Effects of protein and energy supplementation of Guernsey cows on the kinetics of digestion and passage of orchardgrass versus alfalfa.

Four mature, nonlactating Guernsey cows were used in a 4 x 4 latin square experiment to determine nutrient digestibility, rate of particulate passage, ruminal volume and composition of ruminal contents in cows fed diets at 80% of ad libitum intake. Diets consisted of 100% alfalfa (AL), 100% orchardgrass (OG), 85% OG plus 15% soybean meal (OG + SBM) and 70% OG plus 20% SBM plus 10% starch (OG + SBM + S) on an as-fed basis. Particulate passage rates were measured by either meal marking (Ce) or ruminal marking (La). Intake (1.6 vs 1.4% of BW) and digestibility of DM (63 vs 56%) and NDF (66 vs 39%) were greater (P less than .05) for OG than for AL. Dry matter and CP digestibility were improved 6 and 16 percentage units, respectively, when OG was supplemented with SBM + S. Nitrogen retention, higher (19 vs 3 g/d; P less than .02) in animals fed OG than in those fed AL, was reduced (P less than .05) when OG was supplemented with SBM (5 g/d), but it was improved (P less than .05) when OG + SBM was supplemented with S (39 g/d). Particulate passage (-k) measured with Ce (3.8 vs 4.6%/h) and La (4.2 vs 4.9%/h) was slower (P less than .01 and P less than .10) for AL vs OG, respectively. Supplementation of OG with SBM increased (P less than .01) -k and reduced (P less than .01) ruminal t1/2 and total t1/2. Averaged across diets, total (32.3 vs 31.1 h) and ruminal t1/2 (15.1 vs 14.0 h) of Ce were slower (P less than .10) than La. Marking techniques were not different (P greater than .05) in sensitivity for passage characteristics. Total ruminal NDF content was not different (P greater than .05) for AL vs OG (10 vs 9 g/kg BW), but it was lower for OG + SBM and OG + SBM + S (7.8 and 7.9 g/kg BW; P less than .05). Supplementation of OG with protein and starch increased total diet intake but did not decrease extent of NDF and ADF digestion.