applying urea as a foliar spray rather than in the...
TRANSCRIPT
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.
Treatment
Grass growth Grass eaten (kg DM/ha/day) (kg DM/ha)
Dissolved Urea
51 +/-10 618 +/-61
Granular Urea
45 +/-10 548 +/-73
Although the growth rate of grass and the amount eaten on the dissolved Urea
was higher, this was not significant.
Ryegrass samples were taken pre-grazing to simulated grazing height on 3
September 2015 and the kikuyu samples on 23 March 2016. These samples were
analysed for Total N (%), Nitrate-N and Ammonium –N (mg/kg) at the
Environmental Analysis Laboratory (NATA accredited) at Southern Cross
University, Lismore, NSW (See Table 2).
.
Table 2. The content of total N, Urea-N and Ammonium-N in ryegrass leaf, taken
on 3 September and kikuyu leaf taken at the conclusion of the experiment on 29
March.
Analysis
Date Treatment
Total N Nitrate-N Ammonium-N
(%DM) (mg/kg) (mg/kg)
3/09/2015 Dissolved Urea 4.4 1047 439
Granular Urea
4.6 1903 502
29/03/2016 Dissolved Urea 3.19 24 354
Granular Urea
3.50 23 397
The Total N, Nitrate-N and Ammonium –N were higher in the leaves of both
treatments at the start of the study than at the end and this was probably due to
the residual effects of the previous Urea application (see table 3 for comparable
soil data). There was no significant difference between treatments in any of the
parameters measured.
Thirty soil cores, to 10cm depth, were taken across the whole experimental site at
the commencement of the experiment on 18 August 2015 and again at the
completion of the study over each treatment, pooled then sent to the CSBP
laboratory (NATA accredited), Bibra Lake, WA, for analysis of total N, Nitrate-N
and Ammonium-N (see Table 3).
Table 3. Soil Nutrient levels in samples taken over the whole site at the
commencement of the experiment on 18 August 2015 and for each treatment on
30 March 2016.
Date
Soil Nutrient (mg/kg soil) 18-Aug-15 30-Mar-16
Dissolved Granular
Colwell Phosphorus 171 114 167
Colwell Potassium 309 247 309
Sulphur
48 28 35
Organic Carbon
5.1 4.8 5
pH
5.1 5.0 4.9
Nitrate-N
75 22 60
Ammonium-N
9 11 11
The Nitrate-N concentration in the soil on the Spray plots has dropped markedly
since the commencement of the study whereas the granular Urea plots have
remained similar. Although this is expected in order to provide the environmental
benefits it probably necessitates the need to apply foliar N after every grazing as
there is far less N stored in the soil that the plant can access.
Experiment 2
Experiment 2 was undertaken on a monoculture pasture of Short rotation
ryegrass (cv. Feast 2), established on a prepared seed bed in April 2015.
The experimental design comprised 3 treatments: Granular Urea broadcast with a
fertilizer spreader, Urea dissolved in water at a ratio of 2:1 or Urea plus Lime,
both applied with Multi 1000 Tow and Fert spray unit spray (see below) with plot
size for the granular Urea plots being 18m by 85m and for the dissolved Urea 30m
by 85m and treatments were replicated 3 times.
The plots were grazed by milking cows on 18 August and 21 September and 50kg
Granular Urea/ha applied immediately after each grazing or 25kg dissolved
Urea/ha or 25kg dissolved Urea plus 30kg Agricultural lime/ha, about 4 days after
grazing.
The pre and post grazing pasture mass was determined as for Experiment 1 (see
Table 4).
Table 4. The mean +/- se ryegrass growth (kg DM/ha/day) for the 3 treatments
A soil sample, pooled over each of the 3 treatments were taken on 18 August (see
Table 5) and analysed as for Experiment 1.
Table 5. Soil nutrient level for samples taken at the commencement of the study
on 18 August 2015.
Soil Nutrient
(mg/kg)
Colwell Phosphorus 110
Colwell Potassium 227
Sulphur
30
Organic Carbon
pH
5.1
Nitrate-N
14
Ammonium-N
Treatment
Grass growth
(kg DM/ha/day)
Dissolved Urea
44
Dissolved Urea + Lime 51
Granular Urea
38
A pooled pasture sample was taken to simulated grazing height on 3
September (see Table 6) and analysed as for Experiment 2.
Table 6. The content of N, Nitrate-N, Ammonium-N and Calcium in the
leaves of ryegrass in Experiment 2.
Treatment Total N Nitrate-N Ammonium-N Calcium
(%) (mg/kg soil) (mg/kg soil) (%)
Dissolved Urea 3.8 304 490 0.35 Dissolved Urea + Lime 4.1 486 710 0.45
Granular Urea 3.7 555 111 0.36
The interesting issue in Table 6 is the 29% increase in the Calcium content
of the ryegrass sprayed with Lime and this is unlikely to be on the surface as there
was 32 mm of rain fell in the period from application of lime herbage sampling.
Practical implications
In the light of these studies, farmers have started to apply Urea and other N-
based fertilizers as foliar sprays, primarily on cereal crops, but more recently on
dairy pastures. This has led to development of spray units (Tow and Fert,
Metalform Ltd, NZ) which have continual agitation thus allowing lime, as well as
other soil nutrients, herbicides and insecticides and even pasture seed to be
applied at the same time. There are about 200 Tow and Fert machines in use on
NZ dairy farms and quite a few on farms in Victoria and Tasmania.
There are a number of issues to consider when using foliar spray to apply urea to
pastures:
1. The effectiveness of the foliar spray depends on the fluid being applied to
the leaves and not the soil. For this reason, Urea is sprayed on ryegrass
pastures about 4 days after grazing to provide more leaf area. Delaying this
long after grazing is not a big issue as it takes about 5 days for N from
granular Urea to move through the soil to the plant roots anyway. There is
no need for this delay with kikuyu grass or other summer grasses as there
is always enough green leaf/stolon available.
There may also be a need to apply granular Urea on seedling ryegrass
emerging on a worked up seed bed, otherwise too much would be applied
to the soil and its effectiveness diminished.
2. The ratio of Urea to water when applied with the boom spray was about
1:3 but with the continual agitation of the Tow and Fert machines this ratio
can be reduced to 1:2. Although the Tow and Fert Manufacturers
recommend applying the dissolved Urea without any additives, we did
apply non ionic wetting agent at 180 ml/1000L water and this seems to
ensure more spray adheres to the grass leaves but the requirement for this
has not been proven.
3. At temperatures above about 28oC, the foliar application should be made
in late afternoon to prevent burning of the leaves.
4. There may be a need to apply foliar Urea after every grazing, whereas
granular Urea can be applied after every second grazing if the grazing
interval falls below 20 days. This is because with granular Urea, more N as
Nitrate is available in the soil whereas this reserve is not available when
Urea is applied as a foliar spray. Nitrate-N dropped from 75 mg/kg at the
start of Experiment 1 to 22 mg/kg when Urea was applied as a foliar spray
whist with granular Urea it stayed at 60 mg/kg.
5. The N in granular Urea needs soil moisture to be able to move to the root
zone whereas with the foliar spray the N goes directly to the plant,
irrespective of soil moisture and in this case the plant can more quickly
respond to improved soil moisture conditions after a dry spell.
6. A real advantage of the Tow and Fert machines is that they allow other soil
nutrients, lime and insecticides and herbicides to be applied in the one
pass.
Acknowledgements
Thanks to Greg Mcnamara and Micheal Undery for initially kindling my interest in
this topic and to Todd Mcnamara for the excellent cooperation in managing the
plots. The assistance of Haman in the study is also greatly appreciated.
References
Castle M L et al. (2006). An experimental method for varying root
temperature independently of root temperature. N. Z. J. Agric. Res., 46, 157-162.
Dawar K el al. (2012). Applying Urea with Urease inhibitor (N-(n-Butyl)
thiophophoric triamide) in fine particle application improves N uptake in ryegrass
(Lolium perenne L). Soil Sci. Plant. Nutr., 58, 309-318.
Frank W (1967). Mechanism of foliar penetration of solutions. Ann. Rev.
Plant Physiol., 18, 281-300.
Jones C, A et al. (2014). Extension notes, Montana State University, USA.
Middleton K R and Smith J S (1979). A comparison of ammoniacal and
nitrate nutrition of perennial ryegrass through a thermodynamic model. Plant
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Schofield P et al. (2012) Using humic compounds to improve effiecy of
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