Prediction of manure nitrogen and organic matter excretion for young Holstein cattle fed on grass silage-based diets.

The objectives of the present study were to evaluate the effects of sex (steers vs. heifers) of young Holstein cattle on N and OM excretion in feces and urine and to use these data to develop prediction models for N and OM excretion. Data used were derived from a study with 20 autumn-born Holstein cattle (10 steers and 10 heifers) with N and OM intake and output measured at age of 6, 12, 18, and 22 mo, respectively. The cattle were offered a typical diet used on U.K. commercial farms containing a single grass silage mixed with concentrates. In each period, the cattle were housed as a single group in cubicle accommodation for the first 20 d, individually in metabolism units for the next 3 d, and then in calorimeter chambers for the final 5 d with feed intake, feces, and urine excretion measured during the final 4 d. Within each period, sex had no effect (P > 0.05) on N or OM intake or excretion or N utilization efficiency, with exceptions of steers having a greater intake of N (P = 0.036) and OM (P = 0.018) at age of 18 mo and a lower ratio of fecal N:N intake (P = 0.023) at age of 6 mo. A range of regression relationships (P < 0.05) were developed for prediction of N (g/d) and OM (kg/d) excretion in feces and urine. The present data were also used to calculate accumulated N and OM intake (kg) and excretion for the 2 sexes. Sex had no effects (P > 0.05) on accumulated N or OM intake or N or OM excretion in feces and urine or retained N and OM during the first or second year of life. On average for the 2 sexes at first and second year of age, the accumulated N excretions in feces were 11.4 and 21.1 kg and in urine 11.6 and 30.6 kg, respectively, and the corresponding values for accumulated OM excretions were respectively 241.5, 565.7, 30.3 and 81.5 kg. A number of equations were developed to predict accumulated N and OM excretion in feces and urine (kg) using BW (kg; P < 0.001, r(2) = 0.95 to 0.97). The accurate prediction of N and OM excretion in feces and urine is essential for reducing N pollution to ground and surface water and calculating methane and nitrous oxide emissions from manure management of dairy and beef production systems. These data can add novel information to the scientific literature and can be used to improve national inventories of manure N output and greenhouse gas emissions and to develop appropriate mitigation strategies for young Holstein cattle.