Energy efficiency, reproductive performance, and metabolic parameters of grazing Hereford heifers

Abstract

Feed and energy efficiency have been topics of extensive research in recent years, with residual feed intake (RFI) being the preferred index to determine feed efficiency in beef cattle. Nevertheless, the biggest downside of RFI is that its measurement requires accurately determining individual feed intake, which leads to the need to carry out RFI trials in confinement conditions. In this context, residual heat production (RHP), defined as the difference between observed and predicted heat production, presents an alternative to determine efficiency in grazing conditions as it is independent of feed intake. This work aimed to evaluate the repeatability of RHP and study its relationship with metabolic, endocrine, and reproductive variables. Seventy-four Hereford heifers were used, and at the beginning of the experimental period, heifers were 18 ± 0.7-month-old and weighed on average 278 ± 26 kg. Heat production (HP) was determined at the beginning (March to May; fall) and at the end (September to November; spring) of the experiment using the heart rate-O2 pulse technique to classify heifers according with the residual heat production (RHP). Efficient heifers showed lower HP (P < 0.01), ME (P < 0.01), and DM intake (P < 0.01) without differences in retained energy (RE; only in spring). During fall, average daily gain (ADG) was not included in the estimated HP model; therefore, although, as in RFI, RHP should be independent of BW and ADG, a trend for greater ADG and RE was found for efficient when compared with inefficient heifers. Decreased maintenance requirements (NEm and MEm) as well as higher partial efficiency of use of consumed ME (k) were estimated for more efficient heifers (low RHP; HH), while efficient heifers showed an earlier luteal phase and a shorter service-conception interval and calved earlier in the season, probably associated to an earlier development of fat tissue. Excluding glucose, no major differences were found in the plasma metabolites between groups. However, high-efficiency animals showed positive correlations with cholesterol and negative ones with NEFA plasma concentrations during fall.