An evaluation of the use of blood metabolite concentrations as indicators of nutritional status in free-ranging indigenous goats

The aim of this study was to determine whether blood metabolite concentrations in free-ranging indigenous goats are sensitive to expected variations in nutrient supply, and whether they could be used to evaluate different kidding seasons at two locations subject to similar seasonal variations in terms of nutrient supply. Monthly blood samples were taken over a period of one year. At Delftzyl farm, where a winter kidding season (June) was practised, glucose concentrations decreased from Februrary onwards and reached their lowest levels just prior to the kidding season. Plasma glucose concentrations increased sharply after parturition and subsequently decreased until the end of lactation. Glucose concentrations were lower in lactating does than in non-lactating does during the first two months of lactation. In contrast, glucose concentrations during lactation in does at Loskop farm, where kidding took place during spring (October), did not differ from those recorded during the four months following weaning, and neither were there differences between lactating and non-lactating does. Glucose concentrations during lactation at Loskop farm were also higher than at Delftzyl farm. The different responses can be attributed to the fact that lactation at Loskop farm coincided with peak nutrient availability during the summer period of vegetative growth, whereas lactation at Delftzyl farm coincided with low nutrient availability and quality during the winter period of plant dormancy. Plasma urea concentrations were also elevated during the last month of pregnancy and the first two months of lactation at this location, and were higher during lactation than those recorded at the summer kidding site, indicating that body protein reserves may have been catabolized to support gluconeogenesis in these animals. Plasma cholesterol concentrations were higher in lactating goats than in non-lactating goats at Delftzyl farm but not at Loskop farm. Cholesterol concentrations during lactation were also higher at Delftzyl than at Loskop. This suggests that body adipose tissue reserves were catabolized during the winter lactation at Delftzyl farm. These results indicate that lactating does at Delftzyl farm were unable to maintain glucose homeostasis during pregnancy and lactation without significant catabolism of body reserves, and suggests that the winter kidding practised there was inappropriate in relation to the available nutrient supply. It was concluded that the plasma concentrations of all the blood metabolites studied were sensitive to seasonal changes in nutrient supply, and that they could be of use as a management tool in free-ranging farming systems in which conventional methods of nutritional assessment are difficult to apply.