title: Protein supplementation of ruminants consuming low-quality cool- or warm-season forage: differences in intake and digestibility.
abstract: 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 cool-season (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 low-quality 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.
authors.affiliation: Eastern Oregon Agricultural Research Center, Oregon State University, Burns 97720, USA. dave.bohnert@oregonstate.edu
authors.forename: D W
authors.initials: DW
authors.lastname: Bohnert
authors.forename: T
authors.initials: T
authors.lastname: DelCurto
authors.forename: A A
authors.initials: AA
authors.lastname: Clark
authors.forename: M L
authors.initials: ML
authors.lastname: Merrill
authors.forename: S J
authors.initials: SJ
authors.lastname: Falck
authors.forename: D L
authors.initials: DL
authors.lastname: Harmon
mesh_terms: D000824:Animal Nutritional Physiological Phenomena / Q000502:physiology*; D000818:Animals; D001806:Blood Urea Nitrogen; D002417:Cattle / Q000378:metabolism*; D019587:Dietary Supplements / Q000592:standards; D004063:Digestion / Q000502:physiology*; D004435:Eating / Q000502:physiology*; D005243:Feces / Q000737:chemistry; D008297:Male; D006109:Poaceae; D011897:Random Allocation; D012044:Regression Analysis; D012417:Rumen / Q000378:metabolism*; D012621:Seasons; D012756:Sheep / Q000378:metabolism*