Rearing dairy calves on irrigated tropical pastures. 1. Effect of protein level on liveweight gain and blood components

Abstract

Isoenergetic (12 MJ ME/kg DM) concentrate supplements of varying protein contents, were fed to dairy weaner calves grazing irrigated Setaria anceps cv. Kazungula pastures. Calves in each of the 5 treatment groups were given 1 kg concentratelday containing cracked maize and cottonseed meal in the proportions 1:0, 5:1, 4:2, 3:3 and 0:l. Acontrol group received no supplement. Two drafts of animals were used to measure the response during winter-spring (dry season) and summer (wet season). Without supplementation, calves gained 0.34 kg/day in spring and 0.20 kg/day during the wet season. Supplementation increased liveweight gains by 0.25 and 0.20 kg/calf.day in spring and summer, respectively. Substitution of cottonseed meal for maize did not consistently increase liveweight gain. Calves selected a pasture diet of 97% leaf, 16% crude protein and 66% in vitro dry matter digestibility (IVDMD). Mean daily rumen ammonia concentration was 134 mg NH3-N/L in unsupplemented calves, and 89 mg NH3-N/L in calves fed maize (P<0.05). Cottonseed meal was rapidly degraded in the rumen, with peak concentrations of rumen ammonia of 570 mg NH3-N/L 2 hours after feeding. Concentrations of rumen ammonia and plasma urea were closely correlated with supplemental protein intake (R2 = 0.98). Plasma total protein and albumin also increased with protein content of supplements (R2 = 0.73). Concentrations of plasma glucose and total volatile fatty acids in rumen liquor increased with supplementation, but were not influenced by the level of protein in the supplement. Correlations between liveweight gain and plasma glucose concentration were low (R2 = 0.65 spring; R2 = 0.20 summer). Digestible energy intake was the first limitation to liveweight gain of these calves grazing actively growing, tropical grass pastures. Although animals consistently selected leaf, the level of digestible energy intake was well below requirements, and increasing the level of protein in the diet was ineffective in increasing liveweight gain.