Impact of feed efficiency on young beef animals on adult resilience: From field challenge experiments to modelling approaches

Abstract

The relationship between feed efficiency at young ages and resilience as cows has been examined using an experiment that combines residual feed intake (RFI) evaluation of beef heifers and subsequent dietary challenge as cows. Feed efficiency of 569 heifers of 22 months of age was estimated during a 70-day period. Then a dietary challenge was applied on 303 of these animals after their first calving, half of them being fed 30 % below their expected needs from 10 days after calving up to the start of the grazing period (around mid-April), after which all the animals received the same management. When possible, the animals were kept until third lactation, giving a total of 514 lactations. Dam weight, dam BCS and calf weight were measured regularly, dam milk production was estimated three times during the lactation and resumption of cyclicity was recorded as well. The experimental data were analyzed using linear models including the effects of heifer efficiency (RFI <-0.4 = efficient; RFI >0.4 = inefficient) and interaction of heifer efficiency and diet group. The data were also used to calibrate a herd dynamics model representing management-nutrition and reproduction interactions at animal level. The experiment data showed that the efficient heifers were 18 kg heavier as cows when fed ad libitum (P = 0.04) but no difference was observed when restricted. Within both diets, heifers classified as efficient produced between 6 and 12 % less milk as cows than inefficient heifers depending on the lactation stage (P < 0.01), with a subsequent effect on calf weight. Finally, the resumption of cyclicity was one week shorter for the efficient animals when fed ad libitum but the opposite was observed within the restricted group, representing a complete ovarian cycle difference between diet groups within the efficient class (P = 0.001). In the lifetime modelling approach, two clusters of animals with different characteristics, including RFI, were simulated under both a non-restricted and restricted feeding environment. In this simulation, efficient heifers produced lighter cows with lower intake and higher body reserves from second calving until culling. Non-efficient heifers had better productive (kg of weaned calf) performance as cows independently of environment. The simulated reproductive performance of cows was equal for animals coming from efficient and non-efficient heifers under the non-restricted environment whereas efficient heifers perform better in the restricted environment, especially when parity increases. Both approaches showed that the interplay between efficiency at young ages and short- and long-term resilience depends on the environment. It can be concluded that the selection for efficiency at young ages has a negative impact on lactational performance as cows but not necessarily on reproductive resilience.