INFLUENCE OF RUMINAL OR DUODENAL SOYBEAN OIL INFUSION ON INTAKE, RUMINAL FERMENTATION, SITE AND EXTENT OF DIGESTION, AND MICROBIAL PROTEIN-SYNTHESIS IN BEEF HEIFERS CONSUMING GRASS HAY

Six heifers (two Hereford x Jersey, four Hereford x Longhorn; average BW 278 kg) cannulated at the rumen and duodenum and fed a grass hay (fescue/orchardgrass) diet were used in a replicated 3 x 3 Latin square. Treatments were either no infusion (C), 150 ml of duodenally infused soybean oil (DI), or 150 ml of ruminally infused soybean oil (RI)/heifer twice daily for a total daily infusion of 300 ml of soybean oil. Periods of the Latin square included 18 d for adaptation and 5 d for collection. Forage OM, ADF, NDF, and N intakes were not affected (P > .10) by soybean oil infusion. Ruminal (P = .11) and total tract (P < .10) OM digestibilities were decreased by RI compared with C or DI, but ADF and NDF digestibilities were not affected by treatment. Duodenal N (P < .05) and microbial N flows were increased (P < .10) for C and RI compared with DI. Microbial efficiency (g of N/kg of OM truly fermented) was improved (P < .10) by RI compared with DI but did not differ (P > .10) from C. Ruminal pH was lower (P < .05) with RI than with either C or DI. Ruminal NH3 N, total VFA, and acetate were not affected (P > .10) by treatment. Propionate (mol/100 mol) was greater (P < .05) with RI than with DI and C, but the proportion of butyrate did not differ among treatments. These data indicate minimal direct benefits for improving forage usage as a result of soybean oil infusion with a 100% grass diet; however, animals should realize benefits from additional dietary energy provided by infused lipid.