To test the proposition that sheep are able to select a diet that meets their crude protein (N × 6.25; CP) requirements, feeds L, A, B, C and H with the same energy content (11 MJ metabolizable energy/kg feed) but different CP contents (78, 109, 141, 172 and 235 g CP/kg fresh feed respectively) were formulated. In addition, feed U, which was feed L plus 21.4 g urea/kg (CP content 132 g/kg), was also made. The feeds were offered ad lib. either singly (n 4 per treatment) or as a choice between feed H and another feed (pairs LH, AH, BH, CH and UH; n 9 per feed pair) to individually penned Suffolk × Scottish mule wether lambs, over the live-weight range 25–45 kg. On the single feeds the rates of live-weight gain were 273, 326, 412, 418, 396 and 407 g/day (SE of difference (SED) 34; P < 0.01) and protein (excluding wool) gain were 27, 32, 44, 45, 41 and 39 g/d (SED 4; P < 0.001) for feeds L, A, B, C, H and U respectively. When sheep were given a choice between a feed below (L or A) and a feed above their CP requirements (H; as judged by the single-feeding treatments) the CP concentration selected was not different between the two pairs: 131 (SE 4) v. 133 (SE 4) g CP/kg feed for pairs LH and AH respectively. On the choices BH and CH (a choice between two feeds above requirements) the feed lower in CP was constantly preferred (874 (SE 33) and 910 (SE 33) g feed B and C respectively per kg total feed intake; CP selected was 157 and 178 g CP/kg respectively). However, this was not the case with the UH choice on which sheep consumed only 599 (SE 61) g feed U/kg total feed intake, resulting in a selection of a higher CP in their diet (173 g CP/kg). The live-weight gains of the animals given a choice between two feeds were 416, 387, 415, 410 and 383 g/d (SED 37) and protein gains were 45, 40, 46, 50 and 43 (SE 7) for pairs LH, AH, BH, CH and UH respectively, which were comparable with the best performance achieved on a single feed. The results suggest that sheep were able to select a diet that meets their CP requirements and avoid, at least to a certain extent, excess of protein intake. It is also possible that sheep discriminate against a property of feed U, such as an excess of urea, when this feed is paired with a feed high in CP.

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Diet selection in sheep: the ability of growing lambs to select a diet that meets their crude protein (nitrogen × 6.25) requirements

Le pâturage est un systeme alimentaire instable : les variations a court, moyen et long terme de l'offre quantitative et qualitative de fourrage sont frequentes, et determinent la quantite ingeree et les performances animales. D'ou l'interet de quantifier les effets des principaux facteurs de variation de l'ingestion des bovins et des ovins au pâturage Chez les ruminants au pâturage, la quantite d'herbe ingeree s'accroit avec le format et le potentiel de production des animaux, et apres une restriction alimentaire. Un couvert feuillu de bonne digestibilite est une condition essentielle pour assurer des quantites ingerees et des performances individuelles elevees. En pâturage intensif l'ingestion aussi regulee par des facteurs non-nutritionnels comme la disponibilite de l'herbe (quantite offerte et prehensibilite). Les lois de reponse de l'ingestion aux differents facteurs, etablies a court terme, sont coherentes avec les performances animales sur la saison entiere de pâturage. L'integration de ces lois doit permettre de mieux predire les consequences des pratiques et de divers choix sur les performances animales.

Ingestion de l'herbe par les ruminants au pâturage

The main objective of the experiments described was to compare the effects of milling straw and/or soaking or spraying it with sodium hydroxide on its digestibility and acceptability to sheep. Some preliminary trials were made in the laboratory to compare different alkali treatments with respect to losses of organic matter, chemical composition and digestibility in vitro.In a feeding trial of Latin-square design, involving six sheep, the following treatments of barley straw were compared: C, chopped; M, coarsely-milled; OIL, chopped and soaked in 1·5% NaOH (9 g NaOH/100 g straw); CIH, chopped and soaked in 3·0% NaOH (18 g/100 g straw); CS, chopped and sprayed with 16 % NaOH (8 g/100 g straw); MS, milled and sprayed with 16 % NaOH. After treatments OIL and CIH the straw was washed; this caused losses of organic matter of 28 and 32%, respectively. For CS and CM the excess NaOH was neutralized by addition of 7·4 ml propionic acid/100 g straw.The straws were given with supplements of purified soya protein, minerals and vitamins. The digestibility of energy for the straw was (%): C, 40·4; M, 39·1; CIL, 65·0; CIH, 64·4; CS, 57·3; MS, 60·2. Dry-matter intake (g/kg W0·75 per day) was: C, 26·7; M, 36·2; CIL, 37·1; CIH, 44·2; CS, 48·4; MS 53·6.

The digestibility and acceptability to sheep of chopped or milled barley straw soaked or sprayed with alkali

The ruminant animal responds to both the quantity and type of nutrients absorbed. However, in practical terms it is the level of dietary intake which has by far the biggest effect on animal performance. In terms of nutrients, the response by an animal to extra protein is greater when it is accompanied by an increased intake of metabolizable energy (ME). Microbial protein is the major source of absorbed protein for animals consuming forages and the efficiency of microbial crude protein (MCP) production (g/kg digestible organic matter; DOM) varies widely for forage diets, with the concentration of non-structural carbohydrates (NSC) or water soluble carbohydrates (WSC) and dilution rate within the rumen appearing to have most influence on this attribute. C4 plants, in particular, are low in NSC content and are associated with long retention time of digesta within the rumen and low MCP production. In examining the response by animals to various nutrients it is the response of the target tissue (muscle, mammary tissue, epidermal tissue) to specific nutrients which needs to be addressed. There are various strategies by which dietary intake, and hence ME and metabolizable protein (MP) intake, might be increased. Different strategies are suited to different outcomes. For example, reducing protein degradability within the plant is best suited to increasing protein supply from temperate plants. With tropical plants, increasing MCP production appears a more effective strategy to increase protein supply. The inclusion of lowlydegradable protein fractions (eg., sunflower albumin 8; SA8) in the plant, is more suited to a wool production system than for growth or milk production. In an examination of various strategies to improve forage quality, the traditional proposals of legume inclusion and those aimed at increasing intake and digestibility rank highly. Reducing protein degradability was an important strategy for improving protein supply from temperate legumes in particular. However, addressing low efficiency of MCP production was just as effective in improving protein supply and this was sensitive to NSC content, especially in C4 plants. The means by which some of these changes can be delivered may stem from traditional approaches (grazing management, sward structure, legume inclusion, supplements, plant selection) or through molecular approaches (tannin genes, lignin reduction, fibre-degrading bacteria, low-degradable protein fractions, NSC accumulation), and work should proceed on both fronts. The central importance of the rumen in determining the effectiveness of the response to most of these strategies rings a note of caution. Our analysis shows some of these manipulations may not yield the expected response because the rumen microbes are not growing efficiently or are limited in rumen degradable nitrogen, especially in the case of C4 plants.

Forage quality: Strategies for increasing nutritive value of forages

Summary Interrelationships between protein and energy are important in determining digestibility and intake of food and the efficiency with which absorbed nutrients are used for production (growth or lactation). In growing cattle offered fibre-rich diets, digestibility and intake are maximized when dietary crude protein is greater than 85g per kg dry matter. But in lactating cattle crude protein content of the diet up to, or possibly more than, 180g per kg dry matter can affect digestibility and intake. These effects on energy status can be substantial and in much published work ‘protein’ and ‘energy’ effects have been confounded. To establish the nature of responses to protein supplements, especially with a view to developing new protein feeding systems, it would be helpful if feeding trials were always supported by in vivo measurements of digestibility. Growth of cattle offered fibre-rich diets is particularly sensitive to changes in intake of relatively undegradable sources of protein. Source of dietary protein is less critical with cereal-rich and maize silage diets. The importance of choice of protein supplement to provide the right balance of rumen degradable and undegraded protein in a particular production system is emphasized. In lactating cattle supply of protein (amino acids) to the tissues might influence partition of nutrients between body tissue and milk and the efficiency of utilization of metabolizable energy for milk production. The net contribution of amino acids to gluconeogenesis will be small relative to total glucose demands. At very high protein intakes adverse effects on milk production can be expected. Minimal crude protein requirements for increments in milk yield are ˜85g crude protein per kg fat-corrected milk.

Protein–energy interrelationships for growing and for lactating cattle

Summary 1. An experiment consisting of two separate trials was carried out to study the utilization by beef cattle of high concentrate diets containing different amounts of milled barley straw and of protein. For each trial twenty-four Friesian steers, initially weighing about 270 kg, were divided into three groups and the animals within each group then allocated at random to eight dietary treatments. The treatments were based on an all-concentrate diet and three others containing 10, 20 and 30% of milled (1 in screen) barley straw. Four of the treatments consisted of giving the same concentrate mixture with each level of straw, and the other four involved giving concentrate mixtures with increasing levels of protein so that the percentage of protein in the diets was maintained. All the diets were fed ad libitum until slaughter. 2. Performance in terms of live-weight gain was considered as the net result of a number of factors, notably the total intake of dry matter, the digestibility of the dry matter, the efficiency of utilization of the end products of fermentation in the rumen and the composition of the live-weight gain. The inclusions of 20 and 30% of straw in the diet were associated with lower rates of gain than on corresponding all-concentrate and 10% straw treatments but the differences obtained did not attain significance. Total intakes of dry matter were greater on treatments containing 10 and 20% of straw than on corresponding all-concentrate treatments, but then declined with further increase in level of straw to 30%. This trend was significantly curvilinear (P < 0·01), the equation for the relationship being Y = 5·881 + 0·131X - 0·004X2, with Y being the daily intake of dry matter (kg) and X the percentage of straw in the diet. The maximum intake of dry matter was calculated to occur with a level of 16·4% straw in the diet and represents an increase in total dry matter intake of 18·2% over that on an all-concentrate diet. Food conversion ratio, expressed as total dry matter consumed per kg live-weight gain, tended to increase with increasing proportion of straw in the diet. 3. The mean digestibility of organic matter fell sharply with the inclusion of 10% straw in the ration, the decrease being 8·2% where the protein level was not maintained and 9·1% where it was maintained. Further reductions in digestibility occurred on the 20 and 30% straw diets, but the magnitude of the reductions were considerably less than those brought about by the initial introduction of straw into the ration. 4. The molar percentage of acetic acid in the steam volatile acids of rumen liquor increased markedly, and that of propionic acid decreased sharply from the all-concentrate to the 10% straw treatment with a similar level of dietary protein. Increase in the level of straw to 30% gave rise to a further increase in the proportion of acetic acid and reduction in that of propionic acid. Analysis of samples taken at 3, 6, 9 and 12 h after feeding showed appreciable differences in pattern between treatments. 5. Effects of treatment on killing-out percentage were different for each trial. In trial 1 the inclusion of straw in the diet did not cause any reduction in killing-out percentage, but the maintenance of protein level gave rise to a significantly higher killing-out percentage than was obtained with the lower protein groups. In trial 2 the killing-out percentages showed a significantly linear (P < 0·01) decrease with increasing proportion of straw in the ration, and maintenance of protein level did not give any improvement.

The utilization by beef cattle of concentrate diets containing different levels of milled barley straw and of protein.

Summary An experiment was carried out in which the performance of cattle given diets containing 20 and 40% of coarsely chopped straw (1–3 in lengths) was compared with that of similar cattle given an all-concentrate diet. Each diet was offered ad libitum and twelve Hereford cross steers and twelve Friesian steers were used in the experiment. The live-weight gains and food conversion ratios were significantly better for the Hereford cross animals but there was no significant difference in over-all dry matter intakes. When the combined response of both breed types was considered there was a significantly linear decrease in live-weight gain with increasing straw intake, to the extent of 0.95 kg per week for each 10% increase in straw intake up to 40% in the diet. Similarly there was a significant increase in the conversion ratios of dry matter (P < 0·01) and organic matter (P < 0·01) with increasing straw intake. The effect on the dry matter conversion ratio was an increase of 1·67 units for each 10% increase in straw intake. Two digestibility trials were carried out, one with coarsely chopped straw and one with coarsely milled straw, the levels of straw in the diets being the same. There was a slight decline in the digestibility of most constituents associated with milling but reductions were not very pronounced. A treatment was included in the second trial in which milled straw was included at 20% of the diet without maintaining the over-all crude protein content of the diet. The apparent digestibility of the organic matter was 5 percentage units lower with this treatment than with that containing the same level of straw in which the level of protein had been maintained.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0021859600019961

The use of coarsely chopped barley straw in high concentrate diets for beef cattle

A field trial was carried out in which 3 groups of cattle were grazed on a paddock system with and without supplementary barley. The cattle were 6 months old when turned out to grass and were each fed 4 lb/day of barley for one week to prevent any adverse effect from an abrupt change of diet. The trial began on 14 May and continued until 8 October. The mean liveweight gain obtained on the unsupplemented grass was only 0·97 lb/day up to 20 August and owing to the poor growth of the animals a supplement of 4 lb barley/head/day was then introduced. The daily liveweight gain for the remainder of the trial was 1·94 lb. When a supplement of 4 lb barley was provided throughout the trial the mean daily liveweight gain was 1·61 lb. When barley was fed ad lib. intake averaged 12·3 lb/head/day throughout the trial and the mean daily gain was 2·45 lb.



Indoor digestibility and nitrogen retention trials were carried out. The starch equivalents of diets on the three treatments were 63·7, 68·3 and 69·5, respectively. The respective mean daily intakes of dry matter were 11·2, 11·9 and 11·8 lb, and the mean daily retentions of N were 35·0, 35·8 and 33·6g. These results indicate that although the grass was of good nutritive value the poor performance of the animals on grass alone was mainly due to their inability to graze sufficient grass.

Observations on the utilization of grass grazed by young beef cattle with and without barley supplementation

An experiment was carried out in which grass was cut and fed indoors, with or without supplementary barley. Seven animals were allocated to each of the following treatments: (1) grass ad lib. without supplementary barley; (2) grass ad lib. with supplementary barley at the rate of 0.5 lb/100 lb bodyweight; (3) grass ad lib. with supplementary barley at the rate of 1 lb/100 lb bodyweight. Supplementary barley increased total dry-matter intake and improved daily liveweight gain and food conversion rate. The mean daily liveweight gains for the 17-week experimental period were 1.60, 1.91 and 2.20 lb and the corresponding feed dry-matter conversion ratios were 6.18, 5.67 and 5.55 for Treatments 1, 2 and 3, respectively.



The dry-matter digestibility of the diets was slightly higher when the grass was supplemented with barley than when grass alone was given. Although barley depressed the digestibility of protein, the absolute and the proportionate retention of nitrogen was increased. This was particularly marked on Treatment 3, presumably because of the higher net energy value of the diet containing the greater amount of barley. The N-retention data corresponded to the rates of liveweight gain obtained.

The utilization of grass fed indoors to young beef cattle, with or without supplementary barley

An experiment was carried out in which young beef cattle were given diets containing different levels of coarsely milled (1 in screen) barley straw. The proportions of straw used were 0, 10, 20, 30, 40 and 50 % and the performance of animals on these diets was compared with that of similar animals on an all concentrate diet. Each diet was given ad libitum to six animals. There was a significant linear decline in live-weight gain with increasing straw intake, the rate of decline being 0·62 ± 0·133 kg per week for each 10 % increase in the level of straw in the diet. The dry matter conversion ratio increased linearly with increasing straw intake, the rate of increase being 0·65 ± 0·186 units for each 10 % increase in straw in the diet. Dry matter intake was significantly ( P P

A. M. Raven

Papers

The utilization by beef cattle of concentrate diets containing different levels of milled barley straw and of protein.

The use of coarsely chopped barley straw in high concentrate diets for beef cattle

Observations on the utilization of grass grazed by young beef cattle with and without barley supplementation

The utilization of grass fed indoors to young beef cattle, with or without supplementary barley

The use of coarsely milled barley straw in finishing diets for young beef cattle