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27 Mod Nutrient ch26 30/8/00 4:28 pm Page 347
Linear Models of Nitrogen
Utilization in Dairy Cows
26
E. Kebreab,1 R. Allison,2 R. Mansbridge,2
D.E. Beever1 and J. France1
1Centre
for Dairy Research, Department of Agriculture,
University of Reading, Earley Gate, PO Box 236, Reading
RG6 6AT, UK; 2ADAS Bridgets, Martyr Worthy, Winchester
SO21 1AP, UK
Abstract
Linear models describing relationships between nitrogen (N) intake and N output
were constructed based on N-balance data from experiments conducted at the Centre
for Dairy Research (CEDAR). The models were then applied to a second set of data
derived from a series of N-balance experiments conducted at the Agricultural
Development and Advisory Service (ADAS) Bridgets. The CEDAR models described
the data from ADAS Bridgets as well as independently fitted models, despite the fact
that different diets were given to the cows at the two centres. The variance explained
in the ADAS Bridgets data was lower, possibly due to variations in the source and
level of energy intake. The variations indicate the possibility of mitigating N losses
from dairy cows by diet manipulation. Models currently used in practice underestimate N output for the two data sets and the models described herein may be useful
as a basis for better estimation of N excretion. Excretion of N in faeces is preferable,
due to its potentially less harmful effect on N pollution to the environment, particularly with regard to volatilization of ammonia and leaching to groundwater reserves,
and future models will need to estimate faecal output and the proportion of
faecal : urinary N excreted by dairy cows more accurately.
Introduction
There has been an increased use of protein supplements in the UK in order to
improve milk production, particularly milk protein content. Increased use of
protein supplements is also due to their being a cheaper source of energy than
© CAB International 2000. Modelling Nutrient Utilization in Farm Animals
(eds J.P. McNamara, J. France and D.E. Beever)
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E. Kebreab et al.
traditional energy sources. Although the conversion rate of nitrogen (N) to animal product is often as low as 30–35% (Kirchgessner et al., 1994), marginal
milk N responses are still achieved with increase in dietary N above 450 g N
dayϪ1 (Kebreab et al., 2000). This may be because amino acids are also being
used via gluconeogenesis as a source of glucose for the synthesis of extra milk
fat and lactose.
Excess N is excreted in faeces and urine, thus contributing to atmospheric
and hydrosphere pollution (Kirchgessner et al., 1994). Average annual N excreta are estimated to be about 100 kg cowϪ1 (Smith and Frost, 2000), which,
with about 2.5 million dairy cows in the UK (MAFF, 1999), significantly contributes to environmental pollution. Mathematical modelling has been used to
predict the amount of N in milk and potential N losses from dairy farms and
their contribution to environmental pollution. Estimates of ammonia emissions
to the atmosphere are based on linear equations that predict N excretion from
average milk yield and crude protein content of the diet (ECETOC, 1994). The
objective of this chapter is to evaluate alternative linear models developed at the
University of Reading’s Centre for Dairy Research (CEDAR), using independent
N-balance experiments conducted at the Agricultural Development and
Advisory Service (ADAS) Bridgets. These alternative models could ultimately be
used to predict N losses from dairy farms in the UK.
Materials and Methods
Five N-balance experiments using Holstein-Friesian dairy cows in early or midlactation were conducted at CEDAR from 1995 to 1997. The cows were fed 30
different diet types, consisting of ten grass silages and six concentrates. The
silages were made from either directly cut or partially wilted perennial ryegrass
(Lolium perenne), which received either 75 or 150 kg N haϪ1. The concentrates
used were formulated to give higher proportions of either degradable fibre, soluble sugars, high (barley) and low (maize) degradable starch and different protein degradabilities. The data from these experiments were collated and linear
models developed, using the General Linear Modelling Procedure of Genstat
(Genstat 5 Committee, 1994).
At ADAS Bridgets, a series of four N-balance experiments were carried out,
using early and mid-lactation Holstein-Friesian dairy cows as part of the
Minimum Impact Dairy Systems (MIDaS) study (Peel et al., 1997). Each experiment was based on a Latin square design, with each experimental period being
3 weeks in duration. The first 2 weeks of each period were for diet adaptation
and a total collection of faeces and urine was made during the third week. Diets
were formulated using various strategies to improve the capture of dietary N in
milk in order to reduce N losses to the environment. These included reducing N
supply, changing the type of rumen-N supplied, investigating the amount and
balance of rumen-degradable protein and fermentable energy and formulating
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Linear Models of Nitrogen Utilization
349
rations using the Cornell Net Carbohydrate and Protein System (CNCPS) (Fox
et al., 1992).
Results
Over the range of 280–640 g N intake dayϪ1, there was a strong linear relationship between N intake (NI) and N in excreta (Nexcreta) for the experiments
conducted at CEDAR:
Nexcreta = 30 + 0.62 NI
(R 2 = 0.78)
(1)
A similar but weaker linear relationship was established for data collected at
ADAS Bridgets. By setting the value of N excretion at zero intake, as in the above
equation, to aid comparison (which did not significantly affect the residual sum
of squares compared with the ‘best-fit’ equation), the following equation was
obtained for ADAS Bridgets data:
Nexcreta = 30 + 0.63 NI
(R 2 = 0.42)
(2)
Thus equation 1 predicts N in excreta for the ADAS experiments as well as equation 2 (Fig. 26.1).
Similarly, the effect of N intake on milk output was analysed and linear
models from the two sets of data were compared (Fig. 26.2). Based on the
CEDAR data, N in milk (Nmilk) was linearly related to N intake (equation 3). Data
Fig. 26.1. Effect of N intake on the N excreted in faeces and urine for cows from
CEDAR (᭺) and ADAS Bridgets (᭡). The fitted line is given by equation 1.
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E. Kebreab et al.
from ADAS Bridgets gave a similar linear relationship between milk N and N
intake (equation 4).
Nmilk = 38.2 + 0.19 NI
(R 2 = 0.3)
(3)
Nmilk = 38.2 + 0.20 NI
(R2
= 0.3)
(4)
Discussion
It is surprising that empirical equations derived from CEDAR data are so applicable to ADAS Bridgets data. However, this work is consistent with observed
trends in N excretion and N utilization for milk production in intensive dairy
farming systems in the UK. From equations 1 and 2, about 62–63% (plus 30 g)
of N fed to dairy cows in UK is excreted in faeces or urine, thus contributing
directly to N pollution of the environment. This study predicts that only
19–20% of N in the diet is incorporated as milk N. The small gains from increasing N intake beyond 450 g dayϪ1 in terms of milk production are offset by more
than threefold waste of N in faeces and urine. Furthermore, Lockyer and
Whitehead (1990) estimated ammonia volatilization from urinary N to be at
least five to six times higher than from faecal N and, at intakes of 570 g N dayϪ1,
90% of intake would be excreted in a urine : faeces ratio of 4 : 1 (Scholefield et
al., 1991). Management practices and selecting appropriate types of protein and
Fig. 26.2. Effect of N intake on milk N for cows from CEDAR (᭺) and ADAS
Bridgets (᭡). The fitted line is given by equation 3.
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energy sources might improve N retention and utilization. The variability in the
data, especially from ADAS Bridgets, suggests that N utilization under different
treatments might have been affected by feed sources as well as N intake levels.
The currently used method of estimation of N loss from dairy farms is based
on the crude protein content of the diet and average milk yield (ECETOC, 1994).
However, comparison of the estimates with the extrapolation of N output from
the above experiments shows that the method underestimates the annual N loss
from dairy farms, at least in the UK. Although the linear equations reported
here cannot be used directly for estimating annual N excretion, they highlight
the extent of the problem and new models are required to predict N excretion
and ammonia emission more accurately.
Acknowledgement
MAFF Environmental Protection Division is thanked for funding this work.
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