Literature data on utilization of dietary N were analyzed by using meta-analytic procedures for growing milk-fed dairy calves and weaned dairy heifers. The objective was to statistically assess N utilization and excretion in growing dairy cattle when dietary N was altered in otherwise balanced rations at various stages of growth. Studies meeting the selection criteria included data from 16 published papers encompassing 94 distinct observations made on 217 animals. Of these, 6 studied calves were fed milk or milk protein-based milk replacer [milk-fed; 30 to 81 kg of body weight (BW)] with 37 different dietary treatments, and 10 experiments studied heifers receiving diets based on forage, concentrates, or a combination of forage and concentrates (weaned; 56 to 472 kg of BW) with 57 different dietary treatments. Mixed model and fixed effect regression analyses were used to evaluate responses to additional dietary N. True digestibility of dietary N was 100.4% for milk-fed calves and 96.4% for weaned heifers, with corresponding basal fecal N excretion values of 3.05 and 6.51 g of N/kg of dry matter intake. Urinary N (g of N/kg of BW0.75) was consistently greater for milk-fed calves, but the response to increasing N intake was parallel to the response for weaned heifers. Whether using a mixed model approach or a fixed effect approach to account for metabolizable energy intake, BW, and dry matter intake, milk-fed calves retained more N per kilogram of BW0.75 than weaned heifers. However, marginal efficiency of N utilization responded as a continuous function of BW, as opposed to a bimodal response associated with diet type. Gross N efficiency (GNE) responded quadratically to N intake and was greater for milk-fed calves than for weaned heifers. Linear and quadratic coefficients of this function did not differ between diet types, indicating that the response in gross N efficiency to additional N intake was not different between diet groups; rather, the absolute level obtainable differed. Dietary CP concentrations of 18.9% for milk-fed calves and 14.2% for weaned heifers were found to maximize GNE; 22.5% MJ of crude protein/ MJ of ME was found to maximize GNE for both groups. Equations are discussed relative to the requirements to replace basal N losses and efficiency of N utilization.
All Science Journal Classification (ASJC) codes
- Food Science
- Animal Science and Zoology