Dietary Protein Effects on Nitrogen Excretion and Manure Characteristics of Lactating Cows

In the primary experiment, 12 diets were fed to 34 lactating cows with the objective to determine effects of level and source of dietary protein on N excretion, the first estimate needed to budget manure N flow and utilization on dairy farms. Complete collection of urine and feces separately allowed for determination of digestibilities and urine and fecal excretion of N. Equations to predict N excretion in urine and feces were developed using DM intake and N intake as the primary predictors. Including milk yield or body weight in the model did not account for appreciable additional variation. Fecal plus urine N excretions estimated from these equations agreed closely with NRC equations and estimates made assuming that N consumed was either secreted in milk or excreted in urine and feces (diet N minus milk N). Predicted N excretion for 635 kg Holstein cows consuming 17.8 kg/d dry matter (15.3% CP), which is the estimated amount required for 22.7 kg milk/d, was 325 g (150 g urine N and 175 g feces N). Fecal DM excretion averaged 34.3% of DM intake (6.1 kg for typical cow) and was 88.2% volatile solids; daily urine dry matter averaged 0.9 kg/d (20 kg urine/d averaging 0.045% dry matter) and was 47.8% volatile solids. Additionally, compositions and total daily excretions were determined for feces (ADF, NDF, crude fat, Ca, P, Mg, K, Na, Fe, Zn, Cu, Mn, and Mo) and urine (Ca, K, Mg, Na, P). As with N, amounts of various fractions excreted daily were closely associated with DM intake (which varies with milk production) and were much less variable than percentage compositions. Abbreviation key: BM = blood meal, Ca-LCFA = Ca soaps of long-chain fatty acids, CP = crude protein = N 6.25, DM = dry matter, DMI = DM intake, FtM = feather meal, RUP = ruminally undegraded protein, SBM = soybean meal.