Modeling Lamb Weight Changes on Wheatgrass and Wheatgrass‐Sainfoin Mixtures

Abstract

Prediction of animal weight change (CUM) with regression models developed from grazing trials typically uses herbage parameters and CUM measured on the same day. We hypothesized that lamb (Ovis aries L.) CUM recorded at time t was a function of herbage quality and/or quantity measured at a previous harvest t − x (where x = days prior to measurement of CUM). Our objectives were (i) to determine if time series regression analysis (TSR) could be used to model CUM from three irrigated wheatgrasses, ‘Hycrest’ [Agropyron cristatum (L.) Gaertner × A. desertorum (Fischer ex Link) Shulters], ‘Luna’ [Thinopyrum intermedium subsp. barbulatum (Schur) Barkw. and D.R. Dewey], and ‘Jose’ [T. ponticum (Podp.) Barkw. and D.R. Dewey] grown alone and with ‘Renumex’ sainfoin (Onobrychis viciifolia Scop.), and (ii) to test the models. Replicated pastures grown on a fine, mixed thermic Torrertic Paleustolls were rotationally grazed by Rambouillet × Suffolk wether lambs for an average of 77 d in spring of 1987 and 1988. Herbage quality, quantity, and plant parts were estimated from pregrazing, after 2 and 4 d of grazing, and postgrazing (7 d) harvests and used as variables in TSR. For TSR models, lagged variables (t − x) were selected more (67–92% of total) than nonlagged variables (t), supporting our hypothesis. Time series regression models described CUM accurately (average R2 > 0.70), but selected variables were not consistent among treatments, time lags, or years. Model testing indicated poor predictive accuracy (r2 = 0.07−0.51), limiting the usefulness of projecting CUM across seasons and demonstrating the necessity of testing regression models.