A randomized block experiment has been carried out with 42 Friesian heifers over a 2-year period to examine the protein and energy requirements in late pregnancy. Rations were based on good quality hay fed to appetite plus various quantities and combinations of oats and decorticated groundnut meal. Minerals were given daily.

Eight feeding trials on cattle showed that the response in live-weight gain was curvilinear when protein or energy level was varied independently in rations based on poor quality roughages to which cereals and groundnut were variously added to give a range of protein and energy intakes. Live-weight gain was increased by 0-12 lb/day for each 0-1 lb additional digestible crude protein/day at levels of intake below 0-7 lb/day and by 0-03 lb/day at higher levels. It was increased by 0-4 lb/day for each additional 0-1 lb starch equivalent daily/100 lb live weight at total intakes below 0-9 lb daily/ 100 lb and by 0-2 lb/day at higher levels. Over the range 0-5-2-0 lb digestible crude protein daily and 0-7-1-2 lb starch equivalent daily/100 lb live weight the effects of these nutrients were additive.A Latin square nitrogen balance trial demonstrated that a portion of the nitrogen from a protein supplement was retained in the body despite the excretion of the greater part of it in the urine and a further part in the faeces. Nitrogen retention was increased by a supplement of readily available energy through a decrease in urinary nitrogen loss. Nitrogen balance was not increased by a supplement of a fibrous energy source because this induced an increase in faecal nitrogen loss.In a further feeding trial milk yield was found to be affected in a similar manner to growth. At a yield of 30 lb/day the response per lb starch equivalent added to the diet was 0-5 lb milk including 0-05 lb solids not fat and to an increase of 0-1 lb digestible crude protein daily it was 0-25 lb milk including 0-025 lb solids not fat. T~NTR O'DTTrTTO'N' similar result with pregnant heifers. These observations suggest that energy intake orientates the In earlier experiments (Bailey & Broster, 1957; response curve to protein relative to absolute rate of Broster, Tuck & Balch, 1963) we examined the live-weight gain. variation in the rate of live-weight gain of heifers Few of the published experiments on levels of given different amounts of protein in rations calcu-feeding for dairy cows have dealt with the question lated to be isocaloric. The response was curvilinear: of independent variation of protein and energy the increase in rate of gain from additional protein intakes. They have been concerned largely with the decreased as the basal level of intake rose.

The effects, on the yield and composition of the milk of the cow, of additions to the diet of the calcium salts of acetic, propionic, butyric or lactic acids were determined in 3 change-over experiments. In all cases yield of milk was increased slightly and milk fat content was reduced; solids-not-fat (SNF) percentage was depressed by acetate and butyrate. The effects of these treatments differed markedly from those observed previously when dilute solutions of the corresponding acids were infused into the rumen, but the relative effects of the calcium salts were similar to those of the free acids. It was concluded that the specific effects of the acids were overshadowed by a more marked general effect arising from their addition to the diet as calcium salts. This was confirmed in a subsequent experiment with fistulated cows, in which the effect of an infusion of propionic acid into the rumen was compared with that of an infusion of calcium propionate and of calcium propionate given with the diet. The effect of the addition to the diet of calcium in the form of carbonate differed from that of calcium salts of the fatty acids. It was concluded that the salts were not likely to find practical use.In the lactating cow, continuous addition to the rumen of dilute solutions of the short chain fatty acids or of lactic acid, causes changes in milk yield and composition: acetic acid increases milk yield and fat content, but has no effect on the content of milk SNF; propionic acid decreases milk fat and increases SNF; butyric acid increases milk fat, and lactic acid decreases milk fat and increases SNF though to a lesser extent than does propionic acid (Rook & Balch, 1961). The composition of the basal diet affects the extent, but does not change the nature, of the responses to additions of acetic or propionic acid (Rook, Balch & Johnson, 1965).Under practical farming conditions, solutions of acids cannot be administered to cows in amounts sufficient to influence milk yield and composition. A possible alternative is to add to the diet salts of the acids. Sutton (1964) reviewed the more important experiments in which the effects on milk composition of additions of sodium acetate to the diet of milking cows have been examined, and concluded that the daily addition of up to 450 g of sodium acetate (equivalent to 200 g of acetic acid) * Present address: Department of Agriculture, The University, Leeds 2. 13 Dairy Res. 34 200 C. C. B A L C H AND OTHERSto diets low in roughage usually, but not invariably, raised milk fat percentage. A similar conclusion was reached by Balch & Rowland (1959) after experiments in which they infused up to 1-5 kg of sodium acetate into the rumen of cows. More recently, Jorgensen, Schultz & Barr (1965) found that milk fat content, depressed by a diet high in maize and low in roughage, was not restored by a dietary addition of sodium acetate. The addition of larger quantities of the sodium salts to provide amounts of acids equivalent to those infused in the experiments of Rook & Balch (1961) causes ha...

The volatile fatty acids produced during fermentation in the rumen have been shown to vary in their nutritive properties. Armstrong & Blaxter (1957) found that for lipogenesis in the adult sheep the value of the acids increases with increasing chain length. Rook & Balch (1961) showed that the individual acids have characteristically different effects on the synthesis of the various constituents of milk. In consequence the nature of the end products of fermentation must be one factor affecting the utilization of foods for productive purposes.

1. In the winters of 1959–61 three randomized block experiments were carried out to study protein requirements of heifers of 800–1000 lb. live weight. 24 animals were used in each experiment. Half the animals were kept indoors; the remainder stayed out of doors except for 1 hr. per day when they came into covered yards to receive their concenrates ration.2. Rations were based on straw, cereals and roots. The intake of crude protein was varied by replacing cereals by decorticated ground nut meal. The estimated level of energy intake varied from 7·2–8·2 lb. starch equivalent/day between experiments, but the level was constant for all treatments within an experiment.3. At the end of each feeding trial the nitrogen balance was measured for 2 animals from each treatment. The results confirmed the estimated levels of digestible crude protein intakes upon which the experiments were based.4. An increase in intake of digestible crude protein (as determined in the metabolism trials) from 0·35 lb./day to 0·72 lb./day gave a marked response of 0·45 lb./day in the rate of live-weight gain. Further increases in protein intake gave little response in live-weight gain. It was concluded that for heifers of 800–900 lb. live weight the protein requirement for maintenance and a live-weight gain of 1·2 lb./day was 0·70 lb. digestible crude protein/day.5. Comparison of the estimated starch equivalent intakes in the three experiments showed that in heifers receiving 0·52 lb. digestible crude protein per day the rate of gain increased from 0·25 to 0·90 lb./day as the level of energy intake increased from 0·78 lb. starch equivalent/100 lb. live weight per day to 1·01 lb./100 lb. live weight.6. Biological value of the dietary nitrogen decreased as level of protein intake increased. The values for individual animals ranged from 61·3 to 82·4. The mean value was 69·5.7. The weather during these experiments was typical of winters in south-east England with mean minimum ground temperatures about 30° F. and mean maximum and minimum air temperatures of about 50° F. and 35° F., respectively. Snow fell occasionally only; 3½–5½ in. of rain fell in the period 1 January to 31 March. In 2 years out of 3 the outdoor group grew slightly faster than the indoor groups. In the third year this trend was reversed.