Review article
Alternative home-grown protein sources for
ruminants in the United Kingdom
R.J. Wilkins
a,*, R. Jones
baInstitute of Grassland and Environmental Research, North Wyke, Okehampton, Devon EX20 2SB, UK bInstitute of Grassland and Environmental Research, Plas Gogerddan, Aberystwyth, Ceredigion SY23 3EB, UK
Received 1 February 2000; accepted 6 March 2000
Abstract
Improved sources of home-grown protein are required to substitute for animal proteins and soyabean meal in ruminant feeds. The present dominance of grassland feeds for protein supply in Britain is highlighted and possibilities for increasing microbial protein (MP) supply on grass-based diets are considered. There are particular opportunities for improving MP supply and animal performance from the use of grasses with increased content of water-soluble carbohydrates (WSC) and by the prevention of WSC and protein breakdown during ensiling through the use of bacterial inocula or chemical additives to restrict fermentation.
Potential contributions from legumes and kale as alternative forages are reviewed. Whilst white clover and lucerne may give higher levels of MP than grass, this arises largely from high herbage protein concentration and high levels of feed intake, with large quantities of N being lost in excreta. There is evidence of natural protection of protein in red clover from polyphenol oxidase and in lotus and sainfoin by condensed tannins. These attributes may result in improved protein supply to the animal, but further research is required, particularly with silages.
The grain legumes, peas and beans are not ideal protein supplements for grass silage because of their rapid degradation in the rumen, but are more suited as supplements to maize silage with low content of protein and good supply of readily available energy. Progress in plant breeding has opened up the possibility of increased use in Britain of lupins, which have much lower rates of degradation in the rumen than peas and beans and should form an effective complement to grass silage.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Protein; Ruminants; Grasses; Legumes; Silage 85 (2000) 23±32
*Corresponding author. Tel.:44-1837-82558; fax:44-1837-82998. E-mail address: roger.wilkins@bbsrc.ac.uk (R.J. Wilkins)
1. Introduction
There has been much recent focus on protein in ruminant feeding because of restrictions to the use of animal proteins, efforts to reduce costs of ruminant production and concerns arising from the low recovery of N in production systems and loss of N compounds to the environment.
In the UK, prohibition in the use of animal proteins subsequent to the BSE crisis produced a gap in the supply of protein to ruminants. At the same time it was increasingly realised that there were large losses of N to the environment, because only 5±20% of the N consumed by ruminants was being recovered in meat or milk. Jarvis et al. (1996) calculated that the nitrate-N content in water draining from a typical dairy farm was above the EU limit for most of the winter. They also drew attention to high losses to the atmosphere of ammonia and nitrous oxide, with potentially adverse environmental effects. Clearly, approaches to facilitate reduced inputs of N and improve the economy of production may restrict environmental losses.
Table 1 indicates that more than 70% of the crude protein (CP) consumed by ruminants in Britain is from grassland feeds, with some 14% from cereals, 10% from oilseeds and only a trivial quantity from peas (Pisum sativa) and beans (Vicia faba). This predominance of grassland as a source of CP applies throughout the more maritime areas of Europe. We contend that whilst production of CP could be readily increased at low cost, this would not solve the problem of protein supply, because of inef®cient utilisation of N by the animal. For instance, increasing N fertiliser application to grassland from the current average level of 120 kg N/ha to 300 kg N/ha would double CP production from grassland (calculated from Morrison et al., 1980) through increased yields of dry matter (DM) and increased CP concentration in the DM. This extra CP could not, however, completely replace all of the other feed sources of N, because of limitations to feed intake and inef®cient utilisation of CP in grassland feeds, as discussed later. The extra CP from grassland would lead to poor production response and increased environmental losses. This paper will discuss (a) possibilities for improving the ef®ciency of utilisation of CP in grassland feeds, (b) the potential for use of alternative forages, generally to comple-ment grassland feeds, and (c) the potential use of home-grown concentrate supplecomple-ments.
2. Improving ef®ciency of utilisation from grassland feeds
The concentrations of CP in grazed grass, silage and grass hay are normally in the range 150±220, 100±160 and 80±120 g/kg DM, respectively, varying seasonally, with N
Table 1
Supply of crude protein to ruminants in Britain in 1995 (kt) (from Entec, 1997 and personal calculations)
Beans and peas 21
Animal and ®sh 40
Maize gluten 175
Oilseeds 567
Cereals 800
Grassland feeds 4150
fertiliser rate, stage of maturity and the magnitude of losses during conservation. For grassland feeds with high CP contents this CP is broken down rapidly in the rumen, leading to high concentrations of rumen ammonia and low levels of undegraded dietary protein (UDP). Low supply of energy to the rumen often limits microbial protein (MP) synthesis, again restricting the supply of amino acids to grass-fed animals. These effects may be exacerbated in silages, because of the breakdown of true protein (TP) to free amino acids, peptides and ammonia which has already occurred in the silo and the removal during the silage fermentation of most of the readily available energy in WSC. Beever et al. (2000) noted that the yield of MP with silages varied from 13 to 28 g microbial N/kg organic matter apparently digested in the rumen for silages, compared with values of 33±58 for fresh forages.
There is more scope for increasing the ef®ciency of utilisation of CP in grass feeds by increasing MP rather than UDP. MP could be increased by improving the supply of readily available energy in feed and or by improving the synchrony in the supply of N and energy to rumen microbes. In temperate grasses the major source of readily available energy is from WSC with concentrations in fresh grasses varying from 50 to 350 g/kg DM. There are characteristic differences in WSC seasonally (low in autumn), with stage of growth (high during stem development) and between species (higher in ryegrass, particularly Italian ryegrass (Lolium multi¯orum), than in other sown species). Humphreys (1989) demonstrated that WSC content is heritable and varieties of perennial ryegrass (Lolium perenne) have been bred with markedly enhanced WSC content. This would be expected to increase MP yield and improve animal performance. Miller et al. (1999) reported that milk yields were 3 kg/day higher for cows stall fed the high sugar ryegrass (Aberdove) (200 g WSC/kg DM) than normal ryegrass (AberElan) (130 g WSC/ kg DM), but there were also differences between the varieties in DM intake and digestibility.
The addition of readily available energy in sugar or starch to grass silages has had marked bene®cial effects on MP supply and N retention by ruminants (Chamberlain et al., 1985; Huhtanen, 1998). Likewise the use of additives to restrict fermentation during ensiling and thus to retain WSC in the silage has resulted in an ef®ciency of MP synthesis slightly higher than that of barn-dried hay made from the same crop (Jaakkola and Huhtanen, 1993). High retention of TP in the silage may also have contributed to ef®cient protein utilisation.
An alternative approach to improve N utilisation is to reduce the rate of protein breakdown and thus ammonia release in the rumen and achieve better synchrony in supply of N and energy to rumen microbes. For fresh grasses attention has centred on the possible use of grass with the `green gene' mutation in which some of the normal senescence processes are inhibited. Such grasses may retain higher CP contents as they mature and Thomas (1987) demonstrated that the light harvesting chlorophyll protein content was maintained in the grass for 6 days after cutting, whereas there was a four-fold decline with normal grasses. It is, however, not clear whether these changes in protein struc-ture result in improvements in protein utilisation in vivo with either fresh grasses or silages. With silages, there are possibilities to change the composition of the CP through the use of additives. Bacterial inocula have been demonstrated to produce silages with higher proportions of TP than silages made without additive (Merry et al., 2000). This apparently
arises from the inocula promoting rapid pH fall in the early stages of ensiling and thus reducing protease activity in the silo. Many studies, reviewed by Merry et al. (2000), have demonstrated improvements in animal performance with silages made with inoculant and Sharp et al. (1994) reported an improvement of 33% in the ef®ciency of microbial protein synthesis when silages made with inocula were compared with well preserved untreated silage. Additives containing formaldehyde or tannins can have profound effects on N in grasses, as discussed by Beever (1980), resulting in reductions in protein breakdown and ammonia release in the rumen. However, responses depend on the application rate used, and there is a substantial risk of `over-protection' of protein with reduced in vivo digestibility and enhanced faecal loss.
High temperature dehydration of grasses reduces rumen ammonia concentrations compared with fresh or frozen grasses (Beever, 1980). This is associated with N solubility being reduced by high temperatures during drying. Beever (1980) noted substantial overall bene®ts from dehydration on N supply to the animal and attributed two-thirds of the increase to extra dietary protein escaping rumen degradation and one-third to improved MP production. This form of grass conservation is, however, unlikely to be widely adopted, because of high energy and capital costs.
There is potential to increase the ef®ciency of utilisation of N in grassland feeds by appropriate supplementation. Reference has already been made to responses to additional readily-available carbohydrate. In a recent experiment with cows grazing ryegrass swards, the provision of 8 kg/day of a starch-rich supplement with 135 g CP/kg DM compared with a supplement containing 210 g CP/kg DM was calculated to increase the ef®ciency of conversion of feed N to milk N from 0.12 to 0.20 (Gibb, M.J., personal communication). Valk (1994) has shown that the supplementation of grazed grass with maize silage improved milk protein output and halved the loss of N in urine (Table 2).
Positive milk production responses have resulted from the supplementation of grass silage with concentrates of increased CP content which result in higher intakes of grass silage (Aston et al., 1998). Whilst such an approach may be economically attractive, it results in substantial increase in CP intake and increased losses of N to the environment (Table 3).
2.1. Alternative forages
In the UK we see possible contributions from the forage legumes Ð white clover (Trifolium repens), red clover (Trifolium pratense), lucerne (Medicago sativa), lotus
Table 2
Nitrogen utilisation from grass and maize silage (g/day) (Valk, 1994)
Grass Grassmaize silage
N consumed 726 534
N in:
Urine 437 216
Faeces 157 174
Milk 132 144
N in milk as proportion of N consumed 0.18 0.27
(Lotusspp.) and sainfoin (Onobrychis vicifolia) Ð and from kale (Brassica oleracea var. acephala). They may be particularly useful in association with grass silage, because most production systems in the UK are likely to continue to include a substantial proportion of grass silage, made from herbage excess to grazing requirements.
All of these forages are capable of giving relatively high yields of DM and of CP as indicated in Table 4. These forages are characterised by high levels of feed intake (Fraser et al., 1999a, b; Beever et al., 2000). Table 5, from Dewhurst et al. (2000), demonstrates higher intakes and milk yields for silages made from white clover, red clover and lucerne than from grass silage and that levels of intake and production were intermediate when cows were fed mixtures of legume silages and grass silages. Moorby et al. (1998) found higher levels of performance and milk production from cows fed on kale-barley (Hordeum distichon) silage and mixtures of this silage with grass silage than with grass silage alone.
In relation to protein composition and utilisation there are major differences between these forages. White clover and lucerne may have many of the disadvantages of grass silage with low concentrations of WSC and extensive proteolysis during ensiling resulting in silages with low levels of TP and high concentrations of free amino acids, illustrated here for lucerne and red clover in Fig. 1 (Winters et al., 1999). Free amino acids were reduced by use of inoculum, but were still markedly higher in lucerne than in red clover. Davies et al. (1999) using in-vitro rumen simulation technology, concluded that ef®ciency
Table 3
Effect of increasing crude protein proportion in concentrate feed with ad libitum grass silage (from Aston et al., 1998)
Crude protein in concentrate
0.16 0.26
N consumed 388 473
N in:
Urine and faeces 292 367
Milk 96 106
N in milk as proportion of N consumed 0.25 0.22
Table 4
Protein production from forage crops in the UK (adapted from Entec, 1997) CP g/kg DM Yield (t/ha)
DM CP
White clover 220 6 1.3
Lucerne 200 10 2.0
Sainfoin 200 7 1.4
Red clover 180 9 1.6
Lotus 180 7 1.3
Kale 160 6 1.0
Grass 140 11 1.5
of microbial-N synthesis was 34% higher in untreated red clover silage compared to untreated lucerne silage, furthermore, when the red clover silage was treated with a biological inoculant this difference was even greater. Beever and Thorp (1996) concluded that MP yields were higher for silages made from white clover and lucerne than for grass silages, but this arose largely from high CP contents and high levels of feed intake.
With silages made from red clover, lotus and sainfoin, contents of TP may be high and of free amino acids low. The high contents of TP in red clover silages were ®rst noted by Albrecht and Muck (1991) and Jones (1993) and has been attributed to the high content of polyphenol oxidase in red clover.
Lotus and sainfoin contain tannins which may restrict protein breakdown in the silo and in the rumen. This would be expected to increase UDP supply with these forages. Waghorn and Shelton (1997) have provided clear evidence that tannins in Lotus corniculatusgrown in New Zealand improve protein utilisation, with reductions in rumen ammonia concentrations by 27% and increased absorption of essential amino acids from the small intestine of 50%. This resulted mainly through increased UDP. Thomson et al. (1971) demonstrated improved utilisation of protein in dehydrated sainfoin than in dehydrated lucerne and associated this with the condensed tannins in the sainfoin.
Table 5
Effect of legume silages on feed intake and milk production with cows fed 8 kg/day of concentrates (from Dewhurst et al., 2000)
Dry matter intake (kg/day) Milk yield (kg/day) Silage
Grass 11.1 24.9
White clover 12.1 31.5
Red clover 13.5 28.1
Lucerne 13.6 27.7
Grass: white clovera 11.9 27.9
Grass: red clovera 11.0 28.6
S.E.D. 0.80 1.81
aCows fed 1:1 mixtures of silages on DM basis.
Fig. 1. Free amino acids (mol/kg N) in silages made from lucerne and red clover either untreated or treated with a biological inoculant (adapted from Winters et al., 1999).
Nutritional responses are related to the content and structure of condensed tannins. There are, however, marked effects of forage species and conditions of growth on both the composition and content of tannins. Thus, nutritional responses, even for particular species, may be inconsistent, as discussed by Waghorn and Shelton (1997). They concluded, however, that net bene®t was more likely to occur with Lotus corniculatus than withL. uliginosus.
There is a need for much more information on the ensiling of these legumes and impact on the nitrogenous fraction and responses in vivo. Relevant information is being obtained in current programmes in IGER and in the EU project LEGSIL (Wilkins et al., 1998; Pahlow et al., 2000).
The performance of lotus and sainfoin in Britain is rather varied, with some problems both in establishment and persistency. However, the nutritional attractions encourage further research in order to identify approaches to give more reliable and sustained performance.
There is a greater possibility in the short term for increased use of red clover. The crop is widely adapted to the soils and climates of northern and western Europe and in a recent experiment DM yields were higher for red clover than grass with 200 kg N fertiliser/ha at seven out of nine sites in UK, Germany, Sweden and Finland (Halling et al., 2000). The species has limited persistence, but is particularly suited to mixed and organic farming systems. Although of high moisture content and low WSC content, recent research has demonstrated successful ensilage through combination of wilting and use of either biological or chemical additives (Winters et al., 1999; Pahlow et al., 2000). Current breeding programmes promise to produce red clovers with markedly enhanced persistence, capable of sustaining yields for at least 3 years.
Kale can be used in autumn and winter either as fresh material or as silage. With substantial contents of both CP and WSC and high digestibility (Young et al., 1997), the MP synthesis and overall protein utilisation with fresh kale is likely to be high. On ensiling, however, most of the WSC is fermented to organic acids. Kale silage, with low contents of WSC and TP, is unlikely to be a good complement for grass silage, although, as noted earlier, there is evidence of high DM intakes with mixtures including kale silage and grass silage.
Table 6 gives a pro®le of alternative forages as sources of protein for ruminants. When used as supplements for grass silages with moderate to high CP content. characteristics increasing ef®ciency of MP and UDP supply are particularly important. On the other hand, the contribution to CP concentration in the diet will be of increased consequence when forages are used to supplement a protein-de®cient basal diet such as maize (Zea mais).
2.2. Alternative concentrate feeds
Concentrates may either increase MP from the whole ration or increase UDP supply. As noted earlier, with basal-diets containing grass silage of reasonable CP content, it is particularly important to consider the characteristics of the concentrate in relation to those of the grass silage. In many cases, large responses may be obtained with carbohydrate-rich rather than protein-rich concentrates. The situation is different with
maize-based diets where the basal diet may contain <100 g CP/kg DM, but have a good supply of readily-available energy. Here, a wider range of protein supplements are likely to give production responses.
The use of home-produced rapeseed meal is already well established. In addition to rapeseed, the most interesting protein concentrate crops are probably beans, peas and lupins (Lupinus spp.). Beans and peas are traditional crops for much of Britain, with important progress having been made by plant breeders in improving seed yields and harvest index by producing determinant types and, in peas, lea¯ess and semi-lea¯ess varieties. The protein in peas and beans is however readily degraded in the rumen with low levels of UDP (Agricultural and Food Research Council, 1983). Thus, these pulses are not good supplements for grass silages, but could make a major contribution to maize silage-based diets or more widely after heat treatment.
Lupins are not traditionally grown as a ®eld crop in Britain, but progress has been made in identifying determinate types and varieties with a high level of winter hardiness (Fox and Milford, 1996). Thus, varieties suited to signi®cant parts of England and Wales are now available and maps have been produced identifying suitable areas (Milford and Shield, 1996). The grain has a CP content of 440 g/kg DM and Moss and Grundy (1996) found some 0.4 of this to be UDP, approaching the value for soyabean meal. Also, the content of anti-nutritional metabolites is low. There is a need for more direct information on the nutritive value of lupins grown in the UK, but interim results indicate possibilities for substantial replacement of ®shmeal and soyabean meal in the diets of high yielding cows (Mansbridge and Blake, 1998). With prospects for further improvement through breeding and better agronomy, there is potential for this crop to make a major contribution to ruminant feeding in Britain.
3. Conclusions
The supply of CP to ruminants is dominated by grassland feeds, for which protein is often used inef®ciently. There are prospects for improving protein supply from grass by
Table 6
Pro®le of alternative forages as sources of protein for ruminants
Yield of DM CP concentration Contribution to:
Microbial protein Undegraded dietary protein
Grass ***a ** * *
White clover ** *** *(*) *
Red clover *** *** ** *
Lucerne *** *** *(*) *
Lotus ** ** ** ***
Sainfoin ** *** ** ***
Kale ** ** ** *
aMore asterisks indicate increase of merit. For all forages contribution to microbial protein will be higher for
fresh materials than for silages.
either increasing crop WSC content or reducing rate of protein degradation in the rumen. There is considerable prospect for increased use of red clover, which is well adapted to British conditions and has a protein fraction which degrades slowly in the silo and the rumen. For grass silage based rations, the pulse crops, beans and peas, have limitations because of low contents of UDP, but there is high potential for establishment in Britain of lupins as a seed crop with protein quality and quantity similar to that in soyabeans. In addition to responses in animal production, there is need to reduce losses of nitrogen to the environment. Here, approaches to reduce the overall CP content of the diet and obtain good balance between the supplies of N and readily-available energy in the rumen are particularly important.
Acknowledgements
This paper is based on an invited paper presented to the British Society of Animal Science in Scarborough, England in March 1999. The paper makes full use of research in the Institute funded by the Ministry of Agriculture, Fisheries and Food, the Milk Development Council and the European Union. The Institute of Grassland and Environment Research is funded through the Biotechnology and Biological Sciences Research Council.
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