Genetic improvement of feed efficiency of beef cattle: what lessons can be learnt from other species?

10.1071/EA02111 About half of the feed in beef production systems is used to maintain the breeding herd. Of the remaining 50%, about 20% is used by the breeding cow for pregnancy and lactation and 30% is used by the growing calf. There seems little opportunity to improve efficiency of production (lactation and growth) but reasonable opportunity to increase maintenance efficiency. The mean heritability of net feed efficiency from 35 estimates across 7 species/types was 0.25 ± 0.02. Unfortunately, the genetic correlation between net feed efficiency in young bulls and mature cows is likely to be significantly less than 1, thus responding slowly to selection. While it seems clear that improvements in maintenance efficiency can be made, they may come at a cost. In poultry and mice, increased net feed efficiency has been associated with increased fatness but in pigs and beef cattle there is some evidence of the reverse. While a number of studies have predicted that selection for efficiency may result in lower proportions of crucial, metabolically active visceral organ tissues, there are few studies to support this. Poultry and mouse studies both clearly show that increased net feed efficiency is associated with decreased heat production. This decrease is due to both decreased cost of digestion (heat loss) and decreased activity. In poultry, this decreased heat loss resulted in the more efficient line being more affected by high temperatures. Also, it is possible that at low temperatures, the differences in efficiency may not exist. At this stage it is not clear if the decreased activity is an adaptive advantage or disadvantage. In mice and possibly also pigs and poultry, increased net feed efficiency has been associated with a decreased reproductive rate measured as litter size or egg number. This should definitely be of concern when embarking on selection for improved net feed efficiency in the beef industry. It is recommended that since feed intake is both difficult and expensive to measure, a gene or marker test should be developed, and correlated effects on a range of production traits be carefully evaluated