1. Abstract: This study was conducted to determine the effects of reducing rumen degradable protein (RDP) with constant rumen undegradable protein in mid-lactation dairy cow diets on feed intake, milk production, milk composition and body weight. Reduced rumen degradable protein in the diet could reduce manure nitrogen output which will decrease ammonia emissions from manure and improve air quality. Forty mid-lactation Holstein (36) and Jersey x Holstein cross-bred (4) cows were fed a common diet with 18.4% crude protein (CP) during the covariate period (d 1 to 28). From d 29 to 47, cows were randomly assigned to one of four dietary treatments with formulated CP contents of 18, 16.8, 15.7, or 14.5% with formulated RDP contents of 11.3, 10.4, 8.5 and 7.6% of dry matter (DM), respectively. All diets were fed as total mixed rations (TMR). Cows were transitioned to lower CP diets over a period of 3 d. Feed intake, milk yield and body weight were measured daily. Milk composition was measured for 3 d in each of weeks 3 and 7. Individual forage samples, grain mixtures and TMR were sampled weekly. The Proc Mixed procedure of SAS was used to analyze the data using a repeated measures model. Results indicated that treatment period DM intake and CP intake decreased linearly. Treatment period milk yields were observed to be 41.2, 42.1, 40.3 and 36.6 kg/d as dietary CP decreased from 18 to 14.5% with the lowest CP diet trending towards significance. No treatment effects were observed for body weight, milk fat, protein, or lactose content. No significant changes were observed in milk protein yield. However milk fat yield decreased linearly as dietary CP declined. Milk urea nitrogen also decreased linearly as the CP content of the diets declined averaging 20.2, 17.6, 14.3, and 12.4 mg/dl, respectively indicating decreased losses of N via urine. Predicted daily excretion of urinary and fecal N decreased linearly with decreased RDP in the diet indicating decreased ammonia emissions from the manure into air. These results suggest that mid- lactation dairy cows can be fed diets with RDP contents as low as 8.5% of DM, which is less than that recommended by NRC (2001). This indicates that current NRC RDP requirements may be overstated. Effects of reducing ruminally degradable protein in the diets of lactating dairy cows J. Cyriac, A. G. Rius, M. L. McGilliard, M. D. Hanigan Dept. of Dairy Science, Virginia Tech. e-mail jobyjct@vt.edu, Phone : 540-231-7320jobyjct@vt.edu Dept. of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 Objectives: To determine the effect of reducing rumen degradable protein (RDP) with constant rumen undegradable protein in mid-lactation dairy cow diets  feed intake,  milk production,  milk composition, and  body weight  N efficiency Introduction: ■ Reduced dietary nitrogen results in reduced urinary nitrogen output which can lead to reductions in manure nitrogen. ■ Reduced urinary nitrogen should result in decreased ammonia emission from manure and improved air quality. ■ Reduced RDP in the diet increase N efficiency of dairy cows ■ Current rumen degradable protein requirements set by NRC have not been thoroughly tested. Materials and methods: ■ Cows: - 40 mid-lactation (DIM:147 ± 38) cows - Holstein (36) - Jersey x Holstein cross-bred (4) ■ Milk urea nitrogen decreased linearly (P < 0.0001) from 20.2 to 12.4 mg/dl as dietary CP declined. ■ Predicted daily excretion of urinary N was significantly greater for animals fed the high RDP diets (P = <0.0001). ■ Predicted fecal N excretion followed similar trends as in the case of urinary N (P <0.0001) - decreased N in urine and feces should result in decreased emissions of ammonia to the environment. - Frank et al. (2002) found on average a 270% decrease in ammonia release to air from manure recovered from cows fed 14 vs. 19% CP diets. Conclusion ■ Mid-lactation dairy cows can be fed diets with RDP contents as low as 8.5% of DM which is less than NRC (2001) recommendations, without affecting milk and protein yields. ■ Feeding decreased RDP in the diet could decrease N excretion in the manure which should decrease ammonia emissions from manure and improve air quality. ■ Reduced dietary RDP resulted in improved N efficiency. References Christensen, R. A., G. L. Lynch, J. H. a. Clark, and Y. Yu. 1993. Influence of amount and degradability of protein on production of milk and milk components by lactating Holstein cows. J Dairy Sci. 76: 3490-3496. Frank, B., M. Persson, and G. Gustafsson. 2002. Feeding dairy cows for decreased ammonia emission. Livestock Prodn. Sci., 76:171-179. Klusmeyer, T. H., R D. McCarthy, Jr., J. H. Clark, and D. R. Nelson. 1990. Effect of source and amount of protein on ruminal fermentation and passage of nutrients to the small intestine of lactating cows. J.Dairy Sci. 73:3526-3537. Dietary Crude Protein (% of DM) N Efficiency (%) Kg / day Dietary Crude Protein (% of DM) IngredientsCP, % of DM 18.016.815.714.5 Corn silage39.18 Mix grass legume silage 7.79 Whole linted cotton seed 2.87 Rolled high moisture corn grain15.38 Soybean hulls 9.93 8.4811.8715.25 Soybean meal, solvent-ext (48% CP)20.74 7.10 3.55 0.00 Protected SBM (LOL/Purina) 0.00 3.93 7.8511.78 Ground dry corn grain 0.60 1.30 2.00 2.70 Tallow 0.48 0.93 1.39 1.84 Vitamins and minerals 2.59 2.61 2.62 2.65 Figure 1: Dry matter intake and milk yields of cows fed decreasing concentrations of crude protein in the diet Figure 2: N efficiency of cows fed decreasing concentrations of crude protein in the diet. ■ Design: - Covariate period: a common diet of 18.4% CP from d 1 to 28 - Treatment period: diets formulated for 18.0, 16.8, 15.7, or 14.5% CP content from d 29 - 47. - RDP content of the formulated treatment diets were 11.3, 10.4, 8.5 and 7.6% of DM. - RUP content of the formulated treatment diet was 7.1% for all. - DMForage content were same for all diets but grain mixes were formulated to have different CP and RDP content and using protected SBM - Cows were transitioned to lower CP diets over a period of 3 d. ■ Sampling: - Individual feed ingredients and TMR were collected weekly. - Milk yield, feed intake and body weight were recorded daily. - Milk compositional analysis were performed 3 d in last week of covariate and treatment periods. ■ Data analysis: - Proc mixed procedure of SAS Results and Discussion ■ DM content of the diets was 59% across treatments. ■ CP content of diets were 18.4, 16.8, 15.2 and 13.6 % of DM ■ DMI decreased linearly (P=0.01) with decreasing concentrations of CP in the diet although this was driven by intakes on the lowest CP diet ■ CP intake decreased linearly (P<0.0001) with diets ranging from 5.3 to 3.4 kg/day. - decreasing concentration of CP in the diets - decreased DMI contributed. ■ Fiber content of the diet increased with decreasing CP content of the diet. ■ No significant changes in body weight. ■ Milk yield was not significantly affected by treatment. - there was a trend for a linear decline in milk yield (P = 0.09). - RDP at 7.6% of DM may affect milk yield. ■ No treatment effects were observed for milk fat, protein, or lactose content. ■ Milk protein yield ranged between 1.23 and 1.07 kg/d with no significant dietary effects. - lower RDP diet can maintain milk protein yield. ■ Milk fat yield decreased linearly (P=0.02) with decreasing dietary CP. DM: P = 0.002 Milk yield: P = 0.24 P < 0.0001 Table 1: Ingredient composition (formulated) of the experimental diets