Evaluation of supplemental feeding behavior in Katahdin lambs divergently selected for fecal egg count estimated breeding value

Genetic selection and nutritional supplementation are potential methods to mitigate the impact of parasitism in sheep. Understanding feeding behavior of growing lambs divergently selected for parasite resistance (LowFEC) or susceptibility (HighFEC) using the fecal egg count (FEC) estimated breeding value (EBV) may help quantify the interaction of genetic selection and supplemental feeding on parasitism. Over two years [Year 1 (Y1) and Year 2 (Y2)], LowFEC Katahdin ewes (Y1: n = 52; Y2: n = 40) were randomly bred to LowFEC Katahdin rams (Y1: n = 3; Y2: n = 3) and HighFEC Katahdin ewes (Y1: n = 48; Y2: n = 50) were randomly bred to HighFEC Katahdin rams (Y1: n = 3; Y2: n = 3). Prior to weaning (Y1: 76 d; Y2: 92 d), pairs were managed as one contemporary group on fescue-based pasture. At weaning, a subset of LowFEC (Y1: n = 27; Y2: n = 37) and HighFEC (Y1: n = 24; Y2: n = 40) lambs were randomly assigned to a dry-lot feeding trial. During this time, lambs received ad libitum supplementation of a concentrate pellet (16% CP, 76% TDN) via the SmartFeeder (SSFeeder). Harvested mix-grass forages were also provided ad libitum. Throughout the feeding trial in both years, frequency of SSFeeder visits and daily supplemental intake were collected via the SSFeeder and supplemental feed:gain (S:G) was calculated. Fecal egg counts, body weights, average daily gain (ADG), and FAMACHA scores were collected biweekly. Fecal egg counts and FAMACHA scores were collected to verify reduced parasite burden on dry-lot after being managed on pasture pre-weaning. In Y1, LowFEC lambs tended to have greater daily supplemental feed intakes ( P = 0.06), while no differences existed between genotypes in Y2. No differences existed between genotypes for number of daily SSFeeder visits or S:G in either year. In Y2, LowFEC lambs had greater ADG than HighFEC lambs ( P < 0.05). In both years, LowFEC lambs had lower FECs compared to HighFEC lambs ( P = 0.05). These results indicate that selection for LowFEC EBV does improve parasite resistance and FEC EBV had inconsistent effects on supplemental feed intake and growth. • Genetic selection improves parasite resistance by reducing fecal egg counts. • There were no growth performance differences between resistant or susceptible lambs. • Feeding behavior and growth is not impacted by parasite resistance or susceptibility.