Nutrient content, intake and in vitro gas production of diets by Spanish goats browsing a thorn shrubland in North Mexico

The aim of this study was to determine seasonally the nutritive value and fermentability of diets selected by range goats in a thorn shrubland of North México. Esophageal cannulated goats were used to obtain extrusa samples. Samples were collected monthly from February 1999 to December 2000. Intact males fitted with harnesses were utilized for total fecal collection and DM intake determinations. Crude protein, ash and cell wall contents were determined. Macro (Na, Ca, P, Mg, K) and trace elements (Cu, Mn, Zn and Fe) were also estimated. In vitro gas production was performed to estimate rumen kinetics of goat diets. Statistical comparisons were made between seasons. The OM (mean = 84.3%), CP (13.5%), NDF (49.6%), ADF (37.5%) and lignin (12.0%) contents were different (P < 0.05) between periods. Dry matter intake ranged from 58.6 (summer) to 91.7 g kg0.75 (winter). With exception of P, all minerals were different (P < 0.05) between seasons. Moreover, during all seasons, goats selected diets with sustained amounts of Na, Ca, Mg, K, Co, Zn and Fe. However, Cu and Mn appeared to be insufficient. Significant differences between seasons were obtained in the gas production parameters. Mean values for the gas produced from the insoluble but fermentable fraction b and the constant rate of gas production c were 33.0 ml/200 mg OM and 5.6% h−1, respectively. Dietary CP was adequate to meet goat requirements in all seasons. However, a regime of Cu and Mn supplementation must be considered. Negative correlations between cell wall constituents and in vitro gas parameters may indicate the detrimental effect such compounds exert on gas production. Differences registered in cumulative gas production and fraction c permit to identify the differences of nutritive value of the diets selected by animals throughout seasons.