Applying assisted reproductive technology and reproductive management to reduce CO 2 -equivalent emission in dairy and beef cattle: a review.

Methane emission from beef and dairy cattle combined contributes around 4.5-5.0% of total anthropogenic global methane. In addition to enteric methane (CH 4 ) produced by the rumen, cattle production also contributes carbon dioxide (CO 2 ) (feed), nitrous oxide (N 2 O) (feed production, manure) and other CH 4 (manure) to the total greenhouse gas (GHG) budget of beef and dairy production systems. The relative contribution in standard dairy systems is typically enteric CH 4 58%, feed 29% and manure 10%. Herds with low production efficiency can have an enteric CH 4 contribution up to 90%. Digestibility of feed can impact CH 4 emission intensity. Low fertility herds also have a greater enteric CH 4 contribution. Animals with good feed conversion efficiency have a lower emission intensity of CH 4 /kg of meat or milk. Feed efficient heifers tend to be lean and have delayed puberty. Fertility is a major driver of profit in both beef and dairy cattle, and it is highly important to apply multi-trait selection when shifting herds towards improved efficiency and reduced CH 4 . Single nucleotide polymorphisms (SNPs) have been identified for feed efficiency in cattle and are used in genomic selection. SNPs can be utilized in artificial insemination and embryo transfer to increase the proportion of cattle that have the attributes of efficiency, fertility and reduced enteric CH 4 . Prepubertal heifers genomically selected for favourable traits can have oocytes recovered to produce IVF embryos. Reproductive technology is predicted to be increasingly adopted to reduce generation interval and accelerate the rate of genetic gain for efficiency, fertility and low CH 4 in cattle. The relatively high contribution of cattle to anthropogenic global methane has focussed attention on strategies to reduce enteric CH 4 without compromising efficiency and fertility. Assisted reproductive technology has an important role in achieving the goal of multiplying and distributing cattle that have good efficiency, fertility and low CH 4 .