Evaluating and improving the pasture factor based on herbage allowance and nutritive value to estimate the daily herbage intake of stockers grazing bermudagrass

Abstract

Objective

The critical herbage allowance (^cHA), de- fined as the herbage allowance (HA) level below which DMI becomes limiting, is a key variable defining the pas- ture factor used in a daily gain model (DGM) to estimate the daily DMI of stockers grazing bermudagrass (Cynodon dactylon [L.] Pers.) as constrained by HA and nutritive value. Our objective was to evaluate the DGM containing the ^cHA function using a new, comprehensive database.

Materials and Methods

We evaluated the DGM with its ^cHA function, using the observed and model-predicted ADG values involving 1,032 stocker calves in 33 grazing trials conducted at Overton, Texas. Based on evaluation results, we then modified the ^cHA function by incorporat- ing an enhancement factor (κ). By running the DGM with a potential κ value and evaluating it using the ADG data several times iteratively, we derived the best κ value.

Results and Discussion

Evaluation results showed that the DGM containing the original ^cHA function con- sistently underpredicted ADG, as the function was too restrictive to DMI. This issue indicated that the ^cHA func- tion needed to be less restrictive. After multiple iterations of DGM running and testing, we derived 0.75 for κ. With the modification of the ^cHA function through the incor- poration of κ (= 0.75), the DGM performance improved significantly. The modeling efficiency increased by about 31%, the modeling error decreased by about 16%, and the estimated DMI increased from 1.9% BW to 2.1% BW for stockers grazing bermudagrass.

Implications and Applications

By incorporating κ into the ^cHA function in the DGM, this study addressed the issue of consistent underprediction of ADG. By in- cluding the improved ^cHA function in an herbage intake prediction system, the DGM users can more accurately explore the effects of various management and environ- mental factors on daily DMI and ADG of stockers grazing bermudagrass.