Eight multicannulated heifers (average BW 415 +/- 34 kg) were used in a replicated 4 X 4 Latin square to evaluate fluid milk processing wash water solids (WWS) as a dietary N source. Heifers were fed corn/cottonseed hull-based diets containing soybean meal (control, 0% WWS N) or WWS replacing soybean meal at 33, 67, or 100% of supplemental dietary N. Total tract and ruminal DM and OM digestibilities decreased linearly or cubically (P < .05) as dietary WWS N increased. Total ruminal VFA concentration (P < .05) and propionic acid molar proportion (P < .10) were greater in heifers fed 0 vs 100% WWS N. Heifers fed 0% WWS N had the greatest (P < .05) ruminal ammonia concentration at all sampling times. Dietary WWS did not affect (P > .10) ruminal pH, fluid dilution rate, fluid flow, fluid volume, or turnover time. Total tract N digestibility decreased quadratically (P < .10) with increasing WWS N in the diet. Supplemental WWS N did not affect (P > .10) flow of duodenal ammonia N or bacterial N, or efficiency of microbial N synthesis. Diets containing WWS N resulted in a cubic increase (P < .10) in duodenal flow of essential amino acids compared with 0% WWS N; however, there were no differences in small intestinal amino acid disappearance. Data indicate that WWS can replace 33% of the soybean meal N in a corn/cottonseed hull-based diet without decreasing ruminal fermentation, fluid digesta kinetics, microbial efficiency, or small intestinal amino acid utilization.

Twelve ruminally, duodenally, and ileally-cannulated Hereford heifers (average initial BW 313 +/- 20 kg) were used in a replicated experiment to evaluate dairy food processing wash water solids (WWS) as a protein source. Heifers were fed 2.8 kg of chopped (7.6 cm) hay and one of three supplements (1.5 kg/d, DM basis). Supplements were formulated to be similar in energy and contained 1.0 (control), 23.2 (WWS), and 21.6% (soybean meal; SBM) CP on an OM basis. Total N and nonammonia N entering the duodenum (g/d) were greater (P <. 10) for heifers fed WWS and SBM supplements than for controls. Bacterial N flow (g/d) at the duodenum was less (P < .10) for controls (43.9) than for WWS- (63.9) and SBM- (69.9) supplemented heifers. Feed escape N (g/d) was greater (P < .10) for WWS-fed heifers than for those fed SBM (32.1 vs 20.7 g/d, respectively). Total tract N digestion (g/d) was greatest (P < .10) for SBM, intermediate for WWS, and least for control heifers. Microbial protein synthesis (g/kg of OM intake) was enhanced (P < .10) by WWS and SBM supplementation, but efficiency of synthesis (g/kg of OM fermented) did not differ among treatments. Essential amino acid (AA) disappearance in the small intestine (g/d) was less (P < .10) for control than for the other two treatments. Nonessential AA disappearance was greatest (P < .10) for the WWS and least (P < .10) for the control treatment. Based on our short-term feeding data, WWS can be used as a protein source for ruminants, but N availability of WWS seems less than that of soybean meal.