Age and season effects on quality of diets selected by Criollo crossbred goats on rangeland

The objectives of this study were to determine the effects of age of goats (4-week old, 6.2 0.7 kg vs mature, 46.9 5.6 kg) and season (rainy vs dry) on nutrient content of diets selected by Criollo crossbred female goats on an overgrazed Chihuahuan desert rangeland. Two groups of goats, 10 goat kids and 10 non-lactating pluriparous goats from a commercial goat herd were used. Diet quality and dry matter (DM) intake was assessed via repeated collections (3-h periods) of forage from the mouth of goats, which were momentarily restrained using a light short permanent rope tightened to their neck while grazing. Feed intake was assessed by 24-h fecal collection with canvas fecal-collection bags. Mature animals ate more (P < 0.01, 23 g DM/kg bodyweight 7 s.d.) than goat kids (19.5 g DM/kg bodyweight 6 s.d.) across grazing seasons, but DM digestibility of selected diet was greater (P < 0.01) in goat kids than in mature goats (58.5 4.0% vs 55.3 3.5%, respectively) across seasons. Ash (100 16 vs 79 13 g/kg DM), phosphorus (1.36 0.41 vs 1.13 0.36% DM) and crude protein (94.5 4 vs 88.5 5 g/kg DM) contents were greater (P < 0.01) in diets selected by goat kids compared with mature goats. Dietary protein was greater in rainy than in the dry season. Across grazing seasons, herbage selected by goat kids had a lower (P < 0.01) concentration of neutral detergent fibre and acid detergent fibre than did that selected by mature goats. There was an age by grazing season interaction (P < 0.05) for most chemical components of forages selected by goats. In conclusion, both age and season affected diet quality of goats on rangeland, as goat kids ingested a diet richer in nutrients than that of mature goats. This supports the theory that herbage selection is shaped by physiological effort and, consequently, nutrient consumption is driven by higher nutrient requirements for growth, although incomplete development of rumen function and small body mass limited feed intake in preweaning goat kids