Diet selection by steers using microhistological and stable carbon isotoperatio analyses

Two methods of determining diet botanical composition, microhistological (M H), and stable carbon isotope ratio (CR) analyses were used to determine bo tanical composition of ingesta and fecal grab samples in steers grazing rhi zoma peanut-mixed tropical grass pastures. Three pastures were used over tw o grazing seasons, 1992 and 1993, in Brooksville, FL. A weighted-disc doubl e-sampling technique was used to determine forage mass and botanical compos ition, percentage of rhizoma peanut (Arachis glabrata), grass (Paspalum not atum and Cynodon dactlyon), and forb (primarily Chenopadium ambrosioides) o n offer every 28 d throughout the grazing seasons. There was an effect of s ampling date (P < .001), sampling date x pasture (P < .001), and sampling d ate x year (P < .001) on forage mass on offer. There was a pasture x year x sampling date interaction(P < .001) for all botanical components. In 1992 and 1993, using cannulated steers sampled every 56 d, there were interactio ns with year for rhizoma peanut and forb (P < .05), but not for grass with MH analysis (components: rhizoma peanut, grass, and forb). Ingesta and feca l rhizoma peanut (r = .73 and .92 for 1992 and 1993, respectively) and inge sta and fecal forb (r = .86 and .98 for 1992 and 1993, respectively) were p ositively correlated (P < .001). Ingesta and fecal grass mere positively co rrelated (r = .52, P < .001), but the correlation was not as high. With the CR analysis (components: Calvin cycle [C-3] plants and C-4-dicarboxylic ac id pathway [C-4] plants), ingesta and corrected fecal (corrected for in vit ro organic matter digestibility [IVOMD]) Ca plants were positively correlat ed (r = .62; P < .001). Diet composition of fecal grab samples from noncann ulated steers, collected on the same sampling schedule as for hand-clipped pasture samples, differed at times due to the complexity of the award (both rhizoma peanut and forb constituted a single component, C-3, in the CR ana lysis). Based on these results, if there is a substantial contribution of f orb to the diet, fecal microhistological analysis may be more informative t han fecal carbon ratio analysis for estimating diet selection by cattle gra zing tropical pastures.