Investigating the constraints on lamb growth in intensive feeding systems

Intensively feeding lambs high-quality nutrients can enable accelerated lamb growth, irrespective of seasonal variations in feed supply. The cost of feeding a high-quality diet is a substantial limitation, requiring lamb growth rates to be maximised to reduce time to slaughter and total nutrient intake. This thesis investigated potential constraints on the growth rates of intensively fed lambs and it was hypothesised that growth rates could be expected to increase if practical measures to increase nutrient intake were identified and implemented.
A survey of lamb producers investigated the apparent growth rates and feed conversion ratios of lambs being intensively fed. The reported lamb growth rates and feed conversion ratios were highly variable, but on average, were consistent with those expected for lambs fed a high-quality diet. It was concluded that to improve the productivity of lambs in feedlots, feeding strategies need to be identified that minimise social and nutritional issues, and promote maximum intake of nutrients.
A live animal experiment investigated differing nutritional management of lambs prior to feedlot entry. It was hypothesised that exposing lambs to a feedlot ration would expedite acceptance and increase nutrient intake. Providing lambs with a feedlot ration prior to feedlot entry appeared to increase feed intake and growth rates in the feedlot; however, results were likely affected by the low intake of the supplements pre-weaning, and lower than expected lamb growth rates in the feedlot.
A feeding trial used CT scanning to investigate energy transactions in lambs with two feeding levels and at two stages of maturity. Lambs at later stages of maturity retained proportionately more energy as fat compared to younger lambs, which was expected. Lambs fed the lower feeding level in the second feeding period retained proportionately more energy as fat compared to the higher feeding level, which was hypothesised to be because of the reduction in size of visceral organs in response to changes in feeding level.
It was hypothesised that the use of a lamb growth model that predicted carcass and visceral compositional changes would better account for nutritional history thus improving the prediction of animal growth from nutrient supply. The CT scan data was then used to compare two lamb growth models.
The model that separated carcass and visceral components did not improve prediction accuracy. Both growth models substantially underestimated fat gain for the high feeding level at the later stage of maturity. The comparison of models identified specific areas to advance our understanding of ruminant responses to a changing feed supply.
The hypothesis that nutrient intake is the predominant constraint on maximising lamb growth rates in intensive feeding systems was supported by the results of this research. The nutritional management of the adaptation period is likely a key to improving production. The CT scanning of lambs demonstrated its advantages as a non-destructive method to analyse body compositional changes and provides an opportunity for researchers to conduct experiments with greater precision. It is recommended that CT scanning is broadly used to test and improve the precision of predictive growth models.