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 ∆15N, but the treatment groups started to diverge in blood and plasma ∆15N 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 ∆15N 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 ∆15N for plasma, blood, and wool. Overall, the results are consistent with the potential of ∆15N as a rapid, lowcost biomarker of FCE in sheep, despite there being no effects of genetic treatment on FCE and ∆15N