Potential response to supplementation of ewe lambs grazing natural pastures over winter

Lack of information in the efficiency of nutrient use is one of the reasons why concentrate supplementation is little used in southern Brazil. The aim of the present study was to test the efficiency of use increasing levels of concentrate feed to supply (0, 33, 66 and 99%) of metabolizable energy (ME) requirements and rumen degradable protein (RDP) requirements met for growing lambs on winter natural pasture. Eight month old Suffolk lambs were distributed in a completely randomized design with three replications per treatment and results analyzed in a regression analysis. The sward structure was affected by experimental periods, resulting in variables responses to supplementation levels. Average daily gain (ADG) varied between −26.4 and 96.5 g/animal/day for unsupplemented animals. The efficiency of use of supplement was dependent on the energy deficit of the unsupplemented animals, with decreasing linear responses (P < 0.01) in the efficiency of gain as the animals gained more weight. Time spent grazing decreased (P < 0.01) by about 3 min for each unit of requirements met, while the frequency of meals presented quadratic responses to supplement levels, with the greatest number of meals at 66% of requirements met for ME and RDP. Linear responses (P < 0.05) to needs met were also found for glycose and albumin metabolites, with values between 3.87 and 4.53 mmol/L and 23.74 and 27.50 g/L, respectively. This experiment confirmed that while the metabolic profile and ingestive behavior may be indicative of the need for supplementation, the level of supplementation depends on the efficiency of concentrate use, which in turn is dependent on weight gain of unsupplemented animals. These results suggest that supplementation recommendations with concentrates for ruminants should not be determined as a function of nutritional requirements and desired gain but directed for greater efficiency in nutrient use.