Relation among milk production and composition and blood profiles and fecal P and nitrogen in goats on rangeland

Ninety-two multiparous goats of undefined genotype (native × traditional dairy breeds) kept on rangeland were used to assess the association between blood components and fecal P and nitrogen at the beginning of lactation on milk production and composition. The stepwise regression analysis revealed that fecal P was the only significant explanatory variable positively associated with milk production (r2 = 0.08; P < 0.01). Goats with fecal phosphorus >0.21% produced 63.5 ± 21 kg of milk whereas does with <0.21% of this element in feces produced 54.3 ± 18 kg per lactation. Milk yield at the beginning and middle of lactation and total milk production per lactation was not related to serum glucose, urea, creatinine, cholesterol, total proteins, P, Ca, Mg, Cu, Mg and Zn concentrations at the beginning of lactation. Fecal P, determined by the forward set-up selection, showed also the greatest contribution to the variability in milk fat content (r2 = 0.10; P < 0.01), milk protein percentage (r2 = 0.06; P < 0.05), milk casein content (r2 = 0.06; P < 0.05), and lactation length (r2 = 0.12; P < 0.01). It was concluded that metabolic profiles were of no practical use to predict milk yield and composition in goats in this feed-limited environment. These results also indicate that fecal P is a significant predictor of milk yield, milk composition and lactation length of goats on rangeland.