Sixteen multiparous lactating Holstein cows (four with rumen cannulae) were fed diets varying in the content and form of ruminally degradable carbohydrates and N to examine dietary effects on microbial protein synthesis (MPS) and whole animal N efficiency, and to evaluate the use of a model based on milk urea N (MUN) for predicting urinary N excretion and N utilization efficiency (NUE). A replicated Latin square design (consisting of diet and experimental period) was employed. The four diets consisted of two low protein diets with either 20% ground corn (diet LP) or 13.5% ground corn plus 3% sucrose (diet LP sucrose) and two high protein diets with 13.5% corn and 3% sucrose with either urea (diet HP urea) or soybean meal (diet HP SBM) as supplemental rumen-degradable protein sources. The intakes of dry matter and N were increased by increasing dietary crude protein (CP) level. However, the yields of milk and milk protein were not affected by CP level. Yield of microbial protein was reduced by sucrose and increased by CP level. There were no differences between urea and SBM supplementation on DM intake, milk yield, or MPS. Mean urinary N excretion for all cows (252 g/d) was underestimated by 55 g/d or overestimated by 25 or 33 g/d using alternative equations based on MUN. Subsequently, NUE (mean = 22.4%) was underestimated by 7.5, 3.2, or 2.9%, using a previously published set of equations. Urinary N excretion and NUE could be predicted within 10 and 14% of observed values, respectively, using a set of equations incorporating MUN. Therefore, MUN appears to be a useful tool to help assess N losses from lactating cows.

A mixture of field peas and triticale was planted in spring, harvested as silage, and followed by a double crop of pearl millet, which also was harvested as silage. Eighteen Holstein cows were fed diets based on pea with triticale, pearl millet, or alfalfa plus corn silages. Dry matter digestibility of the pea with triticale diet was higher than for control (71.1 vs. 66.9%), but DM digestibility was not different between control and pearl millet diets. Milk production was not affected by diets containing pea with triticale or pearl millet compared with control diets (25.2, 23.2, and 24.5 kg/d). Cows fed pea with triticale produced milk with a higher concentration of fat (4.59 vs. 3.35%) and more FCM (27.3 vs. 22.1 kg/d) than those fed the control diet. However, cows fed the control diet gained more BW than those receiving pea with triticale or pearl millet diets. Partitioning of energy between body stores and milk production was different between cows fed pea with triticale and control diets; however, total energy use was not different (32.4 vs. 30.5 Mcal of NE(L)/d). Differences in energy partitioning may have been caused partly by differences in ruminal fermentation of the respective diets.

Four ruminally cannulated, nonlactating Holstein cows (593 kg) were fed bromegrass hay diets within a 4 X 4 Latin square. Treatments were arranged factorially and consisted of bromegrass fertilized with 0 or 89 kg N/ha and harvested in the late-boot or full-head stage of maturity. Total tract digestibility of DM (57.6 vs 51.1%), NDF (68.8 vs 63.0%), ADF (51.1 vs 46.0%) and cellulose (63.4 vs 53.5%) decreased with increasing maturity (P less than .05). These forages also were placed in dacron bags for determination of in situ digestion kinetics. Rate, extent of digestion, and lag times were determined for DM and fiber components using linear regression of log-transformed data or a nonlinear fitting procedure. Treatment effects generally were consistent between methods of calculating kinetic data; however, numerical results often differed greatly. Extents of in situ ruminal digestion (72 h residue) for NDF, hemicellulose and cellulose were lower (P less than .05) for full-head than for late-boot-stage bromegrass. Extent of digestion calculated using the nonlinear method followed similar trends. Nitrogen fertilization had no effect on extent or total tract digestibilities of fiber components of DM. In situ disappearance rates (calculated using both methods) for fiber fractions and DM generally were almost always affected less by maturity than by fertilization. Particles of N-fertilized bromegrass had more (P less than .05) bacterial N associated with them during the first 12 h in situ than did particles of unfertilized brome. Surface area equations based on lignification of NDF accurately estimated fiber digestion and suggested that lignification was the primary reason DM and NDF digestibilities decreased with advanced maturity. Due to the relatively low DM intakes of these diets (11 kg/d), rate of passage, not rate of digestion, was of principle importance in determining extent of digestion of these diets.

Four nonlactating Holstein cows (593 kg) with ruminal cannulas were fed bromegrass hay that was fertilized with 0 or 89 kg of N/ha and harvested in the late-boot or full-head stage of maturity. Total tract apparent digestibility of N was less for full-head than for late-boot bromegrass (48 vs 64%; P less than .01) and was greater for N-fertilized than for unfertilized bromegrass (60 vs 52%, P less than .01). True N digestibility was greater (P less than .01) by cows fed late-boot hay than by those fed full-head bromegrass (85 vs 81%). Maturity did not affect N retention. Nitrogen fertilization increased N retained, both grams per day (70 vs 20; P less than .01) and as a percentage of N intake (P less than .01). Nitrogen and amino acid disappearance kinetics were determined in situ. Rate constants for N disappearance within rapidly and slowly degraded pools were not different among treatments, but because late-boot had a larger proportion of N in the rapidly degraded pool, overall disappearance rate for N was faster (P less than .05) for late-boot than for full-head bromegrass (12.0 vs 7.1 %/h). Treatment effects observed for individual amino acids were similar to those found for N. Both composition and kinetic data were needed to explain differences in N digestion and balance. Distribution of N within the forage had profound effects on the N economy of the cows.