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Intake, nutrient digestibility and milk yield of dairy cows fed urea and two levels of crude protein in diets with sugar cane


Abstract - The objective of this study was to evaluate the effect of two crude protein (CP) levels and two nitrogen sources in the diets of dairy cows on DM and nutrient intake, total apparent digestibility, ruminal fermentation, milk yield and composition and energy balance using sugarcane as forage. Twelve multiparous Holstein cows, averaging 235 days of milking and 22.0 kg of milk/day, were distributed into three balanced and contemporary 4x4 Latin squares. The study consisted of four experimental periods of 21 days, with 14 days for diet adaptation and the remainder for sampling. The cows were fed isocaloric diets with ad libitum intake and forage based on sugarcane in a 2x2 factorial arrangement of treatments: two main nitrogen sources (soybean meal and urea) and two levels of CP (142 g CP/kg DM and 156 g CP/kg DM). Milk samples for compositional analysis were collected on the 14th to 17th day of each period. Ruminal fluid samples were collected by an esophageal tube, three hours after morning feeding, for pH, short chain fatty acid and ammonia nitrogen analysis. Apparent digestibility was determined by means of an internal indicator (ADFi). No effect of diet was observed on dry matter intake (DMI) (kg/d), net energy for lactation (Mcal/d), or total nutrient apparent digestibility. Interactions between nitrogen source and diet crude protein content were observed on rumen pH (P = 0.007) and acetate-to-propionate ratio (P = 0.003). A tendency was observed (P = 0.052) toward an effect of CP levels on milk protein yield (kg/d), as the cows fed diets with soybean meal produced more milk protein than those fed diets with urea. The results indicate that diets with lower levels of CP do not alter the milk yield and composition of dairy cows in the final third of lactation. Keywords: degradability, nitrogen sources, milk yield and composition, dairy cows (AU)

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