A trial involving the body and carcass analysis of 7885 lambs sired by 15 different breeds of ram and run at two New Zealand locations (Ruakura and Manutuke) over a 10 year period was analysed to determine the significance and size of the year effect on lamb composition. Animal size (growth rate) differed between sites with the Manutuke lambs being approximately 10 larger. Year had a very significant effect on the muscle, fat, and bone measurements investigated, with this effect remaining after adjusting measurements to the same carcass weight. Carcass fatness was strongly influenced by carcass weight which differed between years. At the same carcass weight, lambs differed by around 5 fat as a proportion of carcass weight or around 20 fat as a proportion of total fat, between the most extreme years at each location. Year differences were as large as those associated with identifiable management factors (breed, strains, sex, etc). Unfortunately, the underlying factors contributing to the year effect are at this stage unknown, but may provide a fruitful field for additional research.
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