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 (average initial BW 313 +/- 20 kg) and 27 intact Hereford heifers (average initial BW 256 +/- 17 kg) were used in two experiments to evaluate dairy food wash water solids (WWS) as a protein source in medium-quality hay diets. Heifers received a basal diet of orchardgrass hay (7.4% CP) and were assigned to one of three supplement treatments: control (C;.9% CP), WWS (18.8% CP)-, and soybean meal (SBM 19.1% Cp)-based supplements (fed at 1.5 kg of DM/d). Supplements were formulated to have similar ME concentrations. Ruminal ammonia concentrations were greater (P <.10) for WWS- and SBM-supplemented heifers than for C heifers at most sampling times. Moreover, WWS and SBM increased (P < .10) total VFA (mM) and acetate (mol/100 mol) and lowered propionate (mol/100 mol) at several sampling times. Ruminal fluid volume (liters) was unchanged (P > .10) by treatment; however, fluid dilution and flow rate (liters/h) were less (P < .10) in C heifers than in heifers fed SBM or WWS supplements. Wash water solids and SBM supplementation increased (P < .10) OM, NDF, and ADF digestibilities compared with C heifers. Feeding WWS and SBM supplements increased BW at 84 d (P < .10) compared with C-supplemented heifers. Forage intake at 54 and 84 d by heifers supplemented with SBM or WWS was greater (P < .10) than by C heifers. Control-supplemented heifers had the least, WWS intermediate, and SBM the greatest ADG at 84 d (P < .10; .14 vs .35 vs .48 kg/d, respectively). These data indicate that WWS may be used as a protein source without serious adverse effects in heifers consuming medium-quality hay for 84 d.