Effects of feed intake and genetics on tissue nitrogen-15 enrichment and feed conversion efficiency in sheep

This study investigated the effects of sheep genetics and feed intake on nitrogen isotopic fractionation (Δ15N) and feed conversion efficiency (FCE; live weight gain/DMI), using a 2 × 2 factorial design, with 2 levels of genetic merit for growth (high vs. low) and 2 levels of feed intake (110 vs. 170% of ME for maintenance [MEm]). No effect of genetic merit was detected for live weight gain (P = 0.64), FCE (P = 0.46), plasma urea nitrogen (P = 0.52), plasma glucose (P = 0.78), and Δ15N of wool (P = 0.45), blood (P = 0.09), and plasma (P = 0.51). Sheep receiving 170% of MEm had 175% higher live weight gain (P < 0.001) and 77% higher FCE (P < 0.001) than sheep receiving 110% of MEm. There was no difference among treatments at the beginning of the study for either blood or plasma Δ15N, but the treatment groups started to diverge in blood and plasma Δ15N at 21 and 7 d, respectively. Blood, plasma, and wool samples were enriched in 15N compared with feed. There was a higher blood, plasma, and wool Δ15N for the low feed intake group than the high feed intake group (P < 0.001 in all cases). Across the 4 treatment groups, higher FCE in sheep was associated with lower Δ15N for plasma, blood, and wool. Overall, the results are consistent with the potential of Δ15N as a rapid, lowcost biomarker of FCE in sheep, despite there being no effects of genetic treatment on FCE and Δ15N.