Applying Urea as a foliar spray rather than in the granular form By Bill Fulkerson, Central Field officer, Norco cooperative Ltd, PO Box 486, Lismore, NSW 2477. Conclusion The results of the present study found that Urea applied as a foliar spray gave a similar pasture growth response at 40% of the rate of granular Urea, under the conditions of the study and in a subtropical region. This has implications for both dairy farm profitability and for the impact on the environment. From a farm profitability point of view, on a typical dairy farm in this region milking 150 cows on 80 ha and applying the recommended rate of granular Urea of 100kg Urea/ha per month equivalent with 8 applications annually, the cost of Urea would be about $41, 600. This could be reduced to $17,056 by using Urea as a foliar spray. From an environmental point of view, the N escaping into the environment, as ammonia, leached beyond the plant’s root zone or as the green house gas, Nitrous oxide, would be largely eliminated because the Urea should not enter the soil but is directly absorbed by the leaves.
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Applying Urea as a foliar spray rather than in the granular
form
By Bill Fulkerson, Central Field officer, Norco cooperative Ltd, PO Box 486,
Lismore, NSW 2477.
Conclusion
The results of the present study found that Urea applied as a foliar spray
gave a similar pasture growth response at 40% of the rate of granular Urea, under
the conditions of the study and in a subtropical region. This has implications for
both dairy farm profitability and for the impact on the environment.
From a farm profitability point of view, on a typical dairy farm in this region
milking 150 cows on 80 ha and applying the recommended rate of granular Urea
of 100kg Urea/ha per month equivalent with 8 applications annually, the cost of
Urea would be about $41, 600. This could be reduced to $17,056 by using Urea as
a foliar spray.
From an environmental point of view, the N escaping into the environment,
as ammonia, leached beyond the plant’s root zone or as the green house gas,
Nitrous oxide, would be largely eliminated because the Urea should not enter the
soil but is directly absorbed by the leaves.
Background
There has been growing interest in applying N – based (and other) fertilizers as
foliar sprays, based on the premise that it improves N use efficiency by the
pasture plant. With foliar application the Urea goes directly through the leaves
dermis to the plant rather than through the soil where most of the N losses occur.
Application of fertilizers as a foliar spray has been used in horticulture for many
years, primarily for trace, but also, macro minerals. The arguments against
applying fertilizers, such as Urea, to pastures as a foliar spray was that insufficient
could be absorbed by the leaves and that Urea had to be converted to Nitrate as
the major form in which it is used by the plant.
In the soil, the pathway of N, in granular Urea, to the plant and associated losses
to the environment, is as shown in the diagram below:
Urea is converted to Ammonium by the soil enzyme Urease, immediately it comes
into contact with water. Most of the Ammonium is attached to soil particles and is
therefore stable but some may be converted to Ammonia and volatilized into the
air (4 to 25%). The Ammonium is then converted to Nitrate which is the form of N
most commonly used by the plant, although it can use Ammonium. However, the
Nitrate is not bound and is therefore vulnerable to leaching below the root zone
(nil to 100% under sandy soils and high rainfall), or it can be denitrified to the
green house gas, Nitrous dioxide, under water logged conditions. These pathways
illustrate the potential loss of N in fertilizer as it moves through the soil to the
roots of the plant.
Field studies by Zaman and Blennerhassett (2009) showed that pasture plants
could take up Urea and NH4+ through the leaves as a foliar spray Franke in 1967,
found that the application of Urea to the leaf actually increased the permeability
of the cuticle and hence improved its diffusion into the leaf. Subsequent glass
house studies by Dawar et al (2012) showed that dissolved Urea was about 50%
more efficient in promoting ryegrass growth than Granular Urea. This is in line
with the results of earlier studies by Middleton and Smith (1979) and Castle et al
(2006) who found that less energy is required to synthesise protein from NH4+
when it is directly absorbed from the leaf rather than needing to be converted to
NO3- and then absorbed by the roots.
A recent study in New Zealand by Schofield et al (2012) confirmed the results of
Dawar et al . (2012). They found that the rate of application of Urea could be
reduced by 60% when applied as a foliar spray compared to the granular form.
In the subtropical dairy region, dairy farmers commonly have a summer grass
pasture which provides feed from late spring to early autumn and this is over
sown in early autumn with short rotation ryegrass (Lolium multiflorum) to provide
winter/spring feed. The recommended rate of application of Nitrogen to such
pastures is 46kg N/ha per month as Urea if other soil nutrients are adequate.
The object of this study was to compare the efficacy, under field conditions, of
granular and dissolved Urea for pasture growth in a subtropical dairy
environment.
Methods and Results
The study, comprising 2 experiments, was undertaken on a dairy farm near
Lismore on the far north coast of NSW (lat.28o and Long.153o, elev. 15m).
The long term mean annual rainfall is 1180 mm and the climate is subtropical with
wet summer/ autumns and relatively dry winter/ springs (see Figure 1).
Figure 1.The long term mean monthly rainfall for Lismore and the rainfall in
2015-16 .
Experiment 1
Experiment 1 was located on a kikuyu-based pasture over sown with Short term
ryegrass (Lolium multiflorum, cv. Annual tetraploid ryegrass) in autumn to provide
pasture in late autumn through to mid-spring. The kikuyu-based pasture also
contained some paspalum and couch grass.
The Experiment commenced on the 18 August 2015 and concluded on 30 March
2016.
The experimental design comprised 2 treatments: Urea dissolved in water at a
ratio of 1:3, with 7 replicates (each 9m boom spray width by 55m long) or,
:Granular Urea with 3
replicates (each 18m fertiliser spreader width by 55m long).The pasture mass
0
20
40
60
80
100
120
140
160
Aug Sep Oct Nov Dec Jan Feb Mar
Rai
nfa
ll (m
m)
Month
mean
2015-16
measurements and any pasture sampling was taken in a same transect along the
middle of each plot to minimize edging effects.
The ryegrass pasture was grazed 3 times (18 August to the 15 October) by
Friesian milking cows and the kikuyu pasture 6 times (25 November to the 20
March). The grazing interval was 14 to 22 days and 50kg granular Urea/ha was
applied immediately after each grazing or 21kg dissolved Urea/ha was sprayed on
about 4 days after grazing, to allow adequate leaf regrowth to receive the
dissolved Urea. Thirty seven ml of non ionic wetting agent was also mixed with
the dissolved Urea to maximize retention by the grass leaf. The granular Urea was
applied with a fertiliser spreader and the dissolved Urea with a boom spray.
The dry matter (DM) mass of pasture pre- and post-grazing was measured using an electronic rising plate pasture meter (Farmworks, NZ). From this data, the growth rate (kg DM/ha per day) and the amount eaten by grazing milking cows (kg DM/ha) at each grazing (see Figure 2) and the mean growth rate and amount eaten by grazing milking cows, over the total experimental period is shown in Table 1. Grass growth rate (kg DM/ha)
0
20
40
60
80
100
120
140
160
1 2 3 4 5 6 7 8 9
spray
granular
Grazing number
Figure 2. A comparison of the growth rate of Urea applied as a foliar spray or in granular form
Table 1. The mean (9 grazings) +/- se growth rate of grass (kg DM/ha per day)
and the amount eaten (kg DM/ha) by milking cows in Experiment 1.