feedlot is also a severe problem for feedlot producers. A management tool used more extensively in western US ranching states where management goals are to increase production and performance of feedlot heifers is spaying heifers. Spaying, or ovariectomizing,

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female cattle is the surgical removal of both ovaries so heifers will not exhibit estrus (Klindt and Cruse, 1990).

There are many advantages associated with spaying heifers prior to feedlot entrance. These advantages include maintaining stocker or feeder heifers in an open or neutered status (Klindt and Cruse, 1990), early detection of pregnant heifers accidently bred, prevention of pregnant heifers in the feedlot (Adams et al., 1990), elimination of feeding estrus suppressing feed additives, elimination of checking heifers for pregnancy prior to feedlot entry (Garber et al., 1990), improved average daily gain and feed conversion when spayed heifers are implanted versus intact, implanted heifers (Garber et al., 1990), and ability to graze or feed heifers and steers together and within fence-line contact of herd sires.

With removal of the ovaries, spaying heifers removes endogenous source of gonadal steroids, mainly estrogen and progesterone, which has been related to reduced intake, daily gain, and feed efficiency in the feedlot (Horstman et al., 1982). Therefore, the anabolic effects of ovaries must be replaced via implants in spayed heifers entering the feedlot to improve performance (Adams et al., 1990). Previous research by Garber et al. (1990) suggests spayed heifers respond better to implants containing progesterone rather than testosterone and had more rapid gains and more efficient feed conversion. In the study, implanted spayed heifers had a threefold improvement in average daily gain (0.66 vs. 0.20 lb/d increase) compared to intact, implanted heifers. Carcass characteristics were not affected, and some were even improved due to implanting. Spayed heifers implanted with progesterone tended to have heavier fmal BW and therefore heavier hot carcass weights, decreased backfat thickness, and improved marbling scores and yield grades, suggesting that implanting spayed heifers effectively partitions consumed energy away from fat accretion to increase muscle protein deposition without negatively affecting quality grades (Garber et al., 1990).

Due to heavier feedlot entrance weights, yearling cattle win generally consume more feed and gain more rapidly in the feedlot compared to calf-feds. Also, calf-feds tend to convert feed to gain more efficiently than yearlings, but yearlings typically have leaner carcasses with heavier hot weights and lower percentages of carcasses grading USDA Choice (Sindt et al., 1991). The additional end weight of yearling steers can positively influence gross income when sold either live or on a grid, reduce chance of lightweight carcass discounts in some British breeds, and reduce break-even prices; however, the additional weight in some larger-framed, Continental breeds may actually increase discounts applied to overweight carcasses, so endpoint goals that match the type of livestock being fed should influence choice of stocker cattle program used (Shain et al., 1998).

Deposition of intramuscular fat, or marbling, is a lifetime event and begins at an early stage of growth in cattle. Unlike the use of acetate in subcutaneous fat deposition, glucose is the primary substrate for intramuscular lipid deposition (Smith and Cruse, 1984). The presence of glucose or glucose precursors, (i.e .. propionate), may increase the presence of enzymes and hormones that facilitate fatty acid synthesis and therefore deposition to improve meat quality (Smith and Cruse, 1984). Research indicates feeding high-concentrate, high-energy grain diets in calf-fed systems to early-weaned steers and heifers may effectively improve feed efficiency as well as carcass quality. Cattle placed on high (75%) concentrate diets had higher proportions of ruminal

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propionate in expense of ruminal acetate to allow for increased marbling compared to cattle fed on low (25%) concentrate rations offered ad libitum (Wertz et al., 2001b). The onset of marbling is initiated at an ·earlier age in the calf-fed system compared to calves that were grazed on pasture before entering the feedlot to result in increased intramuscular fat and therefore, greater

. percentages of carcasses grading USDA Choice or higher at the end of the feeding phase (Myers et al., 1999; Loy et al., 1999).

Wertz et al. (2002) studied performance and carcass characteristic differences between heifers fed in an accelerated program versus 2-year old heifers. The results suggest that early-weaning heifers and finishing them in a calf-fed program increased gain efficiency as well as marbling deposition compared to heifers backgrounded on pasture and finished as 2-year olds. The proportion of intramuscular fat relative to subcutaneous fat was increased in the calf-fed heifers which allowed calves to reach higher quality grades and obtain grid premiums before attaining less desirable yield grades compared to heifers finished in long-yearling programs (Wertz et al., 2002).

However, the major goals of the yearling program are to increase frame size and muscle accretion of stocker calves, as well as induce compensatory gain, for the realization of an economic advantage during the finishing phase. Additionally, feed restriction must be severe enough because it is possible to limit feed intake without inducing compensatory gain during the finishing period. Wertz et al. (2001a) did not induce compensatory gain when restricting intake of a wet com gluten feed (WCGF)-based diet to heifers at 83% of ad libitum. The lack of feed restriction, a failure to reduce the gastrointestinal tract volume due to the fibrous nature of the WCGF, or a combination of the two factors may have contributed to why compensatory gain in the feedlot did not occur. However, Sainz et al. (1995) did induce compensatory gain during the finishing phase when cattle were limit-fed concentrate intake to 54% ad libitum during the backgrounding phase. Intestinal capacity and liver size are shown to fluctuate with intake level and may contribute to large proportions of total body energy expenditure (Wester et al., 1995). Therefore, a potential explanation for increased efficiency and subsequent compensatory gain due to intake restriction may be attributed to more energy being partitioned to muscle accretion from red}lced maintenance energy requirements of a smaller GI tract or reduced energy cost of tissue synthesis (Sainz et al. 1995).

Following through to slaughter, hot carcass weight was nearly 30 pounds heavier for heifers consuming the WCGF-based diets ad libitum during the growing phase compared to heifers that were restricted to 83% ad libitum intake (Wertz et al., 2001a). When fed to a common number of days on feed, the limit-fed heifers had lower carcass weights, reduced marbling scores and subsequent lower quality grades, less 12th rib backfat, and therefore, improved yield grades compared to the non-restricted heifers. During the overall feeding phase, heifers fed the WCGF­based diet ad libitum consumed 1.65 pounds more DMI and gained faster during the growing phase than the average of the limit-fed heifers even though overall feed efficiency was not different among heifers. Overall conclusions from this study indicate limit-feeding WCGF and com-based diets to growing beef heifers can allow moderate rates of gain without negatively impacting feed efficiency. Although WCGF can successfully replace portions of com grain in limit-feeding situations, total number of days on feed may increase during the finishing phase compared to heifers grown on ad libitum intake. Thus, the economic advantages of minimizing

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feed costs during the growing phase may be lost due to increased expenses from additional days on feed or reduced income due to lighter carcass weights and lower quality grades at time of slaughter (Wertz et al., 2001a).

Use of growth-promoting agents in feedlot cattle is a common management practice. The majority of implants contain estrogenic, androgenic, or a combination of estrogenic and androgenic properties. Estrogenic implants increase thyroid gland activity and intake (Trenkle, 1997), and implants containing trenbolone acetate decrease maintenance energy requirements to reduce metabolic heat load and increase feedlot efficiency (Hunter and Vercoe, 1987). Anabolic implants primarily increase muscle mass and subsequent bodyweight gain by increasing the rate of protein accretion with little or no negative impact on fat deposition rate. This allows an increase in cattle growth rates to attain similar empty body fat percentages at heavier final BW as non-implanted cattle (Hutcheson et al., i997; Guiroy et al., 2002). Although anabolic implants are effective growth agents and improve feedlot performance in finishing cattle, research suggests certain implant strategies may reduce meat tenderness and consumer acceptability of beefretail products (Platter et al., 2003).

For long-term implant programs, it is generally recommended to administer initial implants that contain low- or moderate-estrogenic activity followed by subsequent re-administration of implants containing estrogenic or androgenic activity every 70 to 100 days thereafter (Mader, 1994). An ideal implant program for young, recently weaned heifer calves is an initial implant containing estrogenic activity followed by re-implant with implants containing a combination of estrogenic and androgenic activity 70 to 100 days prior to targeted slaughter. For yearling heifers, the combination of feeding melengesterol acetate (MGA) and implanting either initially or approximately 60 days after feedlot entrance with implants containing androgenic activity seem to induce superior performance than management strategies that do not involve feeding MGA and initially implanting with estrogenic activity followed by re-implant with estrogenic and androgenic combinations later. An example of this management strategy would be to implant initially with Synovex® C or Synovex® H followed by a re-implant with Synovex® H or Finaplix® H (Mader and Lechtenberg, 2000).

FINISHING PHASE

The major determinant for slaughter in commercial feedlots is visual appraisal of backfat thickness measuring approximately 0.5 inches (Doleman et al., 1998) which generally corresponds to the amount of marbling that meets the targeted USDA Choice grade (Klopfenstein et al., 2000). Consideration of how cattle are finished is also important when determining market dates to not only obtain adequate levels of marbling but also prevent discounts due to lightweight or heavyweight carcasses and undesirable yield grades. At similar carcass composition, cattle placed in calf-fed systems are slaughtered at lighter weights than cattle backgrounded on forage diets and finished as yearlings (Lunt and Orme, 1987).

Supplementing MGA, a progestin that enhances endogenous estrogen production to increase growth, in finishing diets fed to intact, pubertal heifers helps improve gains and efficiencies by eliminating estrus occurrence, activity, and subsequent stress in the feedlot. The mode of action of MGA or other forms of progesterone inhibit the gonadotropic complex through the interaction

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of luteinizing hormone with follicle stimulating hormone to alter ovarian function and eliminate the estrous cycle (O'Brien et al., 1968). By eliminating estrus activity and disruption of normal heifer behavior and also maintaining uninterrupted influence of endogenous estrogens as a natural anabolic agent to increase growth and muscle accretion, feeding MGA allows heifers to gain BW and convert feed to gain similarly to steers in the feedlot (O'Brien et al., 1968). Heifers supplemented with MGA have been observed in estrus in the feedlot; however, this incidence is usually very low and more likely to occur when low (0.3-0.4 mg/d instead of the recommended 0.5 mg/d) doses are fed (O'Brien et al., 1968; Zimbleman and Smith, 1966; Bloss et al., 1966). Compared to non-supplemented heifers, MGA supplemented heifers gained 6% faster and required 9% less feed per unit of gain, so advantages to feeding MGA to intact feedlot heifers are apparent (Utley et al., 1972; Bloss et al., 1966).

Other growth-promoting agents used in the feedlot to improve weight gain and efficiency are ~­agonists. In the United States, the two commonly used ~-agonists are ractopamine hydrochloride (Optaflexx), and more recently, zilpaterol hydrochloride (Zilmax). These agents repartition nutrients toward decreased lipogenesis, increased protein accretion, decreased protein degradation, or a combination of these effects to increase final BW and improve gains and efficiencies during the finishing phase (Byrem et al., 1998). Several research studies confirm that supplementing 200 mg per head per day of ractopamine hydrochloride during the final 28 days on feed effectively improved average daily gain, feed conversion, and hot carcass weight in feedlot heifers (Walker et al., 2006; Sissom et al., 2007) and steers (Greenquist et al., 2007; Gruber et al., 2007; Winterholler et al., 2007; Crawford et al., 2006; Schroeder et al., 2003). In contrast to Sissom et al. (2007), when Quinn et al. (2008) supplemented feedlot heifers with 200 mg per hd/d ofractopamine hydrochloride during the final 28 days on feed, daily intake and gains and carcass characteristics were not different, and there was no effect on fat thickness or marbling score, suggesting ractopamine hydrochloride had no influence on altering intramuscular or subcutaneous lipid deposition. Therefore, an important determinant of how heifers will respond to supplementation of ~-agonists may be influenced by the anabolic status of the heifer caused from payout of the implant strategy employed (Sissom et al., 2007).

A second ~-agonist, zilpaterol hydrochloride, was approved on February 28, 2008 to be fed at a rate of7.5 to 8.3 mg/kg ofDM in combination with monensin, tylosin, and MGA to feedlot heifers during the final 20-40 days on feed with a recommended three day withdrawal period (FDA, 2008). Montgomery et al. (2009) and Robles-Estrada et al. (2009) reported increased average daily gain, feed conversion, final BW and hot carcass weight, as well as improved carcass muscling, increased longissimus muscle areas and lean yields, reduced trimmed fat percentages, and improved yield grades for heifers supplemented with zilpaterol hydrochloride during the final 20 to 40 days on feed. Supplementing heifers with this ~-agonist may reduce quality grade and frequency of carcasses grading USDA Choice while increasing the percentage of carcasses grading USDA Select. However, it appears the negative impacts on quality grade may be remediated by feeding zilpaterol hydrochloride for shorter durations (20 days) at the end of the finishing phase (Montgomery et al., 2009).

The number, as well as potency, of implants administered during the finishing phase not only improves muscle deposition and animal growth performance but also influences carcass quality and beef tenderness traits (Smith et al., 2006; Morgan, 1997). Higher potency implants

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administered more than once during the finishing phase have the most pronounced adverse effects on marbling and beef quality (Morgan, 1997), and some implant programs may result in advanced skeletal maturity (Reiling and Johnson, 2003). When an important goal of feedlot producers is supplying high quality retail products that exceed consumer acceptance, careful consideration of implant strategies is necessary to optimize animal performance without sacrificing carcass quality and meat tenderness.

In a study by Schneider et al. (2007), feedlot heifers implanted with one or two androgenic implants (200 mg trenbolone acetate (TBA) each), a single combination implant (8 mg estradiol (E2) and 80 mg TBA or 20 mg E2 and 200 mg TBA), or two, low-dose combination implants (8 mg E2 and 80 mg TBA) had greater percentage of carcasses grading USDA Choice and Prime than heifers implanted with high, cumulative, combined doses of estrogen and androgens (i.e. two implants of 14 mg E2 and 140 mg TBA, 8 mg E2 and 80 mg TBA followed by re-implant with 20 mg E2 and 200 mg TBA, 14 mg E2 and 140 mg TBA followed by re-implant with 20 mg E2 and 200 mg TBA, or two implants of 20 mg E2 and 200 mg TBA) as was also observed by Berger and Galyean (2000) and Brandt et al. (2000). Additionally, single ingredient implants containing 200 mg of TBA had no negative effects on heifer carcass quality or meat tenderness, whereas the use of implants containing the combination of 20 mg of estradiol and 200 mg TBA reduced marbling score and increased strip loin steak Warner Bratzler shear force values. However, post-mortem aging of carcasses for 28 days may provide a pleasurable eating experience for both implant strategies (Schneider et al., 2007). Therefore, it may be possible to overcome negative impacts of growth-promoting agents on meat tenderness post-mortem.

ASSESSMENT OF THE VALUE OF SPAYED HEIFERS FOR MARKETING

Introduction

Restrictions on interstate travel of intact heifers due to Minnesota's split-state bovine tuberculosis (TB) status of Modified Accredited Advanced (MAA) for the majority of the state and Modified Accredited (MA) in northwest Minnesota are slightly eased for spayed heifers. Intact heifers are required to have proof of an individual (within 60 days of travel) and also whole-herd negative TB test (within the last 12 months prior to travel) before crossing state borders. However, feeder cattle, including spayed heifers, can be exported across most borders to feedlots without any restrictions if they originate from the MAA area, and feeder cattle from the MA zone are only required to have a negative individual TB test within 60 days of interstate travel. Subsequently, spayed heifers may have added value since they can be marketed out of Minnesota to larger cattle feeding states such as South Dakota, Nebraska, and Kansas. Therefore, an experiment was proposed to evaluate the effectiveness of a new management strategy in Minnesota (spaying and implanting feeder heifers) versus the traditional practice of supplementing MGA to finishing heifers on feedlot performance, carcass characteristics, and beef quality.

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Objectives and Hypothesis

The specific goals of this project were to evaluate differences in performance during a backgrounding phase due to reproductive status (spayed vs. intact) of feeder heifers and differences in performance and carcass characteristics during the finishing phase due to reproductive status (spayed vs. intact) and re-implant strategy (moderate vs. aggressive re­implant). The investigators hypothesized that performance of spayed, implanted heifers would be similar to performance of implanted, intact heifers supplemented with MGA, and also that the aggressive re-implant strategy should enhance performance of heifers.

Materials and Methods

Upon arrival to the feedlot, heifers were weighed, tagged with electronic ID, and assigned to an overnight pen where a recorded amount of dry hay and unlimited access to water were offered. Initial processing of all cattle included administration of one dose of Bovishield Gold-V, a 5-way viral vaccination, one dose of One-Shot against Mannheimia haemolytica, and one dose of Ultrabac-7:Somubac, a 7-way clostridial vaccine that includes Histophilus somnus. Heifers were dewormed with Dectomax pour-on and dosed with a metaphylaxis antibiotic (Draxxin). Once cattle were processed and all data was recorded, they were assigned to a home pen and treated as a group. At 21 d, cattle were boostered for the 5-way viral and 7-way clostridial vaccine (including Histophilus somnus) and treated with Valbazen due to the likely possibility of deer fluke (Fasciola magna) infection. Rectal temperatures were measured on d 21, and any heifer with a rectal temperature higher than 103.5°F was treated with Excede. Cattle were observed daily and if respiratory disease was suspected, they were treated with a dose ofMicotil or Nuflor. At 28 d, an experienced veterinarian spayed half of the heifers from each owner within a weight class. Heifers were observed closely for two weeks after spaying to ensure their return to normal health.

The backgrounding experiment comparing only reproductive status (Spayed vs. Intact heifers) began on d 42 after arrival and continued until the heavier blocks weighed approximately 850 lb (after 66 DOF) and the lighter blocks weighed 700 lb (after 86 DOF). Heifers were implanted with Synovex® C (Fort Dodge Animal Health, Fort Dodge, IA) using a standard implanting gun subcutaneously in the middle third of the backside of the ear and assigned to one of 16 pens according to weight and treatment on d 42. Heifers were stepped up to a diet containing 50 Meal NEg/cwt and 14% CP (Table 2) and consisting of com silage, beet pulp, ground com, and supplement (Table 1 ). Heifers were limit-fed diets, and intact heifers were supplemented with MGA (0.5 mg/hd/d) for estrus suppression. Amount of feed delivered and refused was recorded daily, and DM of the diet was measured to determine DMI of pen. Average daily gain and feed efficiency were calculated from DMI and direct or adjusted final weight at completion of the backgrounding phase. Initial and final BW measurements were the average of two consecutive day weights after removing feed for 16 h. Interim BW was measured every 28 d prior to feeding throughout the experiment. During the backgrounding period, interim weights were used to simulate backgrounding end points to represent various marketing times and determine backgrounding profit or loss after various times on feed.

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At initiation of the finishing phase, heifers were randomly assigned to one of two re-implant strategies (Moderate; Synovex® Hor Aggressive; Synovex® Plus re-implant; Fort Dodge Animal Health, Fort Dodge, IA). Re-implant date for each weight class was d 1 of the finishing period. Pens were adapted to the finishing ration over a 28-d step-up period. The finishing ration contained 60 Meal NEg/cwt and 11.4% CP (Table 2) and consisted of high moisture com, com silage, and a supplement containing monensin sodium (360 mg/hd/d) and tylosin phosphate (90 mg/hd/d; Table 1). Intact heifers continued to receive MGA (0.5 mg/hd/d). Cattle were fed for ad libitum intake, and amount of feed delivered and refused was recorded daily to determine DMI of pen. Average daily gain and feed efficiency was calculated from DMI and direct or adjusted final weight at completion of the finishing phase.

Proper degree of finish and endpoint weight (visual assessment of 60% USDA Choice) determined when heifers were marketed by weight class. Heavy heifers were marketed to JBS Swift and Company in Grand Island, NE after 80 DOF, and light heifers were marketed after 88 DOF for humane slaughter according to approved procedures of the abattoir. Final weights were calculated from individual hot carcass weights and common dressing percentage for experimental cattle harvested on that date. Following a 24-h chill, carcass data, including dressing percent, HCW, longissimus muscle area, lib rib fat depth, quality grade, and yield grade were collected. Carcass tags were cross-referenced to individual animal ID tags for identification.

Performance and carcass data were analyzed using PROC MIXED of SAS (SAS Institute, Inc., Cary, NC) with significance declared at P-values :S 0.05 and trends discussed at P-values :S 0.10. Data were analyzed as a randomized complete block design with a 2 x 2 factorial arrangement of treatments with pen as the experimental unit. The model fixed effects were reproductive status (spayed vs. intact) and re-implant strategy (moderate vs. aggressive) and their interaction, and random effect was block. Least squares means were separated using the PDIFF option when the F-test statistic was significant.

Results

Table 1 lists the ingredient compositions and Table 2 lists the analyzed nutrient and energy compositions of the experimental diets fed to the heifers during the backgrounding and finishing phases.

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Table 1. Ingredient composition (OM-basis) of diets fed to heifers during backgrounding and finishing phases.

Backgrounding Finishing

· Ground Com, % 37.0

High Moisture Com, % 83.5

Wet Beet Pulp, % 30.0

Com Silage,% 20.0 11.0

Haylage, % 7.0

Pelleted Supplement1, % 6.0 5.5

1 Two supplements formulated to differ only in melengesterol acetate inclusion (0.5 mg/hd/d) fed only to intact heifers. Supplements contained 9.8% Ca, 1.1 % P, 0.9% Mg, 1.1 % K, 1.8% Na, 1. 1 % S, 754 ppm Zn, 207 ppm Cu, 803 ppm Fe, and 12 ppm Co. Supplements were formulated to contain monensin sodium (360 mg/hd/d) and tylosin phosphate (90 mg/hd/d).

Table 2. Nutrient and energy composition (OM-basis) of diets fed to heifers during backgrounding and finishing phases.

Backgrounding SD Finishing SD

DM,% 34.1 0.7 65.0 2.5

CP,% 14.2 0.4 11.4 0.5

NDF,% 29.1 2.2 12.4 1.7

ADF,% 18.1 2.3 5.46 1.86

Crude Fat,% 2.98 0.13 4.43 0.34

TDN,% 72.5 2.4 80.0 3.7

Ca,% 0.975 0.026 0.441 0.144

P,% 0.248 0.010 0.315 0.030

S, % 0.223 0.017 0.153 0.010

NEm 1, Meal/lb 0.778 0.038 0.896 0.053

NEg 2, Meal/lb 0.500 0.035 0.601 0.046

Net energy requirement for maintenance. 2 Net energy requirements for gain.

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Table 3 lists the effects of reproductive status on heifer performance during the backgrounding phase. Spayed heifers tended (P = 0.11) to consume more feed (16.3 vs. 15.4 lb/d) and gained 10% less (P < 0.01) weight than intact heifers (3.21 vs. 3.55 lb/d). This led spayed heifers to be less efficient (P = 0.01) at converting feed to gain during the backgrounding phase (5.12 vs. 4.36).

Table 3. Effect of reproductive status on heifer performance during the backgrounding phase.

Status

Intact Spayed SE P-Value1

In. BW, lb 619 622 74.8 0.91

EndBW, lb 864 840 18.7 0.01

DMI, lb/d 15.4 16.3 1.04 0.11

ADG, lb/d · 3.55 3.21 0.21 < 0.01

F:G 4.36 5.12 0.18 0.01 1 Significance declared with P < 0.05, and trends discussed with 0.05:::; P:::; 0.10.

Likely the choice of backgrounding phase implant (low potency Synovex® C), and the presence ofMGA, an estrous suppressing feed additive that is shown to enhance performance (but is illegal to use in spayed heifers) contributed to these differences. At the performance difference observed in this study and if feed price is assumed to be $70/ton DM, a producer would need to spend an additional $6.65 to feed a spayed heifer over an intact one during a backgrounding phase. If the cost of spaying a heifer ($18/heifer) is added, the overall investment in growing spayed heifers under a low potency implant strategy approximate $25/hd. A producer feeding and managing spayed heifers during a backgrounding phase would need to receive an additional $0.03/lb to breakeven with one that keeps intact heifers. However, within a MA TB zone, the producer keeping intact heifers would have to spend at least $25/hd to conduct tests on the heifer and whole herd to export the heifer beyond MA zone borders. Additionally, the appeal of selling spayed heifers to feedlots is not reflected by this analysis. As indicated, spayed heifers are desired in feedlots since they can be managed without concern for estrous activity leading to reduced overall performance and efficiency.

Table 4 lists the effects of reproductive status and re-implant strategy on heifer performance during the finishing phase. There were no differences (P > 0.10) in final live BW, DMI, ADG, or feed conversion between spayed and intact heifers as was also observed by Field et al. (1996), Hamernik et al. (1985), Grotelueschen et al. (1988), and Klindt and Crouse, (1990), but these observations are contradictory to earlier reports that suggest spaying has a slight negative effect on rate and efficiency of gain (Dinusson et al., 1950; Kercher et al., 1960; Nygaard and Embry, 1966; Horstman et al., 1982). Thus, it appears the performance advantages gained by intact heifers during the backgrounding phase were not maintained through the entire feeding phase since spayed heifers were able to compensate for gain and efficiency differences. This observation may be partially explained due to inconsistent responses in weight gain, ADG, and feed efficiency for spayed heifers that may be influenced by management factors such as age at

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time of spaying, feedlot entry weight, length of feeding period, nature of the feedlot diet, residual ovarian tissue inadvertently left within the heifer after spaying, and implanting program employed (Adams et al., 1990).

Additionally, the suppression of weight gain induced by spaying may have been reversed by concurrent administration of Synovex® Hor Synovex® Plus. Adams et al. (1990) reported that spayed heifers implanted with Synovex® H had comparable total weight gains over the finishing period as control heifers implanted with Synovex® H, and implants did not affect total weight gain in MGA supplemented heifers either, as was observed in this study. However, heifers that received the moderate re-implant tended (P = 0.07) to consume more DMI but had similar gains and therefore, tended (P = 0.07) to have slightly poorer feed conversion compared to heifers receiving the more aggressive re-implant strategy.

Table 4. Effect of reproductive status and re-implant strategy on heifer performance during the finishing phase.

Status Implant1 P-Value2

Intact Spayed Moderate Aggressive SEM Status Implant S*I

Final Live 1137 1128 1131 1134 29 0.23 0.68 0.80 BW,lb

DMI, lb/d 21.2 20.9 21.4 20.6 0.3 0.41 0.07 0.66

ADG, lb/d 3.34 3.25 3.29 3.30 0.18 0.28 0.89 0.61

F:G 6.40 6.41 6.57 6.25 0.46 0.94 0.07 0.93 1 Moderate implant contained 200 mg testosterone propionate and 20 mg estradiol benzoate per dose. Aggressive implant contained 200 mg trenbolone acetate and 28 mg estradiol benzoate per dose. 2 Significance declared with P < 0.05, and trends discussed with 0.05 ~ P ~ 0.10. Main effects of status and implant are reported with non-significant status by implant (S *I) interaction.

Table 5 lists the effects of reproductive status and re-implant strategy on heifer carcass characteristics. There were no differences (P > 0.10) in carcass characteristics between spayed or intact heifers as was also observed by Field et al. (1996) or in heifers receiving the moderate or aggressive re-implant strategy. Spaying heifers does not improve carcass characteristics as was suggested previously by Garber et al. (1990) or affect meat tenderness or consumer acceptability (Field et al., 1996). In contrast, Adams et al. (1990) reported that surgical removal of ovaries increased marbling and reduced dressing percentage but did not impact backfat thickness or proportion of fat deposition in weight gained. Therefore, it appears implanted, spayed heifers were again able to compensate and perform similarly to implanted, intact heifers receiving MGA during the finishing phase. Additionally, choice of implant should be carefully considered as it does appear to influence some performance variables on both spayed and intact heifers. Although there were no differences in this study, there are concerns of aggressive or long-term implant strategies negatively affecting carcass quality and meat tenderness (Morgan, 1997).

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Table 5. Effect ofreproductive status and re-implant strategy on heifer carcass characteristics.

Status Implant1 P-Value2

Intact Spayed Moderate Aggressive SEM Status Implant S*I

HCW, lb 706 700 702 704 14.2 0.23 0.68 0.80

12th Rib 0.470 0.451 0.463 0.458 0.055 0.52 0.86 0.68 BF, in.

LMArea3, 12.1 12.2 12.1 12.1 0.3 0.82 0.95 0.26

in. 2

Marbling4 582 601 588 595 12.9 0.30 0.67 0.16

Yield 2.95 2.85 2.90 2.89 0.23 0.63 0.94 0.33 Grade5

Carcass 980.82 974.52 984.64 970.70 10.38 0.68 0.38 0.50 Value, $/hd 1 Moderate implant contained 200 mg testosterone propionate and 20 mg estradiol benzoate per dose. Aggressive implant contained 200 mg trenbolone acetate and 28 mg estradiol benzoate per dose. 2 Significance declared with P < 0.05, and trends discussed with 0.05 ~ P ~ 0.10. Main effects of status and implant are reported with non-significant status by implant (S *I) interaction. 3Cold camera estimate of LM area. 4 Marbling score based on 500 = Small00

; 600 = Modest00.

5 Cold camera estimate of USDA yield grade.

CONCLUSIONS

Spayed heifers tended to consume ~ore feed but gained less to result in reduced conversion of feed to gain during the backgrounding phase. Likely, the choice of low potency implant (Synovex® C) to all heifers and inclusion of MGA to intact heifers contributed to performance differences between spayed and intact heifers during backgrounding. During finishing, spaying had no effect on DMI, ADG, feed conversion, or final live BW. Heifers :feceiving moderate re­implants tended to consume more DMI but gained similarly to result in slightly poorer feed conversion than heifers receiving aggressive re-implants. Carcass characteristics were not affected by either reproductive status or re-implant strategy. Therefore, spaying appears to be a viable tool for managing heifers in the feedlot provided implant strategy is carefully considered to compensate for lost endogenous sources of estrogen and progesterone.

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TAKE HOME MESSAGE

Although finishing heifers in the feedlot presents its challenges with nutrition and management, with proper research and education, these challenges can be easily overcome by feedlot producers. There are several key points to consider when finishing heifers to help attain high quality meat retail products effectively and efficiently:

• Preconditioning na1ve, weaned calves increases the likelihood for animals to attain genetic potential for performance in the feedlot.

• Backgrounding is a management strategy that uses mainly forages to increase frame size and muscle in smaller-framed, lightweight cattle prior to feedlot entry.

• In the past, feeding beef heifers has been discriminated against in the industry due to sex effects that act on nutrient partitioning and behavior as well as the possibility of pregnancies in the feedlot.

• Heifers generally produce a fatter carcass and are considered to have reduced feed conversions due to increased stress and disrupted feed intake during estrous cycles.

• Heifers generally mature earlier and deposit fat more rapidly in earlier stages of the finishing period compared to steers.

• Heifers typically need to be marketed at lighter end weights than steers before undesirable yield grades are achieved.

• Heifers finished as calves may have increased 12th rib backfat, increased yield grade, greater quality grade, higher marbling scores and more efficient gains than heifers finished as two-year olds.

• Spaying heifers prior to feedlot entrance has many advantages and may be a viable alternative to feeding MGA to intact heifers for estrus suppression.

• Although there may be reduced performance of spayed heifers during the growing phase, spayed heifers seem to perform similarly to intact heifers during the finishing phase.

• Choice of implant program may affect feedlot performance and should be carefully considered for both intact and spayed heifers for maximum performance but minimal effect on carcass quality and beef tenderness.

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LITERATURE CITED

Adams, T.E., J.R. Dunbar, S.L. Berry, W.N. Garrett, T.R. Famula, and Y.B. Lee. 1990. Feedlot performance of beef heifers implanted with Synovex-H: effect of melengesterol acetate, ovariectomy or active immunization against GnRH. J. Anim. Sci. 68:3079-3085.

Berger, L.L. and M.L. Galyean. 2000. Evaluation of various implant programs on performance and carcass merit of finishing heifers. Revalor-IH Tech. Ref. 1. Intervet Inc., Millsboro, DE.

Bloss, R.E., J.I. Northam, L.W. Smith, and R.G. Zimbelman. 1966. Effects of oral melengesterol acetate on the performance of feedlot cattle. J. Anim. Sci. 25:1048-1053.

Boleman, S.L., S.J. Bolerman, W.W. Morgan, D.S. Hale, D.B. Griffen, J.W. Savell, R.P. Ames, M.T. Smith, J.D. Tatum, T.G. Field, G.C. Smith, B.A. Gardner, J.B. Morgan, S.L. Northcutt, H.G. Dolezal, D.R. Gill, and F.K. Ray. 1998. National Beef Quality Audit-1995: Survey of producer-related defects and carcass quality and quantity attributes. J. Anim. Sci. 76:96-103.

Brandt, R.T., W.K. Rowland, E.G. Johnson, J. Johnson, G.E. Sides, J.P. Hutcheson, W.T. Nichols, and C.D. Reinhardt. 2000. Evaluation of implants containing reduced-dose combinations of trenbolone acetate and estradiol on performance and carcass merit of finishing heifers (Idaho). Revalor-IH Tech. Bull. 1. Intervet Inc., Millsboro, DE.

Byrem, T.A., D.H. Beerman, and T.F. Robinson. 1998. The ~-agonist cimaterol directly enhances chronic protein accretion in skeletal muscle. J. Anim. Sci. 76:988-998.

Coffey, K.P., W.K. Coblentz, T.G. Montgomery, J.D. Shockey, K.J. Bryant, P.B. Francis, C.F. Rosenkraus, Jr., and S.A. Gunter. 2002. Growth performance of stocker calves backgrounded on sod-seeded winter annuals or hay and grain. J. Anim, Sci. 80:926-932.

Crawford, G.I., G.E. Erickson, K.J. Vander Pol, M.A. Greenquist, J.D. Folmer, and M.T. Van Koevering. 2006. Effects of Optaflexx dosage and duration of feeding prior to slaughter on feed conversion and carcass characteristics. 2006 Nebraska Beef Report, pp. 72-74.

Dinusson, W.E., F.N. Andrews, and W.M. Beeson. 1950. The effects of stilbestrol, testosterone, thyroid alteration and spaying on the growth and fattening of beef heifers. J. Anim. Sci. 9:321-330.

FDA. 2008. Freedom of information summary. Original new animal drug application NADA 141-280. Zilmax plus Rumensin plus Tylan plus MGA (Zilpaterol hydrochloride and monensin USP and tylosin phosphate and melengestrol acetate) Type A medicated article for use in the manufacture of Type B and C medicated feed for heifers fed in confinement for slaughter. http://www.fda.gov/downloads/ AnimalVeterinary/Products/ ApprovedAnimal DrugProducts/FOIADrugSummaries/ucm062336.pdf. Accessed July 7, 2009.

129

Ferrell, C.L. and T.G. Jenkins. 1998. Body composition and energy utilization by steers of diverse genotypes fed a high-concentrate diet during the finishing period. II. Angus, Boran, Brahman, Hereford, and Tuli sires. J. Anim. Sci. 76:647-657 .

. Field, R., R. McCormick, V. Balasubramanian, D. Sanson, J. Wise, D. Hixon, M. Riley, and W. Russell. 1996. Growth, carcass, and tenderness characteristics of virgin, spayed, and single­calf heifers. J. Anim. Sci. 74:2178-2186.

Galyean, M.L. 1999. Review: restricted and programmed feeding of beef cattle - definitions, application, and research results. Prof. Anim. Sci. 15: 1-6.

Garber, M.J., R.A. Roeder, J.J. Combs, L. Eldridge, J.C. Miller, D.D. Hinman, and J.J. Ney. 1990. Efficacy of vaginal spaying and anabolic implants on growth and carcass characteristics in beef heifers. J. Anim. Sci. 68:1469-1475.

Greenquist, M.A., K.J. Vander Pol, G.E. Erickson, T.J. Klopfenstein, W.J. Platter, and M.T. Van Koevering. 2007. Performance profile and carcass characteristics of steers fed Optaflexx. 2007 Nebraska Beef Report, pp. 65-67.

Grotelueschen, D., I. Rush, J. Johnson, J. Kinder, K. Nelson, and B. Van Pelt. 1988. Evaluation of ovarian autographs in spayed heifers. Univ. of Neb. Beef Cattle Report MP 53:61.

Gruber, S.L., J.D. Tatum, T.E. Engle, M.A. Mitchell, S.B. Laudert, A.L. Schroeder, and W.J. Platter. 2007. Effects ofractopamine supplementation on growth performance and carcass characteristics of feedlot steers differing in biological type. J. Anim. Sci. 85:1809-1815.

Guiroy, P.J., L.O. Tedeschi, D.G. Fox, and J.P. Hutcheson. 2002. The effects of implant strategy on finished body weight of beef cattle. J. Anim. Sci. 80: 1791-1800.

Hamernik, D.L., J.R. Males, C.T. Gaskins, and J.J. Reeves. 1985. Feedlot performance of hyst~rectomized and ovariectomized heifers. J. Anim. Sci. 60:358-362.

Horstman, L.A., C.J. Callahan, R.L. Morter, and H.E. Amstutz. 1982. Ovariectomy as a means of abortion and control of estrus in feedlot heifers. Theriogenology 17:273-292.

Hunter, R.A. and J.E. Vercoe. 1987. Reduction of energy requirements of steers fed on low­quality-roughage diet using trenbolone acetate. Br. J. Nutr. 58:477-483.

Hutcheson, J.P., D.E. Johnson, C.L. Gerken, J.B. Morgan, and J.D. Tatum. 1997. Anabolic implant effects on chemical body composition and estimated energetic efficiency in cloned (genetically identical) beef steers. J. Anim. Sci. 75:2620-2626.

Kercher, C.J., R.C. Thompson, P.O. Stratton, C.O. Schoonover, J.A. Gorman, and N.W. Hilston. 1960. Comparison of feedlot performance and carcass value of open vs. spayed vs. bred heifers. Wyo. Agric. Exp. Mimeo 127:1.

130

Klindt, J. and J.D. Crouse. 1990. Effect of ovariectomy and ovariectomy with ovarian autotransplantation on feedlot performance and carcass characteristics of heifers. J. Anim. Sci. 68:3481-3487.

Klopfenstein, T., R. Cooper, D.J. Jordon, D. Shain, T. Milton, C. Calkins, and C. Rossi. 2000. Effects of backgrounding and growing programs on beef carcass quality and yield. J. Anim. Sci. 77: 1-11.

Loy, D., D. Maxwell, and G. Rouse. 1999. Effects of early weaning of beef calves on performance and carcass quality. Iowa State Univ. Beef Res. Rep. AS 641, Leaflet Rl 632, Ames. Pp. 22-24.

Lunt, D.K. and L.E. Orme. 1987. Feedlot performance and carcass evaluation of heifers fed finishing diets as weanling calves or as yearlings. Meat Sci. 20:159-164.

Mader, T.L. 1994. Effect of implant sequence and dose on feedlot cattle performance. J. Anim. Sci. 72:277-282.

Mader, T.L. and K.F. Lechtenberg. 2000. Growth-promoting systems for heifer calves and yearlings finished in the feedlot. J. Anim. Sci. 78:2485-2496.

Montgomery, J.L., C.R. Krehbiel, J.J. Cranston, D.A. Yates, J.P. Hutcheson, W.T. Nichols, M.N. Streeter, D.T. Bechtol, E. Johnson, T.TerHune, and T.H. Montgomery. 2009. Dietary zilpaterol hydrochloride. I. Feedlot performance and carcass traits of steers and heifers. J. Anim. Sci. 87:1374-1383.

Morgan, J.B. 1997. Implant program effects on USDA beef carcass quality grade traits and meat tenderness. Pages 14 7-154 in Proc. Symp. Impact Implants Perform. Carcass Value Beef Cattle, Oklahoma State Univ., Stillwater.

Myers, S.E., D.B. Faulkner, T.G. Nash, L.L. Berger, D.F. Farrett, and F.K. McKeith. 1999. Performance and carcass traits of early-weaned steers receiving either a pasture growing period or a finishing diet at weaning. J. Anim. Sci. 77:311-322.

NASS (National Agricultural Statistics Survey), USDA. July 24, 2009. Cattle. Accessed July 25, 2009 from http://usda.mannlib.comell.edu/MannUsda/viewDocumentinfo.do?documented =1017.

Nygaard, L.J. and L.B. Embry. 1966. Response of spayed and nonspayed heifers to diethylstilbestrol and Synovex implants. South Dakota 10th Annu. Beef Cattle Field Day. A.S. Series 66-13:70.

O'Brien, C.A., R.E. Bloss, and E.F. Nicks. 1968. Effect ofmelengesterol acetate on the growth and reproductive physiology of fattening heifers. J. Anim. Sci. 27:664-667.

131

Platter, W.J., J.D. Tatum, K.E. Belk, J.A. Scanga, and G.C. Smith. 2003. Effects ofrepetitive use of hormonal implants on beef carcass quality, tenderness, and consumer ratings of beef palatability. J. Anim. Sci. 81 :984-996.

Quinn, M.J., C.D. Reinhardt, E.R. Loe, B.E. Depenbusch, M.E. Corrigan, M.L. May, and J.S. Drouillard. 2008. The effects ofractopamine-hydrogen chloride (Optaflexx) on performance, carcass characteristics, and meat quality of finishing feedlot heifers. J. Anim. Sci. 86:902-908.

Reiling, B.A. and D.D. Johnson. 2003. Effects of implant regimens (trenbolone acetate-estradiol administered alone or in combination with zeranol) and vitamin D3 on fresh beef color and quality. J. Anim. Sci. 81:135-142.

Robles-Estrada, J.C., A.A. Arrizon, A. Barreras, J.F. Calderon, F. Figueroa-Saavedra, N. Torrentera, A. Plascencia, and R.A. Zinn. 2009. Effects of preslaughter withdrawal period on response of feedlot heifers to zilpaterol hydrochloride supplementation: growth performance and carcass characteristics. J. Anim. Sci. 87:1759-1763.

Sainz, R.D., F. De la Torre, and J.W. Oltjen. 1995. Compensatory growth and carcass quality in growth-restricted and refed beef steers. J. Anim. Sci. 73 :2971-2979.

Schneider, B.A., J.D. Tatum, T.E. Engle, and T.C. Bryant. 2007. Effects of heifer finishing implants on beef carcass traits and longissimus tenderness. J. Anim. Sci. 85:2019-2030.

Schroeder, A.L., D.M. Polser, S.B. Laudert, and G.J. Vogel. 2003. The effect of Optaflexx on growth performance and carcass traits of steers. Optaflexx Exchange No. 1. Blanco Animal Health, Greenfield, IN.

Shain, D., T. Klopfenstein, D.J. Jordan, and R. Stock. 1998. Summer and fall forage grazing combinations: five-year summary. Nebraska Beef Cattle Report. MP. 69-A:66-69. Lincoln, NE.

Sindt, M., R. Stock, and T. Klopfenstein. 1991. Calf versus yearling finishing. Nebraska Beef Cattle Report. MP. 56:42-43. Lincoln, NE.

Sissom, E.K., C.D. Reinhardt, J.P. Hutcheson, W.T. Nichols,.D.A. Yates, R.S. Swingle, and B.J. Johnson. 2007. Response to ractopamine-HCl in heifers is altered by implant strategy across days on feed. J. Anim. Sci. 85:2125-2132.

Smith, G.C., J.W. Savell, J.B. Morgan, and T.E. Lawrence. 2006. Final report of national beef quality audit-2005: A new benchmark for the US beef industry. Natl. Cattlemen's Beef Assoc., Centennial, CO.

Smith, S.B. and J.D. Crouse. 1984. Relative contributions of acetate, lactate, and glucose to lipogenesis in bovine intramuscular and subcutaneous adipose tissue. J. Nutr. 114:792-800.

132

Trenkle, A. 1997. Mechanisms of action of estrogens and androgens on performance of cattle: Hormonal basis. Pages 15-22 in Proc. Symp.: Impact oflmplants on Performance and Carcass Value of Beef Cattle. Oklahoma Agric. Exp. Stn. Puhl. No. P-957. Stillwater.

Utley, P.R., H.D. Chapman, and W.C. McCormick. 1972. Feedlot performance of heifers fed melengesterol acetate and oxytetracycline separately and in combination. J. Anim. Sci. 34:339-341.

Walker, D.K., E.C. Titgemeyer, J.S. Drouillard, E.R. Loe, B.E. Depenbusch, and A.S. Webb. 2006. Effects of ractopamine and protein source on growth performance and carcass characteristics of feedlot heifers. J. Anim. Sci. 84:2795-2800.

Wertz, A.E., L.L. Berger, D.B. Faulkner, and T.G. Nash. 2001a. Intake restriction strategies and sources of energy and. protein during the growing period affect nutrient disappearance, feedlot performance, and carcass characteristics of crossbred heifers. J. Anim. Sci. 79:1598-1610.

Wertz, A.E., L.L. Berger, P.M. Walker, D.B. Faulkner, F.K. McKeith, and S.L. Rodriguez-Zas. 2002. Early-weaning and postweaning nutritional management affect feedlot performance, carcass merit, and the relationship of 12th -rib fat, marbling score, and feed efficiency among Angus and Wagyu heifers. J. Anim. Sci. 80:28-37.

Wertz, E., L.L. Berger, P.M. Walker, D.B. Faulkner, F.K. McKeith, and S. Rodriguez-Zas. 2001b. Early weaning and postweaning nutritional management affect feedlot performance of angus x simmental heifers and the relationship of 12th rib fat and marbling score to feed efficiency. J. Anim. Sci. 79:1660-1669.

Wester, T.J., R.A. Britton, T.J. Klopfenstein, G.A. Ham, D.T. Hickok, and C.R. Krehbiel. 1995. Differential effects of plane of nutrition on visceral organ and hormones in lambs. J. Anim. Sci. 73:1674-1688.

Winterholler, S.J., G.L. Parsons, C.D. Reinhardt, J.P. Hutcheson, W.T. Nichols, D.A. Yates, R.S. Swingle, and B.J. Johnson. 2007. Response to ractopamine-hydrogen chloride is similar in yearling steers across days on feed. J. Anim. Sci. 85:413-419.

Zimbleman, R.G. and L.W. Smith. 1966. Control of ovulation in cattle with melengestrol acetate. I. Effect of dosage and route of administration. J. Reprod. Fertil. 11: 185.

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