The genetic and biological basis of feed efficiency in mid-lactation Holstein dairy cows.

Improvement in feed efficiency in dairy cattle is important as it results in economic benefits through reduced inputs for equivalent output, as feed represents more than 50% of the total cost of producing milk. Further benefits are reduced greenhouse gas emissions and less land and resources needed for the production of feed. Over the past 100 years, cows have become more feed efficient largely through increases in milk production, thereby diluting the proportion of feed used for maintenance. However, because this effect diminishes with each successive increment in production relative to body size and input costs often increase more than milk prices, continued gains via this route are diminishing. This, therefore, warrants studies to understand the genetic basis of feed utilization in lactating dairy cattle. A study by Dr L.C. Hardie and colleagues showed significant progress in this regard. The title of their study, which was published in the Journal of Dairy Science, Volume 100 of 2017, pages 9061 to 9075, was: The genetic and biological basis of feed efficiency in mid-lactation Holstein dairy cows.

The objective of their study was to identify genomic regions and candidate genes associated with feed efficiency in lactating Holstein cows. In total, 4 916 cows with actual or imputed genotypes for 60 671 single nucleotide polymorphisms having individual feed intake, milk yield, milk composition and body weight records were used in this study. Cows were from research herds located in the United States, Canada, the Netherlands and the United Kingdom. Feed efficiency, defined as residual feed intake (RFI), was calculated within location as the residual of the regression of dry matter intake (DMI) on milk energy (MilkE), metabolic body weight (MBW), change in body weight, and systematic effects. For RFI, DMI, MilkE, and MBW, bivariate analyses were performed considering each trait as a separate trait within lactation group to estimate variance components and genetic correlations between them. Animal relationships were established using a genomic relationship matrix. Genome-wide association studies were performed separately by parity group for RFI, DMI, MilkE and MBW using the Bayes B method with a prior assumption that 1% of single nucleotide polymorphisms have a nonzero effect. One-megabase (1-Mb) windows with greatest percentage of the total genetic variation explained by the markers (TGVM) were identified, and adjacent windows with a large proportion of the TGVM were combined and reanalyzed.

Heritability estimates for RFI were 0.14 ± 0.03 in first lactation cows and 0.13 ± 0.03 in multi-lactation cows. Genetic correlations between first and multi-lactation cows were 0.76 for RFI, 0.78 for DMI, 0.92 for MBW and 0.61 for MilkE suggesting moderate carry-over effects. No single 1-Mb window explained a significant proportion of the TGVM for RFI. However, after combining windows, significance was met on Bos Taurus autosome 27 in first-lactation cows, and significance was nearly reached on Bos taurus autosome 4 in multi-lactation cows. Among other genes, these regions contain β-3 adrenergic receptors and the physiological candidate gene, leptin, respectively. Between the second lactation groups, three of the 10 windows with the largest effects on DMI were neighbouring windows affecting RFI, but these were not in the top 10 regions for MilkE or MBW. Nevertheless, two noteworthy quantitative trait loci (QTL) were identified: in first lactation cows, a significant QTL was identified for RFI on BTA 27 that harbours the positional candidate gene ADRB3, and the region of BTA 4 that harbours the gene encoding leptin was identified as a region of interest for RFI and DMI in multi-lactation cows.

In conclusion, the results suggest: (1) a genetic basis for feed intake that is unrelated to energy consumption required for milk production or expected maintenance as determined by MBW; (2) feed efficiency measured as RFI is a multi-genetic trait with no individual region explaining large proportions of the total genetic variation; (3) the genetic basis of the traits is not static throughout the life of the dairy cow, as indicated by moderate genetic correlations between first lactation and multi-lactation cows. Overall, the results illustrate the physiological complexity which underlies the genetic regulation of feed efficiency in lactating dairy cattle.