Effects of Genomic Residual Feed Intake on Performance, Feed Efficiency and Greenhouse Gas Emissions in Holstein Heifers.

The objective of this study was to examine the differences in performance, feed efficiency, and greenhouse gas emissions in Holstein heifers with divergent genomic residual feed intake (RFIg). Holstein heifers (n = 55; initial BW = 351 ± 64 kg) were classified as either low RFIg (n = 29) or high RFIg (n = 26) based on RFIg values determined from their genomic breeding values (Ecofeed index, STgenetics). Heifers were blocked by RFIg, stratified by BW and assigned to 1 of 2 pens, each equipped with 4 electronic feedbunks (GrowSafe Systems), with 1 pen equipped with a GreenFeed realtime gaseous-exchange measurement system (GEM; C-Lock). Heifers were fed a corn-silage based total mixed ration twice daily. Individual dry matter intake (DMI) was collected for 84 d and BW measured weekly. Phenotypic residual feed intake (RFIp) was calculated as the residual from the regression of DMI on average daily gain (ADG) and mid-test BW0.75. As only 1 of the 2 pens had a GEM system, during the 84-day study heifers were rotated between the two pens at 14 day intervals resulting in each group of heifers having three 14-day gaseous exchange measurement periods. A time series analysis of gaseous exchange measurements, with a repeated measure of measurement period, fixed effect of RFIg class and random effect of pen was conducted. A mixed model with RFIg class as fixed effect and pen as a random effect was used for analysis of the other response variables. There were no differences in initial or final BW and ADG for heifers with divergent RFIg; however, the low RFIg heifers consumed 7.3% less (P < 0.05) feed per day than their high RFIg counterparts. Consequently, low RFIg heifers exhibited a more favorable (P < 0.05) RFIp, with a RFIp of -0.188 and 0.211 kg/d for low and high RFIg heifers, respectively. There were no RFIg class by measurement period interactions (P > 0.10) for the greenhouse gas emissions measured. Low RFIg heifers had 7.7% less (P < 0.05) methane emissions (g/d) and 6.1% less (P < 0.05) carbon dioxide production (g/d) than their high RFIg counterparts. However, methane yield (g methane/kg DMI) was similar (P > 0.05) for low and high RFIg heifers. These results indicate that selection for low RFIg has the potential to identify cattle with favorable feed efficiency phenotypes. Additionally, selection based on RFIg provides the opportunity to produce cattle with fewer greenhouse gas emissions. Further research is warranted into selection based on RFIg through genomic indexes to improve feed efficiency and mitigate the environmental impact of cattle production. [ABSTRACT FROM AUTHOR]