Supplemental soybean oil or corn for beef heifers grazing summer pasture: Effects on forage intake, ruminal fermentation, and site and extent of digestion

Nine Angus x Gelbvieh heifers (average BW = 347 +/- 2.8 kg) with ruminal an d duodenal cannulas were used in a split-plot designed experiment to determ ine the effects of soybean oil or corn supplementation on intake, OM, NDF, and N digestibility. Beginning June 8, 1998, heifers continually grazed a 6 .5-ha predominantly bromegrass pasture and received one of three treatments : no supplementation (Control); daily supplementation of cracked corn (Corn ) at 0.345% of BW; or daily supplementation (0.3% of BW) of a supplement co ntaining cracked corn, corn gluten meal, and soybean oil (12.5% of suppleme ntal DM; Oil). Soybean oil replaced corn on a TDN basis and corn gluten mea l was included to provide equal quantities of supplemental TDN and N. Three 23-d periods consisted of 14 d of adaptation followed by 9 d of sample col lections. Treatment and sampling period effects were evaluated using orthog onal contrasts. Other than crude fat being greater (P = 0.01) for supplemen ted heifers, chemical and nutrient composition of masticate samples collect ed via ruminal evacuation did not differ (P = 0.23 to 0.56) among treatment s. Masticate NDF and ADF increased quadratically (P less than or equal to 0 .003) and N decreased linearly (P = 0.0001) as the grazing season progresse d. Supplementation did not influence (P = 0.37 to 0.83) forage OM intake, t otal and lower tract OM digestibility, ruminal and total tract NDF digestib ility, or total ruminal VFA; however, supplemented heifers had lower rumina l molar proportions of acetate (P = 0.01), higher ruminal molar proportions of butyrate (P = 0.007), and greater quantities of OM digested in the rume n (P = 0.10) and total tract (P = 0.02). As the grazing season progressed, total tract OM and N and ruminal NH3 concentrations and NDF digestibility d ecreased quadratically (P 0.04). Microbial N flow (P = 0.09) and efficiency (P 0.04) and postruminal N disappearance (P = 0.02) were greater for Contr ol heifers and declined linearly (P less than or equal to 5 0.02) as the gr azing season advanced. Depressed microbial N flow seemed to be more pronoun ced for Oil than for the Corn treatment. Although total digestible OM intak e increased with supplementation, metabolizable protein supply was reduced in supplemented heifers. Therefore, feeding low levels of supplemental grai n with or without soybean oil is an effective strategy to increase dietary energy for cattle grazing high-quality forages, but consideration should be given to reduced supply of metabolizable protein.