Intake and digestibility of meat ewes belonging to two contrasting feed efficiency genetic lines, during their two first production cycles

Feed is the largest cost in animal production, therefore improving feed efficiency (FE) is one of the main priorities when redesigning farming systems. The objective of this work was to evaluate the effects of a divergent genetic selection for FE in Romane rams [i.e. efficient (RFI-) or inefficient (RFI+)] on the feed intake and nutrient digestibility of their daughters (ewes), reared in different conditions. Different to their sires, which were fed high concentrate-diets, the ewes were fed ad libitum total mixed ration (TMR) composed of silage (65%), hay (25%) and barley (10%). A series of in vivo digestibility trials were conducted using Romane ewes (n =20; 10 RFI- and 10 RFI+) during their 2 first production cycles. The BW, metabolic BW (MBW), body condition score (BCS), DM intake (DMI) and nutrient intake and digestibility were individually monitored. All parameters were significantly affected (P < 0.05) by the production cycle. The BW, MBW, BCS as well as the feed intake and nutrient digestibility were higher in the cycle 2 compared to cycle 1. The exception was the CP digestibility, which was higher in younger ewes (cycle 1). The BW and MBW was, as expected, higher in older ewes thus increasing the feed intake capacity in cycle 2 compared with cycle 1. Slight differences in TMR nutritive composition between the years of the study were also probably responsible of differences in CP digestibility. The RFI+ ewes showed a better BCS than RFI- ewes (3.0 vs. 2.7; P < 0.0001). In the conditions used in this study, we could not demonstrate any differences in intake and digestive efficiency between the two ewe FE lines, probably because the selected rams were tested with one diet (concentrate-based diet) whereas their daughters (ewes) with another (forage-based diet). Further research is warranted to evaluate RFI testing in animals fed a more fibrous diet closer to that of ewes under range conditions whereas also assessing responses to multiple-generation divergent selection on RFI.