Maximising lamb growth rates

Abstract

A study was undertaken to evaluate the relative importance of milk and pasture to lamb growth during lactation under a high performance lamb production system. A total of 285 East Friesian x Romney (EFxR), Finn x Romney (FxR) and Romney (R) maiden 2-tooth ewes lambed following insemination with semen from six high growth rate Poll Dorset and composite rams. Mean lambing date was 16 July 1998. Breed groups were run together in either single or twin mobs and fed at high levels of nutrition (pasture covers were maintained at 1400 kg DM/ha or greater throughout lactation). Average lamb growth rates from birth to 15 weeks of age were 343 g/d and 292 g/d for single and twin lambs, respectively. Single and twin lambs reared by EFxR ewes grew significantly faster than lambs reared by the other ewe breeds. Ewes were milked at 3-weekly intervals using oxytocin with EFxR ewes producing significantly more milk than the other breeds throughout lactation. Milk composition varied significantly between ewe breeds with FxR ewes producing significantly higher fat % and total milk solids (P<0.001) than the other breeds. Reduced overall milk solids from EFxR ewes lowered a 30% advantage in milk volume to approximately 20% in terms of total milk energy production. Although positive, the correlation between milk production and lamb liveweight was generally poor, indicating that non-milk factors had an additional influence on lamb liveweight gain. The relative importance of milk to lamb growth was examined by calculating theoretical metabolisable energy (ME) requirements for single and twin lambs and comparing them with the ME supplied in the milk produced by the ewes. Assuming that lambs harvest all of the milk produced by the ewes, it was found that even in an optimum grazing situation, twin lambs needed to acquire over a third of their energy requirements from pasture by the time they were 6 weeks of age. At this age, lambs were unlikely to have a fully developed rumen and the opportunity for the lamb to select high quality, highly digestible pasture components would be critical for maximum growth. Therefore, in a situation where feed is limiting, competition between ewes and lambs for highly quality feed is likely to restrict lamb growth rate. This is likely to occur even at peak lactation with well fed, high milk producing ewes. Keywords: East Friesian, ewe milk production, Finn, lamb growth rates, Romney, 2-tooth