Seasonal variation in chemical composition of native forages, grazing behaviour and some blood metabolites of Small East African goats in a semi-arid area of Tanzania

A study was conducted to evaluate the chemical composition and digestibility of 12 most preferred forage species by goats in a semi-arid area of Tanzania. The forages representing grasses, forbs and browses were collected during three seasons (rainy: February–May, mid dry: July–August and late dry: October–November) in 2008. In the same periods, sixteen Small East African does (25 ± 1.2 kg body weight) under free ranging grazing conditions were observed for their grazing behaviour, body weight changes, worm counts and blood metabolites. There was a wide variability in chemical composition between species and seasons. Crude protein (CP) content ranged from 24.4 to 191.0, 45.7 to 241.6 and 97.5 to 228.4 g/kg DM for grasses, forbs and browses, respectively. Unlike forbs and browses, the mean CP of grasses declined below the critical maintenance level for goats during mid and late dry seasons. In vitro organic matter digestibility (IVOMD) coefficients and predicted metabolizable energy (ME) density were in the range of 0.21–0.61 and 3.5–8.1 MJ/kg DM for grasses, 0.47–0.76 and 6.2–10.2 MJ/kg DM for forbs and 0.23–0.69 and 3.8–10.7 MJ/kg DM for browses, respectively. Mineral concentrations varied among forage species and were all low in phosphorous concentration. Most plants were rich in Ca. Except for forbs and to some extent grass species, browses were deficient in Zn while Cu was only deficient in grasses. Goats showed high flexibility in feeding behaviour with shifts in diets and grazing/non grazing activities in different seasons. In the rainy season, goat diets were largely composed of herbaceous vegetation. As the season advanced from rainy to dry, more browses and forbs were consumed while grasses contributed much less to the diets of goats. The proportion of feeding time increased from rainy (0.57) to late dry (0.68) but that of idling in the same seasons decreased from 0.048 to 0.009. Highest (35 g/d) and lowest (15 g/d) mean weight gains were recorded during mid and late dry seasons, respectively. On the other hand, serum urea concentrations and faecal egg counts decreased from rainy to late dry season. The concentrations of minerals and biochemical indicators determined in the blood showed seasonal cycles but generally within the physiological limits. It is concluded that seasonal changes in the quality of forages elicited shifts in dietary preference and the time spent for grazing by goats and that supplementation of nitrogen and essential minerals is crucial to grass based diets since grasses cannot provide enough of these nutrients especially in the dry months.