Nutrient partitioning and response to insulin challenge at different planes of nutrition during lactation in goats of high vs. low milk production potential

The aim of this experiment was to determine the effect of genetic selection for milk production on nutrient partitioning at a high vs. a low plane of nutrition. Twelve Indigenous goat does and eight Saanen x Indigenous crossb red does were allocated to either a basal diet formulated to provide energy for maintenance or the basal diet supplemented with maize so as to maintai n a plasma glucose concentration of 3.5 mmol/l throughout lactation. Milk y ield was not reduced by the low plane of nutrition in either genotype. The sustained rate of milk production at the low level of nutrition was achieve d by increased mobilisation of endogenous nutrients from body reserves, evi denced as a greater loss of body mass at the low plane of nutrition in both breeds. The Saanen crossbred genotype, which produced twice as much milk a s the Indigenous genotype, lost 19% of initial mass by week 10 of lactation , whereas mass losses in the indigenous goat never exceeded 7%, with the re sult that repletion of initial (week one) mass was achieved by week six of lactation. Plasma glucose concentrations were depressed to a smaller extent by insulin in the Saanen crossbred than in the Indigenous goat at weeks 3, 7 and 11 of lactation. Although there was evidence for a genotype x nutrit ion interaction during early lactation, this did not persist throughout lac tation. The majority of data derived from this study indicates that the two genotypes reacted in a similar manner to plane of nutrition, with the main differences between milk yield potential being occasioned by differences i n the extent of mobilisation of endogenous body reserves mediated by differ ences in insulin sensitivity.