A review of the energy, protein and phosphorus requirements of beef cattle and sheep.
Defra Project WQ 0133
A report prepared by
Dr Bruce Cottrilla
Dr Lynne Dawsonb
Dr Tianhai Yanb
Dr Bai Xueb
a ADAS, Woodthorne, Wergs Road, Wolverhampton, WV6 8TQ
b Agri-Food and Biosciences Institute, Hillsborough, County Down, BT26 6DR
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1 Executive summary
Energy, protein and phosphorus (P) are essential nutrients for beef cattle and sheep, but failure to balance supply with requirements can lead to reduced productivity with possible welfare implications in the case of under-supply, or increased environmental pollution where intake is in excess of requirements. Over-feeding of protein results in excess nitrogen excretion, which contributes to ammonia and nitrous oxide emissions. Similarly, P supplied in excess of requirements is excreted in manure, predominantly in faeces, contributing to groundwater pollution. In order to improve feeding standards in the UK and to help minimise the environmental impact of cattle and sheep, this study has examined current information on their energy, protein and phosphorus requirements.
Nutrient requirements for beef cattle and sheep in the UK were published in the 1980’s and reviewed again in the 1990’s. Since then, feeding standards have been published by expert groups elsewhere, including other European countries, the USA and Australia. This report has examined the recommendations contained in these reviews, together with other published and unpublished studies, and provides recommendations on changes to the current UK feeding standards
Because an accurate assessment of intake is a fundamental part of rationing, the review also examined the accuracy of published methods of predicting intake by beef cattle and sheep. Finally, feeds used in ration formulation vary markedly in their nutrient composition, and a reliable database of feed composition is an essential tool in diet formulation. Tables of feed composition have therefore been included in the same format to those produced as part of the Defra LINK ‘Feed into Milk’ project, thus facilitating their use on mixed ruminant livestock enterprises.
Feed intake: The ability to accurately predict the amount of feed an animal consumes is a pre requisite to effective ration formulation. For both beef cattle and sheep, near infra-red spectroscopy is an effective tool for predicting intake of silages, but there is still considerable uncertainty when it comes to estimating intakes of grazed grass. Intake prediction equations for grazing livestock have been developed in Australia, and these need to be examined for applicability under UK conditions.
Energy: There are differences between published rationing systems in their estimates of the energy requirements of cattle and sheep which are predominantly due to differences in estimates of requirements for maintenance. Energy requirements for both beef cattle and sheep in the UK were last revised in 1993, but are broadly in line with those published subsequently in the USA. However, research undertaken since then suggest that current standards underestimate requirements for maintenance of suckler cows and beef cattle by up to 30%, and for sheep by up to 28%. The effect of these differences is to overestimate productivity from a given energy intake. There is an urgent need to revise maintenance requirements for cattle and sheep in line with the increased maintenance requirement recently adopted for dairy cows as part of the Defra LINK ‘Feed into Milk’ project.
Protein: The Metabolisable Protein (MP) system is widely used for ruminant ration formulation in the UK. A number of reviews have concluded that the UK standards underestimated requirements for MP, a view that was supported by the publication of revised standards in North America in 2000. The discrepancy between the two systems, which is largely due to the higher estimate of requirements for maintenance in the latter model, is sufficiently substantial to justify an increase in the estimates of maintenance requirements for growing cattle.
For sheep, this review has identified a number of areas where further refinement is needed, particularly in estimates of requirements for pregnancy and for wool. Current estimates of requirements for MP by growing lambs are ~20% higher than those published in Australia and the USA, and there may therefore be scope for reducing MP intake by these animals without compromising growth rates. However, these estimates are based on relatively few studies, and do not take account of breed effects, and therefore any reduction in supply should be undertaken cautiously.
Estimating the supply of MP from feeds requires information both on the potential degradability of the protein in the rumen – which is now available as part of routine analysis of forages – and the amount of time the feed remains in the rumen and available for digestion. This residence time is influenced by both the nature of the diet and the level of feeding, and there is some uncertainty over the correct value to adopt in certain situations, particularly for pregnant ewes and grazing lambs.
Phosphorus (P): There are large differences between published systems in estimates of phosphorus requirements of cattle and sheep, particularly for older animals. These are largely the result of differences in estimates of requirements for maintenance, but exacerbated by lack of data on the availability of dietary P. Recent research indicates that actual requirements are substantially lower than currently recommended in national systems in Europe and North America, and that there may be scope for reducing P intake. In practice, however, significant reductions in P intake may be difficult to achieve, particularly where diets include cereals or cereal by-products which have naturally high P contents. Availability of P is variable, influenced both by animal and plant factors. Reliable methods of estimating P availability in feeds would provide ruminant livestock producers with the confidence needed to reduce dietary P concentrations.
All of the nutritional models reviewed here have used a factorial approach to estimating requirements. A particular failing of this approach is that it fails to adequately describe interactions between feeds or nutrients, or the effect of these on the composition of animal products. There is therefore a need for the development of more mechanistic models that predict responses to nutrients, and in particular partitioning of nutrients between the different metabolic processes. However, the amount of information required to develop such models is considerable. A start has been made in the USA with the publication of a mechanistic model for sheep based on the Cornell Net Carbohydrate and Protein System, and it is proposed that the applicability of this to UK conditions should be investigated.
2 Recommendations
Intake
Prediction of grass silage intake
· To date, NIRS provides the most reliable, rapid and cost effective method of analysing grass silage, and intake predictions based on NIRS should be used where possible.
· In the absence of NIRS, grass silage intake for growing and finishing beef cattle can be predicted using published models. The model of Steen et al (1998) is used routinely as part of the Hillsborough Feeding Information System and is recommended for wider use.
· The model developed by McNamee et al. (2001) should be used to predict the intake of grass silage/concentrate-based diets for growing and finishing cattle.
· There is a paucity of scientific data which can be used to accurately predict intake of beef cows and further research is required in this area.
Prediction of non-grass silage intake
· Limited information is available on which to base an accurate prediction of non-grass silage intake. Further work is required in this area, particularly in the view of the fact that an increasing number of beef producers are including alternative forages in the ration for beef cattle. Forage intake potential defined by Rymer and Agnew (2004) could form the basis for an intake prediction for beef cattle.
Prediction of grazed grass intake
· In the absence of models for the prediction of grazed grass intake under UK conditions, the approach recommended by NRC (2007), based on models adopted in Australia (CSIRO, 1990) should be further investigated for their applicability under UK conditions.
· Although NIRS has been shown to estimate grazing intake with a 14% error, it is the most reliable method currently available to predict grass intake and merits further investigation.
· In the absence of models to accurately predict the intake of grazed grass by sheep under UK conditions, intakes of prediction of grazed grass proposed by NRC (2007), based on the Australian models, should be examined for their applicability under UK conditions.
Energy
· Energy feeding systems for beef cattle and sheep based on the UK (AFRC, 1993) recommendations should be revised. In particular, this review recommends an increase in the ME requirement for maintenance for both beef cattle and sheep, based on published calorimetric data.
· There is little information available in the literature on the effects of animal, dietary and management factors on energy requirement for growth or pregnancy, or the efficiency of ME utilisation for maintenance, lactation, growth or pregnancy for beef cattle or sheep. However, these parameters can be evaluated using data from specially designed production studies. A similar approach has been successfully used for dairy cows in the Defra LINK project “Feed into Milk”. It is recommended that a series of specially designed production studies is commissioned to produce improved prediction of the effects of a range of animal, dietary and management factors on energy requirements and efficiency of ME use in beef cattle and sheep production systems. Such information is essential for accurate feeding models to be developed for beef cattle and sheep systems.
Metabolisable Protein (MP)
· It is recommended that the requirement for MP for maintenance of beef cattle be increased from 2.3 g/kg0.75 (as proposed by AFRC, 1993 to 3.8 g MP/kg0.75. This would bring UK standards more into line with standards adopted in the UK for dairy cows and those proposed by NRC (2000) for beef cattle.
· Current UK estimates of the MP requirements for growing lambs are some 20% higher than in other published systems, and there may therefore be scope for reducing MP intake by these animals without compromising growth rates. However, these estimates are based on relatively few studies, and further studies, to include examination of breed effects, are required.
· There is considerable variation between systems in estimates of requirements by sheep for MP in pregnancy, largely due to differences in the assumed efficiency of metabolisable protein use. Given the productivity and welfare implications of underfeeding MP at this critical time, research is needed to define requirements more clearly, including closer scrutiny of potential species differences.
· The residence time of feed in the rumen, or outflow rate, has a large impact on the extent of protein degradation in the rumen, and hence on the supply of MP supply from feed. There is uncertainty over the correct values to adopt in a number of situations, particularly for pregnant ewes and grazing lambs, and these warrant further evaluation.
Phosphorus
· Maintenance requirements for P are currently based on measurements of endogenous faecal loss arising from failure to reabsorb salivary P, when animals are fed P at or near requirements. The use of salivary P secretion and absorption rates should be examined as an alternative method of estimating requirements.
· Cattle and sheep have extensive abilities to buffer against P deficiency through mobilization of phosphorus reserves. As a result of this buffering ability, P requirement for growth may be less than potential retention. Improved estimates of P requirements for gain may reduce P over-feeding, while separate estimates of retention will result in improved estimates of P excretion.
· It has been widely assumed that the availability of organic P in the diet is a constant, but recent research has suggested that there may be significant difference in the availability of P both in forages and concentrates. The development of a rapid and reliable laboratory method for assessing the availability of P in feeds should be given priority.
· Water-soluble P in faeces has been shown to be highly correlated to total P in faeces and to P intake by dairy cows (Powell et al., 2001), and this has led researchers in the USA to propose the concept of a ‘faecal P indicator’ of P adequacy, based on measurements of soluble P in faeces (Dou et al., 2002). It is recommended that this approach be examined as a means of assessing the P status of cattle and sheep. If successful, it would provide a simple management tool for farmers to assess the adequacy of P intake of their livestock, and thereby reducing the need for unnecessary P supplementation of rations.
Future development of nutrient requirement models
· All of the nutritional models reviewed in this report have used a factorial approach to estimating requirements. Further enhancements will come from the development of mechanistic models that predict responses to nutrients, and in particular partitioning of nutrients between the different metabolic processes. It is recommended that the mechanistic model for sheep, based on the Cornell Net Carbohydrate and Protein System (Cannas et al., 2004) should be examined for its applicability to UK conditions.
Contents
1 Executive summary 2
2 Recommendations 4
3 Introduction 8
4 Predicting the voluntary intake in beef cattle and sheep 9
4.1 Prediction of intake by beef cattle 9
4.1.1 Prediction of intake from the chemical composition of the diet 10
4.1.2 Prediction of intake from Near Infrared Reflectance Spectroscopy 13
4.1.3 Prediction of intake based on animal and feed factors 13
4.1.4 Summary and recommendations 15
4.2 Prediction of feed intake by sheep 17
4.2.1 Background 17
4.2.2 Prediction of grazed grass intake 17
4.2.3 Prediction of silage intake 19
4.2.4 Prediction of intake of non-silage diets 21
4.2.5 Discussion and recommendations 22
5 Energy and protein rationing systems for beef cattle and sheep – an introduction 23
5.1 Energy Feeding Systems 23
5.2 Protein Feeding Systems 24
6 Energy feeding systems for beef cattle and sheep 27
6.1 Beef cattle 28
6.1.1 Energetic efficiencies 28
6.1.2 Energy requirement for maintenance 29
6.1.3 Energy requirement for live weight gain 30
6.1.4 Energy requirements for lactation 31