Phenotypic correlations among alternative definitions of feed efficiency in grazing lactating dairy cows across parities and lactation stages

摘要

The objective of this study was to estimate correlations among alternative measures of energy conversion efficiency and energy balance (EB) across parities and lactation stages in grazing lactating dairy cows. Individual measurements of net energy intake (NEI) (n=8,183) from pasture and concentrate from 2,693 lactations on 1,412 cows were available. Residual energy production (REP) was defined as net energy of lactation (NEL) minus predicted energy requirements; energy conversion efficiency (ECE) wasdefined as NEL divided by NEI; and EB was defined as the difference between NEI and energy required for both maintenance and lactation. Residual energy intake (REI) was defined as NEI minus predicted energy requirements. A strong correlation existed between REI and EB (0.88; P<0.05) for parity three or greater early lactation, weakening in parity one (0.70; P<0.05). These differences in correlations between REI and EB for animals across parities in early lactation, indicated primiparous animals were using body energy for milk production and growth; nevertheless, these strong correlations suggested negative REI animals (i.e., more efficient) are also in more-negative EB. Our findings suggest REI could be a valuable tool for future genetic improvement,however, selection on REI could have detrimental effects on reproduction and health traits, thus, further research is warranted.