Fattening lambs with divergent residual feed intakes and weight gains: Unravelling mechanisms driving feed efficiency

Abstract

Important achievements have been made to improve feed efficiency of farm livestock, but molecular mechanisms determining feed utilization remain unclear. The objective of this work was to identify the reasons behind divergent feed efficiencies in fattening lambs. Twenty newborn male Assaf lambs were fed milk replacer (MR) ad libitum using an automatic feeder, and weaned progressively during the last 7 days of the artificial suckling period (50 days in total). Then, all the lambs were offered ad libitum a complete pelleted diet during the fattening phase. During the fattening period, lambs were weighed twice weekly and daily dry matter intake was measured. These data were used to estimate residual feed intake (RFI), residual weight gain (RWG), and residual intake and body weight gain (RIG). Animals were ranked according to RIG, and then assigned to two experimental groups (n = 10 per treatment): group EFFI including the most efficient lambs (with negative values for RIG) and group NEFFI with the least efficient lambs (positive values for RIG). The most relevant results revealed that divergent values for feed efficiency traits in these fattening lambs were not associated to differences in digestibility or ruminal fermentation parameters (P > 0.05). However, the NEFFI lambs presented lower concentration of glucose (95.4 vs. 88.2 mg/dL; P < 0.05) and cholesterol (62.8 vs. 55.1 mg/dL; P < 0.05) in the serum. These effects might be related to the lower gene expression of enzymes involved in key metabolic pathways in the liver (e.g., PDHA1OA, CYP3A4). Moreover, bone formation and growth in NEFFI lambs also might have been compromised by a lower expression of genes involved in protein synthesis, such as those encoding ribosomal proteins (e.g., RPL22L1), these results being aligned with the lower albumin concentration also observed in the serum of these animals (35.56 vs. 33.89 g/L; P < 0.05). A pro-inflammatory profile at both locations, liver and ileum, having the last one a down regulation of genes encoding anti-inflammatory cytokines involved in the innate immune response (e.g., IL-4 and TGF-β) can also explain, at least partially, an increased expenditure of energy and protein redirected from growth and protein accretion towards the alleviation of systemic inflammation.