1. Thirty Ayrshire bull calves were used in an experiment to study the effects of diets containing different proportions of concentrates to hay on rumen development at 12 weeks of age. In addition, six milk-fed calves were slaughtered at 3 weeks of age for comparison. 2. Restriction of the maximum daily allowance of concentrates to the lower levels was accompanied by an increase in the voluntary intake of hay, but the performance of calves from 3 to 12 weeks of age was significantly better in calves given the high-concentrate diets. 3. The weight of contents of the reticulo-rumen increased from 0.7 kg at 3 weeks of age in the milk-fed calf to 7.7 kg and 10.9 kg at 12 weeks in calves given the high-concentrate and high-roughage diets respectively. Between 68 and 79% of the total contents of the alimentary tract was contained in the reticulo-rumen of the ruminant calf. The weight of contents of the omasum was significantly greater in calves given 1.36 kg concentrates/day than in those given either 0.45 or 2.27 kg concentrates/day. Equations were developed from the results by which the weight of contents of the various parts of the alimentary tract, and hence empty body weight in the live animal, can be determined from a knowledge of the live weight and daily consumption of concentrates and hay, 4. Volume displacement of the reticulo-rumen tended to increase with increase in the proportion of hay in the diet. The volume displacement of the omasum increased in response to concentrate intake up to a maximum of 1.36 kg/day and thereafter declined, but the nature of the diet had no significant effect on the volume displacement of the abomasum. 5. Calves given the high-concentrate diets had a significantly greater weight of reticulo-rumen tissue at 12 weeks than those given the high-roughage diets. 6. Whereas the thickness of the muscular wall of the rumen did not differ significantly between treatments, there was an increase in the length and density of papillas, particularly in the anterior dorsal and ventral sacs of the rumen, as the intake of concentrates was increased. The results show the marked relationship between the performance of the animal and an advanced stage of development of the rumen papillas.

I . Twenty-four Ayrshire bull calves were reared on either high-concentrate or high-roughage diets to study the effect of diet on the subsequent ability of the calf to digest either concentrates or hay when given alone. The rearing diets were known to produce different types of rumen development. 2. Although there were significant differences between the apparent digestibility coefficients of the high-concentrate and high-roughage rearing diets and between concentrates and hay when given alone, the subsequent ability of the calf to digest either concentrates or hay was unaffected by the type of diet given during the rearing period to 13 weeks of age. 3. The amount of nitrogen retained was significantly greater in calves reared on the highconcentrate diet than in those given the high-roughage diet, but similar amounts of N were retained/Ioo g gain in body-weight in both groups. After the change in diet, the amount of N retained was significantly greater in calves given concentrates after a rearing diet high in concentrates than in those given concentrates after a rearing diet high in roughage, although in both groups 32% of the N intake was retained. Four of the six calves given hay after a rearing diet high in concentrates were in negative N balance, but only one calf lost weight. Only 19% of the N intake was retained in calves given hay after being reared on a highroughage diet. 4. The concentration of total volatile fatty acids (VFA) in the rumen contents rose to reach peak levels at about 3 h after food was offered; the concentration then declined. VFA concentrations tended to be higher in calves given the rearing diet high in concentrates than in those given the one high in roughage, and differences reached significance when the diet was changed to one of all concentrates or all hay, with no obvious carry-over effect of the rearing diet. The proportion of propionic, butyric and higher acids tended to be greater in calves given high levels of concentrates than in those given high levels of hay and also tended to rise, irrespective of diet, as the concentration of total VFA rose. 5. A change of diet at 14 weeks from one high in concentrates to one consisting entirely of hay resulted in a significant decline in the viable counts of streptococci, lactobacilli and coliform organisms in the rumen liquor at 16 weeks. The numbers of bacteria were unaffected by the other dietary changes. 6. The concentration of total VFA in plasma of peripheral blood tended to be higher in calves given the high-roughage or all-hay diets. A fall in the concentration of plasma VFA occurred when calves were changed from a high-roughage diet to an all-concentrate diet and a rise in concentration was observed when an all-hay diet replaced one containing a high proportion of concentrates. 7. During the initial rearing period the mean blood plasma glucose concentration was 92.8 mg/Ioo ml in calves given the high-concentrate diet, a value significantly higher than the mean value of 64.5 mg/roo ml found in calves given the high-roughage diet....

1967). The protein requirement of the ruminant calf. I. The effect of protein content of the concentrate mixture on the performance of calves weaned at an early age.

I . Seventy-two bull calves, comprising forty-eight Ayrshire and twelve each of the Friesian and Jersey breeds, were used in a randomized block experiment. They were reared from birth on liquid diets, offered in two feeds daily, at either restricted or ad lib. levels. The restricted level of feeding was such that sufficient diet was fed to allow for a weight gain of I kg/d, and the calves given this level of feeding were slaughtered at a weight 77 kg above the mean birth weight of the breed. The calves given the diets ad lib. were slaughtered at 22 % of mature cow weight of the breed.2. Three milk substitutes, based on spray-dried skim-milk powder, spray-dried whey powder and margarine fat, were compared. Two of these were high-protein diets (26-29 % protein) designed to contain either 20 % fat (LFHP) or 30 yo fat (HFHP) and the other was a low-protein diet (19 % protein) designed to contain 30 yo fat (HFLP).3. Age at slaughter was lowest for diet HFHP and highest for diet HFLP. Total dry-matter intake did not differ between treatments, but daily dry-matter intake was lower on diet HFLP. Relative weight gain did not differ between diets LFHP and HFHP but was much greater than that for diet HFLP. After adjustment for differences between treatments in mean daily drymatter intake, relative weight gain tended to be highest for diet HFHP. No difference occurred between treatments in the incidence of diarrhoea and of a high rectal temperature ( > 39.33'). However, mean rectal temperature was higher for calves given the HFHP diet than for those given the HFLP diet. 4.Digestibility and nitrogen and calcium balance trials were made on six of the replications of Ayrshire calves at 4 and 10 weeks of age. Dry-matter intake at 10 weeks of age was lower for calves given diet HFLP than for those given the other diets. Apparent digestibility of fat tended to be lower for the HFLP diet. True digestibility of protein did not differ significantly between treatments. Apparent digestibility of lactose was highest for the HFLP diet.5. No difference occurred between the two high-protein diets (LFHP and HFHP) in N or Ca retention, but N and Ca retention by calves on the low-protein diet (HFLP) was markedly lower. Daily faecal N excretion was unaffected by treatment, but daily urinary N excretion was lower and the biological value of the protein was higher for diet HFLP than for diet LFHP, with the corresponding values for diet HFHP being intermediate between the two. N retention/Ioo g weight gain was lower on diet HFLP, lower at 4 weeks of age than at 10 weeks, and lower at the restricted than at the ad lib. level of feeding at 4 weeks of age. Metabolic faecal N and endogenous urinary N at 4 weeks of age were estimated as 0.192 g/Ioo g dry-matter intake and 192.9 mg/kg0.7a live weight respectively.6. Haematological findings showed that the high-fat diets (HFHP and HFLP) resulted in a slower rate of decline in packed cell volume and haemoglobin content with age. The lowprotein diet (HFLP) resulted in a lower rate of increase with age in...

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