Increasing ewe genetic fecundity improves whole-farm production and reduces greenhouse gas emissions intensities 1. Sheep production and emissions intensities

Greenhouse gas (GHG) emissions from livestock constitute the largest proportion of Australian agricultural GHG emissions, necessitating development of strategies for mitigating GHG emissions from the livestock sector. Here we simulate a self-replacing prime lamb enterprise to examine the effect of increasing ewe genetic fecundity on whole farm GHG emissions, animal production and emissions per animal product (emissions intensity; EI).

Breeding ewes were a cross-bred genotype containing the Booroola (FecB) gene with average lambing rates of 1.5–2.0 lambs per ewe. Lambs were born in winter on pastures of phalaris, cocksfoot and subterranean clover, and were sold at the beginning of summer. Flock dynamics were simulated using the model GrassGro and whole-farm GHG emissions were computed using equations from the Australian National Greenhouse Gas Inventory.

Increasing ewe fecundity from a baseline of 0.96–1.54 lambs per ewe reduced EI from 9.3 to 7.3 t CO₂-e/t clean fleece weight plus liveweight (CFW + LWT) and GHG emissions per animal sold by 32%. Increasing fecundity reduced lamb sale liveweights and increased lamb mortality rates at birth, but this was offset by an increase in total liveweight turnoff. Greater ewe fecundity increased whole-farm productivity without increasing GHG emissions. For the same stocking rate as an enterprise running genotypes with lower fecundity, high fecundity genotypes either increased annual production from 449 to 571 kg CFW + LWT/ha with little change in net emissions, or reduced emissions from 4.2 to 3.2 t CO₂-e/ha for similar average productivity. In both cases, EI decreased by ca. 2.1 t CO₂-e/t CFW + LWT.

A foremost advantage of using high fecundity breeds is greater intra-annual variation in flock number, because such genotypes give birth to more lambs. This necessitates a reduction in the number of adult breeding ewes to maintain average annual stocking rate and benefits whole farm emissions, because breeding ewes contribute the largest proportion of farm emissions (77–80%), particularly enteric methane. We conclude that increasing ewe fecundity offers a win–win opportunity for the sheep industry by allowing sustainable intensification through greater production and lower emissions intensity, without adversely affecting net farm emissions or increasing stocking rate. High fecundity genotypes also present an opportunity for sheep producers to reduce stocking rates while maintaining current levels of farm production, thereby reducing labour and flock nutritional requirements.