Abstract
This study assessed the effect of crude protein level in the diet on animal performance, feed conversion ratio and N utilization, in early-weaned lot-fed calves. Four iso-energetic rations were formulated (ME: 12.0 MJ/kg dm) differing in the crude protein content (CP: 12%, 15%, 17% and 21%, DM basis), and offered in a random experimental design to 24 Hereford calves (107.2 ± 21.5 kg). Animals were individually housed and fed ad libitum twice a day for 84 days. Increasing CP levels in the diet promoted a quadratic response (P<0.01) for dry matter intake (kg/d) and liveweight gain (LWG, kg/d), with maximum LWG of 1.36 kg/d for CP= 17.5%, without differences in subcutaneous back fat thickness (3.70 ± 0.73 mm, P = 0.25) or feed conversion ratio (4.44 ± 0.26, P> 0.10). As CP increased in the diet, N consumption increased quadratically (P <0.01), and an improvement was observed for CP (P = 0.053) and ndf (P = 0.020) apparent digestibility. Daily excretion of N in feces tended to decrease (P = 0.06), while urea-N concentration both in blood (P = 0.013) and urine (P = 0.046) increased. These results show that a level of 17% CP in the diet would optimize the performance of lot-fed early-weaned beef calves while controlling the excretion of N to the environment.
Highlights
Weaning of calves at two months of age significantly improves the reproductive performance of primiparous and multiparous cows in poor body condition at the beginning of the service period(1)(2)(3), but it challenges the nutrition of the earlyweaned calf
Calves' early weaning associated with lot feeding with concentrated diets allows, in addition to improving cows' reproductive rate, to significantly increase calves' live weight at 180 days of age, compared to that achieved in grazing(9), positively impacting productivity and economic result of the cow-calf system(10)
Given the early development stage of the earlyweaned calves, protein requirements for the expression of growth potential are maximum compared to more advanced development stages(11), which leads to increasing the Crude protein (CP) concentration in diets offered to this category
Summary
Weaning of calves at two months of age significantly improves the reproductive performance of primiparous and multiparous cows in poor body condition at the beginning of the service period(1)(2)(3), but it challenges the nutrition of the earlyweaned calf. Supplementation with high energy-protein ration (ME 12 MJ/kg, CP 19%) on temperate pastures in British breed calves weaned at 60 days of age has been enough to achieve weight gains in the range of 0.6 to 0.7 kg/d(7)(8), and a bodyweight of around 150 kg at 6 months of age, similar to those reported for lactating calves on native pastures(7)(8). Calves' early weaning associated with lot feeding with concentrated diets allows, in addition to improving cows' reproductive rate, to significantly increase calves' live weight at 180 days of age, compared to that achieved in grazing(9), positively impacting productivity (total kilograms of weaned weight/cow exposed to bull) and economic result of the cow-calf system(10). Grazing animals need concentrates with 18-19% CP to complement the lower quality forage base, probably, when feeding high concentrate total mixed rations the daily supply of metabolizable protein (MP) needed to meet calves’ requirements to gain around 1.0-1.2 kg/d is achieved with less CP in the ration(14)
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