BRIEF COMMUNICATION: Modelling liveweight change to inform residual feed intake models in growing lambs

Forty non-pregnant ewe lambs, approximately nine months of age, were individually penned undercover and fed for two periods (P1 & P2) of 42 days each. During P1 all animals were fed a ryegrass sward harvested the previous day. During P2, half remained on the ryegrass sward and the other half were fed pure lucerne pellets, following a transition period. The animals were weighed twice-weekly, in the morning prior to the new day’s feed being offered. Only 38 animals completed P2 of the trial, with two dying due to misadventure. Regression models were fitted individually for each animal using the regression procedure in SAS (SAS, 2004). A series of combinations of different data sub-sets and models were used for liveweight data collected from P1 and P2 respectively: linear model fitted using the twice-weekly collected data for the first four (9 weights), five (11 weights) and all six weeks (13 weights) worth of data; a linear model fitted using the twice-weekly collected data for all six weeks, but excluding data from day 18 (12 weights); a linear model fitted using once-weekly data for all six weeks (7 weights); a quadratic model fitted using the twice-weekly collected data from all six weeks (13 weights); a quadratic model fitted using the twiceweekly collected data from all six weeks, but excluding data from day 18 (12 weights).