Protein Supplementation of Ruminants Consuming Low-Quality Cool- or Warm-Season Forage: Differences in Intake and Digestibility
Four steers (252 ± 8 kg BW; Exp. 1) and four wethers (38 ± 1 kg BW; Exp. 2) were used in two 2 × 2 factorial design experiments to determine the influence of protein supplementation of low-quality cool- (C3; bluegrass straw) and warm-season (C4; tall grass-prairie hay) forage (6.3 and 5.7% CP, respectively) on intake and nutrient digestion. Steers and wethers were allotted to 4 × 4 Latin squares with 20-d periods. Animals were provided forage at 120% of the previous 5 d average intake. Soybean meal (SBM; 52% CP) was used as the CP supplement. In Exp. 1, feed and digesta were collected on d 14 through 18 for estimation of nutrient digestibility and ruminal fluid was sampled on d 20. In Exp. 2, feed, feces, and urine were collected on d 16 to 20 for calculation of N balance. Contrasts were: 1) supplemented (SUPP) vs not supplemented; 2) C3 vs C4; 3) SUPP × forage type. A SUPP × forage type interaction (P < 0.01) was noted for forage and total DMI in Exp. 1, with supplementation increasing intake of C4 by 47% but only 7% for C3 forage. DM digestibility responded similarly with a SUPP × forage type interaction (P = 0.05; SUPP increased digestibility 12% with C4 and 9% with C3 forage). Also, SUPP × forage type interactions were noted for ruminal liquid retention time (P = 0.02; SUPP decreased retention 3.6 h with C4 and only 0.6 h with C3 forage) and particulate passage rate (P = 0.02; SUPP increased particulate passage 46% with C4 and 10% with C3 forage). As in Exp. 1, a SUPP × forage type interaction (P = 0.01; SUPP increased digestibility 18% with C4 and 7% with C3 forage) was observed with DM digestibility in Exp. 2. In contrast, only supplementation effects were noted for N balance (P = 0.002) and N digestibility (P < 0.001), which increased with supplementation. These data suggest that intake and digestion of low-quality C3 and C4 forages by ruminants are not similar and, more importantly, the physiological response of ruminants differs with protein supplementation of C3 versus C4 forages.