Feeding extra protein as an attempt to increase amino acid flux to the intestine may increase lactational performance of dairy cattle. The objective of this study was to compare lactating dairy cow diets containing 16% crude protein (CP), adequate in rumen degradable protein (RDP) and metabolizable protein (MP) according to NRC (2001), with diets containing 17.5% CP. Forty-two Holstein cows (27 primiparous and 15 multiparous, with 172 days in milk) were used in a 3 ´ 3 Latin Square design with 14 replicates. Control diet consisted of 16% CP, adequate in RDP and MP . Crude protein content of diets was increased to 17.5% by feeding extra soybean meal and cottonseed meal (SBCS17.5) to increase diet MP, or extra urea (U-17.5) to increase diet RDP. The experiment was carried out during 60 days with three periods. Animals were group-fed a total mixed ration and milked twice a day. Dry matter intake was higher for the U-17.5 diet (p < 0.15) than for the control diet. Milk (p < 0.01) and 3.5% fat corrected milk (FCM-3.5%) (p < 0.05) yields were increased by extra soybean and cottonseed meal (SBCS-17.5) but not by extra urea (U-17.5). Milk fat content and yield were not affected by treatments (p > 0.05). Milk protein content decreased (p < 0.01) by feeding extra urea (U-17.5). However, higher (p < 0.01) protein yields were observed for the SBCS-17.5 treatment. For cows milking around 29 kg d-1, increasing diet CP content to 17.5%, by feeding extra soybean and cottonseed meal, beyond recommendations for RDP and MP, increased yields of milk, yields of 3.5% fat corrected milk and yields of milk protein.
cottonseed meal; metabolizable protein; protein sources; rumen degradable protein; soybean meal