Variation in Grazing Intake of Prime Lamb Dam Genotypes

Summary

The feed cost for maintenance of the ewe flock and replacements accounts for over 65% of the feed required for production of lamb carcass weight. A reduction in this feed requirement through genetic improvement of efficiency of feed utilisation would have a dramatic impact on carrying capacity and both biological and economic efficiency in sheep flocks. 
There is currently no information on the genetic variation in feed requirements for maintenance or the relative feed efficiency among meat sheep. To include feed intake or efficiency in the breeding objective for meat sheep, reliable estimates of its heritability and genetic correlations with other production traits are required. These genetic parameters would allow estimated breeding values (EBVs) for feed intake to be calculated by Sheep Genetics Australia (SGA) and included in breeding programs. 
The project aimed to estimate feed intake at grazing pasture for mature ewes in the MCPT project (LAMB.325A) and estimate heritability and genetic correlations between feed intake and production (growth, carcass, meat quality, wool and reproduction) traits for maternal genotypes. These genetic parameters are required to allow breeders to include reduction in feed requirements for maintenance as an objective in their breeding programs. 
The project has demonstrated there is considerable genetic variation for intake for maintenance among maternal meatsheep genotypes. The high estimated heritability (0.41 - 0.07) for intake together with the large range in sire progeny means indicates potential for rapid progress from selection. However intake at maintenance, especially under grazing, is very difficult and expensive to measure with current technology and requires large numbers of progeny per sire to provide accurate EBVs. The genetic correlations with a range of production traits were all close to zero. Hence the use of other production traits for indirect selection to reduce maintenance requirements is not feasible. 
However on the positive side it also means that breeding programs aimed at improving production traits such as liveweight, carcass or reproduction, will not be antagonistic and will have little effect on intake, other than through the direct effect of increasing liveweight. The project has established the potential for genetic improvement of feed efficiency and other modelling research has shown it can have a high impact on sheep enterprise profitability. The outcomes stress the importance of pursuing new technologies for measuring feed intake directly and/or determining the relationships with underlying physiological traits and use of genomic technologies. 
Recommendations include: 
- The genetic parameter information obtained from the project be made available to SGA. 
- The proposed studies to investigate the relationship between IGF-1 and intake among a sample of the MCPT ewes should be pursued. 
- The development of alternative technology for measuring feed intake of grazing animals be supported as well as the pursuit of physiological and genomic markers.