Influence of stocking rate on nutritive intake of steers grazing a short-duration grazing system

Abstract

A short-duration grazing system was simulated to evaluate dietary quality and forage intake of steers grazing at four stocking rates. Above normal precipitation in 1981 created non-restrictive herbage allowances at all stocking rates. In 1982, an increase of 50% in stocking rate and below normal rainfall resulted in restriction of available forage during the late season. When herbage allowances were not restrictive to potential animal consumption throughout the growing season (1981) and rainfall was well distributed, differences in dietary crude protein (CP) could not be seen across stocking rates until the fall. However, dietary in vitro organic matter digestibility (IVOMD) was depressed at the extreme stocking rate until fall. Under these conditions, greater stocking rates enhanced forage structure to allow selection of diets higher in CP and IVOMD during fall and early winter. When greater grazing pressure was applied the following year (1982) and rainfall was below normal, higher dietary CP was selected from the greater stocking rates throughout the year except in early spring. Dietary IVOMD was generally enhanced throughout the growing season by greater stocking rates. Although herbage allowances were reduced by increased stocking rates across seasons and years, organic matter intake (OMI) was generally not affected. This was due to compensating affects of enhanced forage quality while forage availability declined. However, during fall in the below normal year, OMI could not be sustained by ameliorated forage quality at the greater stocking rates.Regression models based on dietary CP, IVOMD and herbage allowance were used to account for variability in OMI within grazing periods and across years. The regression of herbage allowance on OMI was linear with variable slope coefficients in all periods where forage quantity was not restrictive. When forage availability limited intake, the quadratic regression of herbage allowance on OMI explained up to 75% of the variation in intake. A double reciprocal transformation of herbage allowance and OMI also was significant when forage availability limited intake. In contrast to within grazing period models, a linear regression with positive slope of IVOMD to OMI was found across all grazing periods and years. Erbium and ytterbium indigestible markers were compared and it appeared that either marker could be used with confidence on rangeland. Two methods of estimation of indigestibility were evaluated. In vivo estimates were more variable than in vitro estimates. This reflected a greater amount of animal variability associated with the in vivo based indigestibility estimates.