Protein supplementation of ruminants consuming low-quality coolor warm-season forage: Differences in intake and digestibility

An in situ study (Exp. 1) using 4 ruminally cannulated steers (343 ± 11 kg of BW) in a completely randomized design was used to compare ruminal degradation characteristics of low-quality coolseason (C3; Kentucky bluegrass straw; Poa pratensis; 6.3% CP; DM basis) and warm-season (C4; tallgrass prairie; 5.7% CP; DM basis) forage. Four ruminally cannulated steers (252 ± 8 kg of BW; Exp. 2) and 4 wethers (38 ± 1 kg of BW; Exp. 3) were used in two 2 × 2 factorial arrangements of treatments to determine the influence of supplemental CP (CPSupp; soybean meal; 0.09 and 0.19% of BW, CP basis, for steers and lambs, respectively) on nutrient intake and digestion of C3 and C4 forages. Steers and wethers were allotted to separate 4 × 4 Latin squares that ran simultaneously with 20-d periods. In Exp. 1, C3 had a greater A fraction (fraction of total pool disappearing at a rate too rapid to measure) and effective degradability of DM and NDF compared with C4 (P < 0.01). In addition, C3 had a greater (P < 0.01) A fraction and effective degradability of N, whereas the C fraction (fraction of total pool unavailable in the rumen) was less (P < 0.01) than those for C4. Consequently, RDP accounted for 84.7% of total CP in C3 as compared with 66% for C4 (P < 0.01). In Exp. 2, a CPSupp × forage interaction (P < 0.01) was noted for forage and total DMI, with CPSupp increasing intake of C4 by 47% and intake of C3 forage by only 7%. Dry matter digestibility responded similarly, with a CPSupp × forage interaction (P = 0.05; CPSupp increased digestibility by 21% with C4 and by 9% with C3 forage). In addition, CPSupp × forage interactions were noted for ruminal liquid retention time (P = 0.02; CPSupp decreased retention by 3.6 h with C4 and by only 0.6 h with C3 forage) and particulate passage rate (P = 0.02; CPSupp increased passage by 46% with C4 and by 10% with C3 forage). As in Exp. 2, a CPSupp × forage interaction (P = 0.01; CPSupp increased digestibility by 18% with C4 and by 7% with C3 forage) was observed with DM digestibility in Exp. 3. In contrast, only N balance (P < 0.01) and N digestibility (P < 0.01) were affected by CPSupp. These data suggest that intake and digestion of lowquality C3 and C4 forages by ruminants are not similar and, more important, that the physiological response of ruminants to protein supplementation of low-quality forage is dependent on forage type.