Correlation of steer average daily gain with diet quality and forage phenology in an improved annual grassland

Abstract

Management of legume-improved annual range forage is made difficult by the rapid declines in nutritive quality and animal gains as the plants mature. An improved ability to predict occurrence of the critical spring grazing period (CSGP) when these declines begin would help in making livestock management decisions. Objectives of this study were to construct a model to describe seasonal changes in steer average daily gain (ADG); to observe changes in nitrogen concentration ([N]) and in vitro organic matter digestibility (IVOMD) related to time of season and ADG; and to relate the phenological progress of maturation of rose clover (Trifolium hirtum L.) to ADG, [N], and IVOMD. Data from 5 years of a grazing experiment were used to construct the ADG model, which consisted of 3 season-related zones which were described by a series of linear and quadratic function. Data for [N] md IVOMD from 2 spring seasons of sampling with esophageally fistulated steers, and from 1 season of hand-cut sampling of rose clover and other plant species from annual range were related to the CSGP. Nitrogen content of the forage was a more useful predictor of rapid ADG change during the CSGP than was IVOMD. The CSGP midpoint coincided with an approximately 0.5:0.5 mixture of 2 well-defined maturation stages of rose clover.