The genetic relationship between feed efficiency and host resistance to parasites: insights from experimental infections in ewe lambs from divergent lines.

Breeding for disease resistance or for feed efficiency can support sheep farm strategies to reduce reliance on feed or medical inputs. However, genetic improvement in those two types of traits may be hampered by trade-offs. Based on the resource allocation theory, immunity-conferring host resistance may use feed nutrients at the expense of production processes and then lead to reduced feed efficiency. Conversely, a high feed efficiency may reduce the nutrient budget for immunity and then lead to increased susceptibility. Finally, direct infection costs due to high susceptibility should compromise feed efficiency during infection. We investigated this theory using the same standardised protocol of artificial infection with the gastrointestinal parasite Haemonchus contortus in two independent experiments. The first experiment (Exp. 1) included ewe lambs from lines divergently selected for residual feed intake (RFI), whereas the second (Exp. 2) included ewe lambs from divergent lines on host resistance. In each experiment, about two-thirds of the animals were infected while the others were kept uninfected as controls. Our results were consistent with each divergent selection: in the control treatment of Exp. 1, the efficient line (LRFI) had a lower RFI than the inefficient line (HRFI), while in the infection treatment of Exp. 2, the resistant line (R) had a lower parasite faecal egg count than the susceptible line (S). However, our results were inconclusive regarding the expected trade-off, as neither comparison of host resistance between infected ewe lambs from HRFI and LRFI lines, nor of RFI between controls from the R and S lines, revealed a consistent pattern. In both experiments, evidence for a deterioration in the feed efficiency of infected animals relative to controls was also inconclusive. Our findings do not constitute evidence for the absence of a genetic relationship between feed efficiency and host resistance to parasites, but it provides effect sizes for planning future studies of the trade-off between those traits. The challenge conditions required to detect trade-offs and simultaneously measure feed efficiency warrant further investigation.