Digestive physiology of steers grazing fertilized and non-fertilized blue grama rangeland

Abstract

Eight field trials [early August 1983 (EAUG83), late August 1983 (LAUG83), early November 1983 (ENOV83), early January 1984 (EJAN84), May 1984 (MAY84), late July 1984 (LJUL84), late August 1984 (LAUG84), late November 1984 (LNOV84)] were conducted on blue grama (Bouteloua gracilis) rangeland in southern rangeland in southern New Mexico to examine relationships among nitrogen (N) fertilization of forage, stage of plant growth, diet botanical and chemical composition, forage intake, digesta kinetics, and ruminal fermentation in beef steers. A fertilized pasture (45 kg N/ha) was evaluated during the year of and year after fertilizer application and compared with an adjacent nonfertilized pasture. Two esophageal- and 4 ruminally cannulated steers/pasture were used in a split-plot design. Dietary organic matter percentage was not affected by fertilization; however, fiber components increased as plants approached dormancy on both fertilized and nonfertilized rangeland. Dietary crude protein levels were numerically higher in the fertilized pasture within all trials. Fertilization had no consistent effect on rate or extent of in vitro organic matter digestibility. Fertilization increased (P<0.05) ruminal ammonia ( NH3) concentrations in all but one trial and levels were adequate for maximal microbial protein synthesis; however in the nonfertilized pasture, ruminal NH3 levels were potentially inadequate during periods of dormancy. Ruminal pH was numerically higher for steers on the fertilized pasture than for those on the unfertilized pasture each sampling trial except LNOV84. Fertilization had little effect (P>0.05) on total volatile fatty acid (VFA) concentration or molar proportion of individual acids. Total ruminal VFA concentration was highest in steers during periods of active plant growth. Voluntary organic matter intake was usually unaffected (P>0.05) by fertilization except in EJAN 84 when intake was higher (P<0.05) in the fertilized pasture and LNOV84 when intake was higher (P<0.05) for steers grazing the nonfertilized pasture. Organic matter intake by steers averaged 21.8 g/kg body weight (BW) and 21.6 g/kg BW across the 8 trials for fertilized and nonfertilized pastures, respectively. Intake in both pastures declined with advancing season. Particulate passage rate (PPR) was not different between treatments (P>0.05) during ENOV83, MAY84 and LNOV84. However, PPR was faster (P<0.05) for steers grazing the fertilized than in the nonfertilized pasture during the remaining 5 sampling periods. Correspondingly, retention time of digesta in the gastrointestinal tract was reduced for steers grazing the fertilized pasture during these 5 trials. Estimated gastrointestinal fill was unaffected (P>0.05) by treatment except during the EAUG83 and LAUG84 trials when steers grazing fertilized pasture had reduced (P<0.05) fill compared with steers grazing nonfertilized pasture. Fluid passage rate (FPR) did not differ (P>0.05) between treatments for any trials except in LAUG84 when steers in the fertilized pasture had a lower (P<0.05) FPR than steers in the nonfertilized pasture.