Effects of protein concentration and source on nutrient digestibility by mature steers limit-fed high-concentrate diets

Five ruminally fistulated 3-yr-old mature Holstein steers (average BW 691 /- 23 kg) were used in a 5 x 5 Latin square experiment with a 2 x 2 + 1 fac torial arrangement of treatments. Effects of protein concentration and prot ein source an nutrient, digestibility, excretion of DM and fecal N, ruminal fluid volume and dilution rate, ruminal characteristics, and in situ DM di sappearance of whole shelled corn, ground corn, and orchardgrass hay were m easured in steers limit-fed high-concentrate diets at 1.5% of BW. A negativ e control basal diet (NCI; 9% CP) was supplemented to achieve either 11 or 14% CP; supplemental CP was either from soybean meal (11 and 14% SBM) or a 50:50 ratio of CP from urea and soybean meal (11 and 14% U). Dry matter and OM digestibilities were 5% greater (P < .07) for steers fed the SBM diets than for those fed the U diets. Starch digestibility did not differ (P > .1 0) among steers fed any of the diets. Nitrogen source did not, affect (P > .10) apparent N digestibility or fecal N excretion; however, steers fed the NC diet had the lowest (P < .10) apparent N digestibility compared with th ose fed all other diets. Ruminal fluid volume was lower (P < .06) when stee rs were fed the NC diet compared with all other diets; there were no differ ences (P > .74) among diets for ruminal fluid dilution rate. In general, ru minal ammonia N and VFA molar proportions were not affected by protein sour ce or concentration. Although CP concentration affected (P < .06) insitu DM disappearance of ground corn, CP concentration did not (P > .48) affect to tal tract digestion of DM or OM. This indicates that CP concentration may h ave affected site of digestion, but not extent of digestion. When mature ru minants were limit-fed acorn-based diet to meet primarily a maintenance fun ction, protein source and concentration had little effect; on measures of n utrient digestion.