Diagnosis and Management of Rumen Acidosis and Bloat in ...

Diagnosis andManagement of Rumen

Nathan F. Meyer, MS, MBA, PhD, DVMa,b,*, Tony C. Bryant, MS, PhDa,c

KEYWORDS

� Acidosis � Bloat � Feedlot � Diagnosis � Rumen

KEY POINTS

� Ruminal bloat and ruminal acidosis represent the most common digestive disorders infeedlot cattle.

� Prevention of digestive disorders focuses on proper grain adaptation, sufficient ration fi-ber, ionophore inclusion, and minimizing feed variation.

� Diagnosis of digestive disorders should include a thorough feed and treatment history,evaluation of animals in their home environment, and complete necropsy.

� Treatment of digestive disorders depends on the specific digestive disorder and severity.A large number of animals may be affected so triage becomes critical to minimize impacts.

� Numerous animal and operational variables determine the digestive disorder prevalence,preventive techniques, treatment options, and overall impacts of digestive disorders in thefeedlot.

INTRODUCTION

Most beef cattle in North America are raised on pasture for most their lives and thenare finished in a feedlot on a high-concentrate diet composed of cereal grains such ascorn, wheat, or barley. Economics favor a grain-finishing production system becauseof several factors, including reduced cost per unit of energy and resulting improvedgrowth efficiencies associated with grains compared with roughages; availability ofgrains and land; logistical, storage, and operational efficiencies of transporting andhandling grain; consistency of the nutrient profile of grains; and the quality and flavoraspects of beef produced from grain-fed cattle. The rumen is remarkably adaptable to

Disclosure: The authors have nothing to disclose.a JBS Five Rivers Cattle Feeding, LLC, 1770 Promontory Circle, Greeley, CO 80634, USA;b Department of Clinical Sciences, Colorado State University, 1678 Campus Delivery, FortCollins, CO 80523, USA; c Department of Animal Sciences, Colorado StateUniversity, 350WPitkinStreet, Fort Collins, CO 80521, USA* Corresponding author. 1770 Promontory Circle, Greeley, CO 80634.E-mail address: [email protected]

Vet Clin Food Anim 33 (2017) 481–498http://dx.doi.org/10.1016/j.cvfa.2017.06.005 vetfood.theclinics.com0749-0720/17/ª 2017 Elsevier Inc. All rights reserved.

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digest both forage and grain, and health challenges are common to cattle in each pro-duction phase. During the traditional feedlot phase of production, digestive disorderssuch as acidosis and bloat can result from the rapid fermentation of grain in the rumen.Understanding contributing causes and management factors that can help mitigatethese potential maladies is important in order to optimize cattle production and health.

PREVALENCE OF DIGESTIVE-RELATED MORTALITY

Compared with other causes of mortality in the feedlot, digestive-related mortalitiescomprise 19.5% to 28.4% of all mortalities.1–3 An internal database was examinedthat represented 4,487,364 head of cattle marketed between the years 2014 and2016 in 11 feedlots ranging from southern Idaho to southern Arizona. In these feedlotsand over this time frame, digestive mortality represented 0.073% (percentage ofmonthly occupancy) with a range from 0.064% (2016) to 0.082% (2015).2 In addition,digestive mortality accounted for 26.9% of all mortalities with a range of 25.3% (2016)to 28.4% (2015). This prevalence was similar to that found by Vogel and Parrott,3 whoreported an average monthly digestive mortality of 0.06% (range, 0.05%–0.08%) anddigestive mortality comprising 25.9% of all mortalities. In a more recent publication byVogel and colleagues,1 an analysis was performed from an industry feedlot databasewith closeout records from 2005 to 2014. This analysis revealed that 19.5% of mortal-ities were digestive related, with a range of 0.039% to 0.049%monthly digestive mor-tality, and the average day-on-feed at death was day 99. In addition, digestivemortality tends to be positively correlated with days on feed and with the beta-agonist feeding period.1,4,5 Anecdotally, the investigators have observed that Hol-steins and cattle housed in feed yards at higher elevations are more at risk of bloat.In addition, it has been observed that cattle in feedlots north of 38� north latitudehave a higher prevalence of digestive mortality.2 This finding could be a function of ge-netics, weather, or other factors associated with more northern latitudes.An analysis of mortalities diagnosed as bloat or acidosis provides the relative contri-

bution of each diagnosis to total digestive-related mortality.2 From the years 2014 to2015, 96.3% of all digestive mortalities were diagnosed as bloat compared with 3.7%diagnosed as acidosis. From a mortality standpoint, the contribution of bloat todigestive-related mortality represents most cases.

RUMINAL ACIDOSIS REVIEW

Ruminal acidosis in feedlot cattle can occur when rumen osmolality increasesbecause of accumulation of lactate, short-chain fatty acids (ie, volatile fatty acids[VFAs]), and glucose. As a result, rumen pH decreases and the body reacts in a pro-tective fashion by reducing feed intake and reducing acid absorption.6,7 Lactate accu-mulation predominates in acute acidosis from the increased rate of production ofglucose and reduced use of glucose, causing lactic acid–forming bacteria to prolifer-ate.8 The significance of ruminal lactate concentrations in subacute and acuteacidosis seems to differ.9 Researchers have indicated that concentration of totalorganic acids is of greater significance in subacute acidosis10 and lactate may be ofgreater significance in acute acidosis.11 As a result of ruminal acid accumulation,the ruminal osmotic pressure exceeds that of blood, resulting in a concentrationgradient and a net flux of water into the rumen. This high osmotic pressure and influxof water can cause diarrhea and dehydration as well as damage to the rumen epithe-lium causing rumenitis (Fig. 1).6

During repair, the rumen wall can be thickened, and ruminal papillae can be altered,resulting in parakeratosis.12 This resulting damage to the rumen epithelial wall can

Fig. 1. The cause of acidosis in cattle. (Adapted from Owens FN, Secrist DS, Hill WJ, et al.Acidosis in cattle: a review. J Anim Sci 1998;76(1):277; with permission.)

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increase permeability, which can have long-term effects on both nutrient and toxin ab-sorption.13,14 The rumen microbial population also changes with the changing rumenpH and with increased lactate accumulation so that lactate producers such as Strep-tococcus bovis and Lactobacillus spp proliferate and protozoa and cellulolytic mi-crobes decline.8,15–19 Entodiniomorph protozoa are known to engulf and sequesterstarch and ferment it at a slower rate than bacteria; however, the reduction of theseciliates during acidosis may provide for a less stable rumen environment.18,20,21 Thealtered microbial composition, reduced salivation, and reduced ruminal pH can alsodecrease fiber digestibility, which can exacerbate the issue further.17,22–24

Endotoxins such as lipopolysaccharides and amines such as histamines can also bereleased from microbes because of the lysis of microbial cells that occurs at low pH,and these are then absorbed more easily across the damaged rumen wall.14,19,25–28

These products, along with the damaged rumen epithelium, can affect rumen motilityand can contribute to other ailments associated with acidosis, such as bloat, laminitis,liver abscesses, polioencephalomalacia, and death.7,25,29 If the rumen pH cannot beequilibrated, the acids will be absorbed into the blood. Consequently, if the bicarbon-ate buffering capacity of the body is overwhelmed, systemic acidosis can result.

CLINICAL FINDINGS

Ruminal acidosis can vary from a mild indigestion to subacute acidosis to an acuteand often fatal metabolic acidosis. When acidosis is suspected, a thorough historyshould be collected, including ration composition, milling records, and feed records.In addition, feed transition schedules and feed step-ups can aid in understandingthe risk of ruminal acidosis. Severity of ruminal acidosis is a function of time belowa specified threshold pH (5.2) and magnitude below the threshold. Animal variationalso exists in tolerance to acidosis, as shown by intraruminally placed pH metersand challenge models.30 In cases of mild or subacute ruminal acidosis, cattle mayshow signs of colic and anorexia, and loose stools are commonly present. When anentire group of animals is affected, it is common to see a reduction in feed intake,and, as the home pen surface is evaluated, stools will be seen across the surfacethat have a shiny appearance and are mostly liquid. In addition, cattle commonlyhave an abnormal amount of manure smeared across their hindquarters. Mild or

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subacute acidosis is commonly seen during ration transitions, weather alterations,and mild feeding errors.In severe cases of ruminal acidosis, the first indication of a problem is often a large

decrease in feed consumption noticed by the feed caller. In addition, pen checkersnotice cattle showing clinical signs consistent with ruminal acidosis. Cattle may berecumbent, some may be staggering, and severe anorexia is present. Recumbent an-imals often lie with their heads tucked in their flanks, similar to parturient paresis.Further examination reveals an absence of ruminal contractions, diminished or absentpalpebral reflexes, severe dehydration, and animals that are severely lethargic. A pro-fuse and malodorous diarrhea develops and grain is typically present in the stools atan abnormal quantity from the increased rate of passage and decreased digestibility.Postmortem examination at the time of death reveals severe dehydration (eye

recession), copious amounts of fluid in the gastrointestinal tract, and a low ruminalpH (<5.0). The mucosa of the ruminal papillae is brown, friable, and easily detaches,revealing a diffuse and severe rumenitis (Fig. 2).31 Ruminal contents have an increasedproportion of fluid and undigested grain and a sour smell.

TREATMENT

Once a presumptive diagnosis of acidosis has been made, the current feed should beevaluated and bunk scooped or ration altered to prevent cases from progressing ornew cases developing. Cattle that are recumbent have a guarded to poor prognosis.Severely compromised cattle should be humanely euthanized.Standing cattle should be offered grass hay and water and monitored until they are

stable. Systemic antimicrobials, nonsteroidal antiinflammatory drugs, thiamine, fluidsupport, and oral magnesium hydroxide are often beneficial to address the varying ef-fects of the insult.32 Limited research is available on stepping cattle back up to a

Fig. 2. Acute acidosis. Note the sloughing of the ruminal papillae revealing a diffuse andsevere rumenitis.

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finisher ration after an acidotic bout. It is common to wait a week after the initial insultand then begin a slow ration step-up program.Laminitis, polioencephalomalacia, and liver abscesses are sequelae that can occur

in cattle that recover from rumen acidosis. It is also the investigators’ observation thatcattle performance is severely affected following a severe acidosis insult and rarelydoes the animal or pen return to previous intake and performance. Salvage slaughtershould be considered for cattle that do not respond to treatment or have a poorrecovery.

RUMINAL BLOAT REVIEW

Bloat (ruminal tympany) results when excess gas accumulates in the rumen, resultingin increased intraruminal pressure and distention of the left dorsal abdominal wall,becoming apparent with protrusion of the paralumbar fossa area. Ruminal fermenta-tion of feedstuffs by microbes generates gases, primarily carbon dioxide andmethane. These gases are expelled from the rumen by absorption through the rumenwall, passage through to the next stomach compartment, or eructation through theesophagus; eructation through the esophagus is the predominant route for gas expul-sion from the rumen.33,34

Eructation is a complex process involving integration of numerous organs by thecentral nervous system.33 Ruminants use organ reticulorumen contractions to mixcontents and for eructation. The primary contractions consist of 2 contractions ofthe reticulorumen fold and then a contraction that moves caudally. Eructation is asso-ciated with the secondary contractions and comprise contractions of the dorsal cor-onary pillar, caudodorsal blind sac, and dorsal sac, and relaxation of the caudoventralblind sac.35 Contractions of the longitudinal and cranial pillars prevent digesta fromfilling the cranial sac and allows free gas in the dorsal sac to be expelled from the car-dia. The multiple-stage eructation event is induced by increased ruminal gas and fillpressure, which is sensed by tension receptors located in the reticulum and cranialsac of the rumen.35,36 The information from the tension receptors is transmitted viathe afferent vagus nerve fibers to the central nervous system. If the distention is se-vere, then ruminal contractions are inhibited and result in ruminal atony.37 During eruc-tation, gas passes out of the rumen through the cardia into the esophagus and thenpharynx. As the nasopharyngeal sphincter is closed, the gas is forced through the tra-chea and then into the lungs, where some gas is absorbed and the remainderexhaled.33,36

If the rate of gas production exceeds the rate of elimination, then bloat can result. Inextreme situations, the increased gas accumulation and resulting ruminal distentioncan exert pressure on the diaphragm and lungs, which can inhibit respiration andmay result in death by asphyxiation if not alleviated. Bloat can occur in cattle grazingpasture or in cattle housed in a feed yard and can occur in the forms of free-gas bloator as frothy (foamy) bloat. Free-gas bloat usually results from one of the followingcauses: a physical obstruction (eg, potatoes, beets, hay twine) anterior to the rumen;damage to tissue such as the vagus nerve, cardia, or rumen; or changes in rumenmotility.34,36,38,39 Although not as common, free-gas bloat can also occur duringlateral recumbency or be caused by hypocalcemia, acid indigestion, or esophagealstenosis from lesions, tumors, or inflammation.36,40 If observed and diagnosed in atimely manner, free-gas bloat can typically be alleviated quickly by removing theobstruction from the esophagus or by intubating the rumen to expel the gas. Inmany cases, free-gas bloat arising from damaged tissue can become a chronic, recur-ring issue.

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The predominant type of bloat in feedlot cattle is frothy bloat.34,37 The rumen con-tents of feedlot cattle are normally stratified by the density of digesta, with the smallerand denser particles in the ventral rumen progressing dorsally to longer, less denseparticles and then a gas layer in the dorsal sac that forms as gases produced by mi-crobial activity rise as bubbles through the digesta.41 Frothy bloat arises when a stablefoam forms and then traps gases throughout the fluid phase of the rumen so that thedigesta are less stratified.38 The foam can be very persistent and can occupy the entirereticulorumen. The increased volume of rumen digesta can potentially impair theclearing of the cardia and inhibit eructation of gases.33

The formation of a stable foam is thought to be related to the release of excessstored mucopolysaccharides (or slime) from encapsulated rumen bacteria, releaseof other molecules including nucleic acids and carbohydrates during lysis of rumenmi-crobial cells, and digestion of feedstuffs.41–44 The increased quantity of nucleic acidsand carbohydrate increases the viscosity of rumen fluid and is characteristic of feedlotbloat.43–46 Saliva has been shown to have antifoaming characteristics.47,48 Specif-ically, it was postulated that salivary mucin may be an antifoaming agent, and re-searchers showed that catabolism of mucin by mucinolytic bacteria was associatedwith bloat.49 Consequently, decreased saliva production may be correlated to bloatoccurrence.Acidosis and bloat are interrelated disorders, and bouts of acidosis can predispose

cattle to bloat. Ruminal contractions and motility are reduced by lactic acid, VFAs, en-dotoxins, and histamines, levels of which have been shown to increase duringacidosis.25 Some consequences of acidosis and increased rumen osmolality includedecreased salivation, reduced rumen and intestinal motility and absorption, increasedlysis of microbial cells, a change in the rumen microbiome, and decreased bacterialdigestion of starch and fiber.6,8,25 Altogether, rumen stasis and stagnation andreduced abomasal motility can cause gas to accumulate in the rumen and can leadto bloat.

CLINICAL FINDINGS

Primary ruminal bloat is one of the more straightforward diagnoses to make. In earlystages of the disease, the left paralumbar fossa is mildly distended and abdominaldistention is present. As bloat progresses and intraabdominal pressure increases,the distention in the left paralumbar fossa becomes more apparent, the rectum mayprotrude, and pressure on the diaphragm increases until the animal shows signs ofrespiratory distress, and a reluctance to move (Fig. 3).In addition to clinical signs, other factors can aid in a presumptive diagnosis of bloat.

In the feedlot, peak prevalence occurs at 99 to 120 days on feed,1,2 and is seen mostfrequently during ration changes, weather alterations, and following variations in mill-ing and feed delivery. During a bloat event, many of the factors that trigger bloat affectthe entire group so it is common to see a range of clinical severities within in a groupfrom mild to moderate abdominal distention to clinical bloat.50

A thorough individual and pen-level history should be collected at the time of death.In addition, noting the location and presentation of the animal at death and timely nec-ropsy aid in an accurate diagnosis. Non–digestive-related causes of bloat mortalitysuch as cast and a prolonged period from death to the time of necropsy may haveidentical postmortem lesions to bloat. A complete postmortem examination shouldbe performed, examining all major organs and ruling out other causes of death.Gross necropsy findings associated with bloat may include, but are not limited to, (1)

congestion, hemorrhage, and edema of the anterior portion of the carcass, especially

Fig. 3. Severe bloat. Note the severe distention in the area of the left paralumbar fossa.

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in the area of the cervical muscles; (2) pallor of the semimembranosus and semitendi-nosus muscles of the hindquarters; (3) edema, often with emphysema between themuscles groups of the hindquarters, scrotum, and area of the mammary gland; (4)small and pale liver; (5) compressed lungs; and (6) presence of a tenacious froth orgas in the rumen.50–52

TREATMENT

Passage of a stomach tube is indicated for the initiation of the treatment of abdominaldistention. The largest-bore tube of sufficient length to reach the dorsocaudal ruminalsac should be passed. Attempts should be made to clear the tube by blowing and thetube should be moved back and forth within the rumen to locate areas of gas that maybe relieved.50 If no gas is present, the tube should be removed and presence of frothwithin the tube should be examined. If the animal is not in respiratory distress orextremely colicky, surface-active agents such as mineral oil or dioctyl sodium sulfo-succinate may be administered.Severely compromised animals require surgical intervention. A trocar and cannula in

the left paralumbar fossa should be used to initiate immediate relief. With frothy bloat,a standard size instrument is often not sufficient, so a larger-bore (2.5-cm diameter)instrument or rumenotomy should be performed.50 Salvage slaughter is a humane op-tion for cattle that fail to respond to treatment and chronically bloat.

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RUMINAL DIGESTION OF GRAINS AND FORAGES

Grains and forages are ingested, and the starch and fiber components within eachfeedstuff are digested by rumen microbes to form short-chain fatty acids (VFAs),including acetate, propionate, and butyrate. In addition, lactate and gases such ascarbon dioxide, methane, hydrogen, nitrogen, oxygen, and hydrogen sulfide can bebyproducts of ruminal digestion. The short-chain fatty acids are then absorbedthrough the rumen wall and are used by various tissues for production of glucose, fattyacids, and ketones. Grain processing methods such as steam flaking or rolling orgrinding followed by fermentation, which results in high-moisture corn, are commonlyused to help solubilize the protein matrix surrounding the starch molecules in the ker-nels, which then makes the starch more available to rumen microbial action, therebyincreasing the efficiency of starch use. The cell wall components of forages such ascellulose and hemicellulose are digested more slowly than the starch in grains. Thisdifference in the rate of digestion can potentially result in digestive and health chal-lenges such as acidosis and bloat.

FACTORS AND MANAGEMENT CONSIDERATIONS AFFECTING ACIDOSIS AND BLOAT

A key challenge in management of digestive disorders is the acute to peracute natureof the disease resulting in failure to detect and treat in a timely manner. Records showa low level of detected morbidity attributed to bloat and acidosis compared with a highmortality.2,31,53 The National Animal Health Monitoring System estimated that 71% offeed yards surveyed were affected by digestive problems, and the respondents onlyobserved digestive-related issues in 4.2% of the cattle.53

Compared with other disease states, more than 500,000 treatment records werereviewed examining morbidity and mortality attributable to infectious pneumonia.2

An average mortality/morbidity ratio of 3.79% was observed for cattle diagnosedwith infectious pneumonia. Alternatively, when examining records of diseases attribut-able to bloat and acidosis, a different observation was made. For disease attributed tobloat and acidosis, an average mortality/morbidity ratio of 158.7% was observed.Clearly, focusing on prevention of digestive-related diseases is paramount to reducingoverall mortality prevalence.

RATION TRANSITIONS

Digestive disorders such as acidosis and bloat are interrelated and are complex dis-ease states. Before shipment to feed yards, most cattle are on a forage-basedgrowing program. In a typical grazing scenario, intake is regulated predominantly byphysical fill; however, on a high-concentrate diet, chemostatic regulation becomesthe primary method of modulating intake.6 Consequently, after arrival into a feedyard, cattle are normally transitioned slowly over a period of 3 or more weeks, startingwith a diet of 45% to 55% concentrate andmoving to an 85% to 95% concentrate dietin order for both the animal and the rumen microbial population to have ample time toadapt to diets containing readily fermentable carbohydrates.54

Cattle that are rapidly adapted to high-concentrate diets have more variable ruminalpH response, lower intakes, and increased incidence of acidosis.55–57 If cattle areadjusted to diets up to 90% concentrate in less than 14 days, acidosis and poor per-formance can result.54 In addition to time, managing energy intake level can be impor-tant for proper adaptation of cattle to high-concentrate diets, and many studieshave shown that providing ad libitum intake during days 5 to 14 of the period of tran-sition to high-concentrate diets usually results in dramatic reduction in intake.54,58

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Consequently, a properly designed adaptation program for managing dietary changesand intake levels is an important aspect of transitioning cattle to diets containing highlevels of readily fermentable carbohydrates.

BUNK MANAGEMENT

Although cattle are fed and managed as groups with multiple animals in a pen, largevariation in intakes and rumen environments exists within a pen of cattle on any givenday. Consequently, bunk management entails managing the bell curve of individualswithin a pen of cattle by assessing cattle behavior, time at which all feed distributedin a bunk has been consumed, fecal consistency, and the health of the group. As aresult, feed yard nutritionists tend to design feeding programs to find an optimal bal-ance between productivity for most of the animals within the pen and trying to mitigatethe risk of digestive disturbances for those cattle that are more susceptible to meta-bolic disorders. The capacity of animals to cope with grain and acid loads is highly var-iable, and on any given day animals within a pen are at different physiologic stages.57

Animals that are more acid tolerant and that have the capability to adapt more quicklyto dietary and ruminal changes likely consume more feed and are more productive.54

Many feed yards use a slick-bunk (or clean-bunk) feeding program in which the dailyintake assignment for a pen is determined based on a bunk-scoring methodology thatfactors in assessments of cattle behavior, the amount of feed remaining in a bunk on agiven day immediately before feeding, or the time at which cattle have consumed allthe feed that was delivered to them on a given day. The goal of a slick-bunk feedingprogram is to maximize average intake over the feeding period while also minimizingthe daily wastage of feed that can occur in an ad libitum feeding system. In a slick-bunk program, the feed yard and nutritionist strive to have most of the bunks on afeed yard void of feed immediately before the delivery of the subsequent day’s feedallotment.The challenges to managing a slick-bunk feeding program result from the daily vari-

ation that exists in the rumen environment, cattle behavior, pen conditions, weatherchanges, logistical hurdles that exist from milling and delivering feed each day, andingredient and dietary composition. Consequently, alterations in any of these factorscan result in a cascade of physiologic changes in the rumen that may predispose an-imals to digestive issues such as acidosis and bloat. Because most feed yards stockcattle in pens to provide 22 to 30 cm (9–12 inches) per animal, not all cattle can fit atthe bunk at any one time.59 Therefore, animals must spread their meals out throughoutthe day. In a feed yard setting, cattle tend to eat 8 to 12 meals per day, but there islarge animal-to-animal variation as well as day-to-day variation in meal frequencyand size.60–63 Consequently, if there is a dramatic change in the proportion of animalsthat desire to consume feed at a given time, then the rate and quantity of feedconsumed can increase dramatically, resulting in rapid changes in the rumenmicrobialand pH environments.58 These engorgement and binge-eating events can be trig-gered by behavioral changes associated with weather and barometric pressure pat-terns or with variation in the time at which feed is delivered caused by logisticalchallenges. Therefore, social dominance structure and competition can be a factorin meal size and frequency and thereby affect the potential for digestive disorders.The assignment of the pen’s daily feed amount can have bearing on cattle behavior

and meal patterns. Many feed yards use a methodical and objective feed-calling sys-tem to apply an intake assignment to a pen based on visual assessment of the bunksthroughout the time period between when cattle are fed for the last time on one dayand for the first time on the subsequent day; the objective of these bunk assessments

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is to determine when cattle completed eating all the feed delivered to their pen. Otherprograms are more subjective and rely on assessments of cattle behavior and of theproportion of animals in each pen that come to the bunk immediately after feed isdelivered to the pen. In either case, if the intake assignment is drastically out ofsync with the wants of the cattle, digestive disturbances can result. Nutritional pro-grams are made even more complex with differences in cattle breeds, quality, age,and background; time of year; weather patterns; and changes in ingredient sources,moistures, and nutrient composition. If possible, the cattle populations that aremore susceptible to bloat should be fed first in a sequence. Examples of thesehigher-risk populations may include Holsteins, cattle with more relative days onfeed, and those on a beta-agonist. In order to minimize digestive upsets, a bunk man-agement and nutritional program must strive for consistency in all aspects of intakeassignment, ingredient quality, feed manufacturing, and feed delivery.

GRAIN TYPE AND PROCESSING

The grain type and degree of processing can also influence the rate and extent ofdigestion and hence the rumen environment and predisposition to digestive upsets.Grains vary in average starch content and are ranked in decreasing order as cornand sorghum (71%–76%), wheat (62%–65%), barley (57%–59%), and oats (44%).64

Each grain type also has unique features related to the protein matrix that surroundsthe starch; consequently, the rate and extent of ruminal degradation depend on thestructure and type of the protein-starch matrix and the ability of rumen microbes tobreak down the starch within this matrix. In addition, several grain processingmethods exist that can aid in this process. For example, the grain can be ground ordry-rolled, resulting in a smaller particle size and more surface area for microbesand enzymes to attack the starch. Grain can also be fermented into products suchas high-moisture corn; this process breaks the pericarp, reduces particle size, and sol-ubilizes the protein matrix that encompasses the starch molecules within the endo-sperm. Another common method of grain processing, steam flaking, entailsgelatinizing the starch via application of steam and pressure. The total quantity andrate at which starch molecules are converted to ruminal short-chain fatty acids(VFAs), lactate, and gas are determined by the interaction of starch content, the grainprocessing method, and degree of processing. Taken altogether, these factors alongwith dietary inclusion levels of grain, roughage, and other ingredients can determinethe risk to digestive upset. Therefore, it is difficult to assign the order of risk to graintype and processing method; however, because of their high starch content and rapidrate of ruminal fermentation, wheat and high-moisture corn can be feedstuffs that canelicit a rapid change in the rumen environment and a predisposition to acidosis orbloat in cattle. Similarly, although less ruminal fermentation occurs with steam-flaked corn than with wheat and high-moisture corn, a rapid fermentation rate canoccur with steam-flaked corn that is processed to a higher degree. Total ruminalorganic acid production is less with grains containing less starch and that are pro-cessed to a lesser degree; this results in a shift of starch digestion to the abomasumand small and large intestine for products like dry-rolled corn. However, dry-rolling ofcorn results in lower animal productivity and efficiency than products such as steam-flaked and high-moisture corn.65 For example, recently it was shown that starchesfrom whole, dry-rolled, high-moisture, and steam-flaked corn have ruminal disappear-ance rates of 75%, 70%, 91%, and 85%, respectively, and total tract disappearanceof starch was 85%, 91%, 99%, and 99% for whole, dry-rolled, high-moisture, andsteam-flaked corn, respectively. Consequently, a balance is sought between animal

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productivity and ruminal health; a slow rate of fermentation is better for prevention ofacidosis, but a faster rate of degradation is better for energetic efficiency.6 Assess-ments of degree of processing of grains include visual appraisal, particle size distribu-tion, density, protein solubility, gas production, refractometry, enzymatic conversionof starch to glucose, and fecal starch content. For dry-rolled grains, the primarymethods of quality assessment are visual appraisal and particle size distribution. Fer-mented products such as high-moisture corn are typically appraised for particle sizedistribution and protein solubility. The predominant methods for assessing the qualityof steam-flaked corn are on-site bulk density and laboratory enzymatic starch avail-ability, and most nutritionists target a starch availability of 52% to 71%.66 Starch avail-ability can be affected by moisture content and age of the grain, retention time in thesteam chest, quantity of steam applied, roll settings, roll corrugation and wear, andstorage of the grain after processing. As mentioned previously, as particle size de-creases, more surface area is exposed to microbial enzymes; as a result, particlesize of grain is negatively correlated to rate of digestion and viscosity or rumen con-tents.34,67 Diets containing finely ground grain have also been associated withreduced salivation, increased gas production, and increased occurrence of bloat.33,68

Grain type, variety, and degree of processing are important considerations for meta-bolic maladies such as bloat and acidosis and must be managed accordingly.

DIETARY FIBER

Although most feed yard diets are primarily composed of grain, numerous aspects ofdietary forage play a critical role in ruminal and animal health and in minimizing risk fordigestive upset. Roughage sources used in most feed yard diets have different phys-ical and chemical profiles, which makes determining roughage value and equivalencydifficult.69 Numerous attempts to assess roughage quality have been made, but themost commonly used system of categorizing roughage components, the neutraldetergent fiber (NDF) method, was devised by Van Soest,70 in which forages aredivided into a readily digestible fraction and an incompletely digestible fraction.Most feed yard nutritionists use NDF or acid detergent fiber (ADF) composition of for-ages for dietary formulation and roughage equivalency purposes. Before the NDF sys-tem, crude fiber content was commonly used; however, crude fiber underestimatesthe plant cell contents.71 The NDF fraction is considered to be the incompletely digest-ible fraction of the feed and contains cellulose, hemicellulose, and lignin, whereas theADF fraction contains cellulose and lignin. Dairy nutritionists further defined effectiveNDF (eNDF) on the ability of a feed to replace roughage in a diet so that the percentageof fat in milk produced by cows eating the ration is maintained.72 The eNDF is basedon particle size as well as adjustments for numerous other subjective factors; as aresult, the eNDF values for feedstuffs is arbitrary.64 Physically effective NDF (peNDF)is related to the physical characteristics of fiber, such as particle size, that stimulateschewing activity and establishes the biphasic stratification of ruminal contents with apool of liquid and small particles sitting below a floating mat of large particles.72 In thepeNDF system of roughage comparison, long-grass hay is used as the reference feedand the peNDF of other forages are expressed as percentages (known as the physi-cally effective factor) relative to long-grass hay, which is multiplied by the NDF contentof the roughage. In high-roughage diets of dairy cows, particle size and peNDF havebeen positively correlated to increased chewing time, salivary output, and ruminalpH.73–75 Although feedlot cattle chew and salivate less than cattle on increased foragediets, mixed results have been observed in correlating particle length of roughagesand ruminal pH in feedlot cattle.76,77 Limitations to the use of peNDF for dietary

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formulation also include reliance on book values for the physically effective factor, andthis factor was generated from limited research data.64 In addition, peNDF does notaccount for ruminal degradation of the roughage and may not be a good predictorof rumen pH.64 Consequently, use of peNDF for dietary formulation purposes hasnot been widely adopted by feed yard nutritionists.66 Despite the variable resultsassociated with peNDF, most nutritionists strive for longer chop lengths on hay(75–125 mm [3–5 inches]) and silage (13–19 mm [0.5–0.75 inches]); however, longerchop lengths on corn silage result in more whole kernels and increased variation inparticle size unless it is kernel processed.In terms of dietary formulation, both source and concentration of roughage affect

intakes of feedlot cattle. In addition, a recent review and analysis of published datarevealed that dietary NDF and eNDF supplied by roughage account for 92% and93%, respectively, of intake responses observed in feed yard cattle, whereas dietarydry matter inclusion of roughages only accounted for 70% of the intake variation asso-ciated with roughage source and level.69 Many feed yard nutritionists formulate dietsbased on the NDF solely supplied by the dietary roughage sources (ie, forage NDF orroughage NDF). Forage NDF has been correlated to ruminal pH, which may partiallyexplain the relationship observed between forage NDF and intake.69,73 Cliniciansmust be cognizant of the extreme variation that can exist in NDF or ADF content offeeds caused by roughage source, variety, maturity, and growing and harvest condi-tions. In addition, the fiber content of a single roughage can vary between sources (ie,farmer to farmer) and then be combined into 1 roughage supply pile or pit. Conse-quently, monitoring and adjusting for changing fiber content of feeds on a daily basisis extremely challenging; as a result, nutritionists tend to formulate for the average andassess and adjust for fiber changes over time. Because of the low inclusion ofroughage in most feedlot diets, maintaining a minimal level of roughage in order tomaintain a healthy and sustainable rumen environment is very important. Because di-ets are manufactured on an as-fed basis, extreme changes in moisture content ofroughages caused by weather events or suppliers can have substantial impacts onthe fiber composition of the diet. For example, if a roughage becomes extremelywet, the fiber content of the diet becomes reduced because of dilution with water ifdiets are not adjusted for moisture content of those ingredients. Therefore, monitoringfor swings in moisture content of roughages is a key action that feed yards can do toensure that the roughage requirements of cattle are met and to avoid digestive distur-bances. Similarly, preceding and during storm events, cattle intakes can changedramatically; moreover, cattle intakes often increase in anticipation of a weather front.Consequently, ensuring that cattle have ample feed supplied to them as well as suffi-cient roughage is important. Most feed yards use so-called storm diets that containhigher inclusions of roughage for this purpose. Cattle must then be transitioned slowlyoff these diets and back to the regular diets.In recent years, with the increased production of ethanol and other corn-derived

byproducts for human consumption and use, the abundance of corn byproducts forcattle feed has increased, and wet corn gluten feed and corn distillers’ grains havebecomemore common in feedlot diets. Because of the reduction in total dietary starchassociated with the use of these byproducts in place of cereal grains, it has been hy-pothesized that these corn byproducts may thereby reduce the prevalence of acidosisin feedlot cattle. Some studies have shown that use of wet corn gluten feed increasesruminal pH or reduces variation in ruminal pH, but less conclusive results exist with theinclusion of distillers’ grains.78–81 Consequently, these byproducts may have positiveeffects on rumen environment but should not be used as a replacement for roughageor for mitigation of digestive maladies.

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FEED ADDITIVES

Feed additives are also used as aids in productivity and ruminal health in feedlot cattle.Ionophores such as monensin have been used extensively and others, including lasa-locid and laidlomycin, have been investigated and used commercially. Ionophores area unique set of antibiotics that affect bacterial cellular membranes, thus inhibitingcellular growth and replication.82 In general, monensin selectively inhibits gram-positive bacteria because of the absence of a complex outer membrane. However,monensin is also a potent inhibitor of Butyrivibrio fibrisolvens, a gram negativebacterium.83

Researchers have shown that monensin decreases meal size and quantity and dailyfeed intake variation while increasing meal frequency.84,85 As a result of the changes inmeals and bacterial population, monensin can increase ruminal pH and moderatechanges in pH.85 The modes of action of laidlomycin and lasalocid are similar to mon-ensin, but at currently approved feeding levels these compounds seem to have lessprofound effects on the ruminal levels and intake patterns.7

Regardless of the primary role of lactate in acidosis, a reduction in lactate produc-tion is beneficial in prevention of acute acidosis and improving feed efficiency. Primarylactate-producing bacteria isolated include S bovis and Lactobacillus spp, and inhibi-tion of these bacteria has been beneficial in reducing in vitro lactate production.86

Nagaraja and colleagues11 experimentally induced lactic acidosis by ruminallyinfusing glucose or finely ground corn. Cattle treated with monensin had higherruminal pH and lower concentrations of the D (�) and L (1) isomers of lactate forthe glucose challenge but did not prevent lactic acidosis in the fine-ground corn chal-lenge. Differences could have been attributable to the amount of carbohydrate infusedin the rumen (fine-ground corn was greater). Burrin and Britton10 observed an increasein ruminal lactate concentrations without a concurrent decrease in pH when cattlewere fed monensin. These observations could be attributed to a subacute acidoticstate and a low correlation between lactate concentration and ruminal pH during sub-acute acidosis.10 In general, monensin is effective at inhibiting lactate-producing bac-teria, especially in acute acidosis, but the role of lactate inhibition in performanceresponse observed with monensin feeding remains elusive.Bartley and colleagues87 described the effects of monensin and lasalocid on feedlot

bloat. Cattle bloated on a high-grain diet were tested with a dose of 1.32 mg/kg BW(body weight) of lasalocid or monensin with bloat being reduced by 92% and 64%,respectively. In addition, cattle dosed with 0.66 mg/kg BW lasalocid prevented bloatfrom developing in cattle fed high-grain diets.Most other feed additives and ingredients that have been researched have had no or

inconclusive effects. For example, the addition of sodium bicarbonate to feedlot dietshas had mixed results.88,89 The use of detergents such as poloxalene have beenshown to reduce pasture bloat but have not been effective in feedlot diets.33 The addi-tion of high levels of salt to the diets of feedlot cattle has hadmixed outcomes.90,91 Theaddition of mineral oil to feed yard diets has been shown to reduce bloat, and additionof soybean oil increased bloat, whereas tallow was neutral.92 Current researchfocused on rumen health and cattle productivity is centered on the use of direct-fedmicrobials and probiotics such as Lactobacillus acidophilus andMegasphaera elsdeniiand yeast fermentation products.

SUMMARY

Cattle feeding has progressed to an efficient and sustainable production system thatconverts available feedstuffs into beef products that satisfy consumers’ needs.

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Prevention of digestive disorders such as ruminal acidosis and bloat is achievablethrough a whole-systems approach. Dietary adaptation to high-concentrate diets,bunk management, grain source and degree of processing, roughage source andlevel, and feed additives can all affect productivity, rumen health, and digestive distur-bances in feedlot cattle. Although prevention is a cornerstone, treatment and manage-ment of animals that are affected should be thorough and focus on the welfare of theanimals. Numerous research opportunities exist in prevention and treatment of diges-tive disorders, especially as feeding programs and management systems change andnew technologies become available.

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