Impact of winter near-maintenance or non-restricted feeding on growth and intramuscular fat content of yearling rams Abstract

There is increased interest in genetic improvement in intramuscular fat in lamb due to its positive association with meat quality. Live-animal measures such as ultrasound or computed tomography scanning can enable genetic selection. The optimum time at which such measurements should take place is unknown. This study investigated levels of intramuscular fat in year-old rams. During the winter of 2020, ram lambs were fed on either a just-above-maintenance pasture diet (JAM) (n=119) with a target weight gain of 50 g/day, or non-restricted feeding (GAIN) (n=120) with a target weight gain of 180-200 g/day, with all rams slaughtered after 10 weeks of differential feeding. Both groups of rams were fed a ryegrass-dominant pasture. The target for the GAIN group was achieved with an average growth rate of 182±42 g/day (range 76 to 293 g/day), however, for JAM group the average growth rate was 24±64 g/day (range -212 to 126 g/day), with a third of the animals losing weight. The JAM group were split according to whether they had gained (JAM – Gain) or lost weight (JAM – Loss) for the analysis. There were differences (P<0.001) at the time of slaughter in early spring in carcass weight between groups (23.7±0.35, 19.5±0.40 and 17.0±0.47 kg for GAIN, JAM – Gain and JAM – Loss respectively). The levels of intramuscular fat were low across all groups compared to levels normally observed in 6-8-month old ram lambs and with further differences (P<0.001) between the three groups (2.1±0.08, 1.9±0.09 and 1.4±0.10 kg for GAIN, JAM – Gain and JAM – Loss respectively). More work is required to understand management systems that allow improved expression of the trait. Additionally, careful consideration needs to be given to winter grazing management of all young stock, as there is considerable variability in individual animals handling of winter maintenance feeding.