Interactions of fecal egg count estimated breeding value and housing system on Katahdin lamb post-weaning performance

Genetic selection based on fecal egg count estimated breeding values (FEC EBV) and controlled environmental systems (confinement housing, nutritional supplementation, etc.) are potential methods to mitigate the impact of parasite burden in lambs. The objective here was to evaluate the result of divergent selection for parasite resistance (LowFEC vs. HighFEC) based on sire’s and dam’s EBVs on performance in two different management systems (confinement vs. pasture). Over two years [Year 1 (Y1) and Year 2 (Y2)], LowFEC EBV Katahdin ewes (Y1: n = 52; Y2: n = 40) were randomly bred to LowFEC EBV Katahdin rams (Y1: n = 3; Y2: n = 3) at the Virginia Tech Southwest Agricultural Research and Extension Center. HighFEC EBV Katahdin ewes (Y1: n = 48; Y2: n = 50) were randomly bred to HighFEC EBV Katahdin rams (Y1: n = 3; Y2: n = 3). Ewes across both genotypes in each year ranged from 2 to 7 years in age. Rams across both genotypes in each year ranged from 2 to 8 years in age. For genetic connectedness between years, one LowFEC sire and two HighFEC sires from Y1 were utilized in Y2. Prior to weaning (Y1: 76 d; Y2: 92 d), pairs of ewes and their lambs were managed on fescue-based pasture. After weaning, a subset of the lamb crop were randomly assigned to a dry-lot (Y1: LowFEC: n = 27, HighFEC: n = 24; Y2: LowFEC: n = 37, HighFEC: n = 40) or pasture (Y1: LowFEC: n = 26, HighFEC: n = 25; Y2: LowFEC: n = 37; HighFEC: n = 40) group. The dry-lot group received ad libitum supplementation of a concentrate pellet (16% CP, 76% TDN) via the C-Lock, Inc Super SmartFeeder (SSFeeder) while the pasture group was supplemented 2% body weight via standard trough feeders. After 63 days in Y1 and 71 days in Y2, lamb eye muscle depth (EMD) was determined via ultrasound. Throughout the feeding trial, body weights, ADG, FEC, packed cell volume (PCV), and FAMACHA scores were collected approximately every 14 d. Statistical analyses were conducted using SAS with Proc MIXED with fixed effects of genotype (LowFEC vs. HighFEC), housing, sex, and time within year (the repeated factor). In Y2, LowFEC lambs had greater ADG than HighFEC lambs ( P < 0.05) and dry-lot housed lambs had greater ADG than pasture raised lambs ( P < 0.01). As anticipated, dry-lot lambs had greater EMD than pasture raised lambs in both years ( P < 0.01). In both years, LowFEC EBV lambs had lower FECs compared to HighFEC EBV lambs ( P = 0.05), and dry-lot lambs had lower FECs compared to pasture lambs ( P < 0.01). No differences existed in the percentage of lambs dewormed across housing or genotypes. In Y2, approximately 24% more lambs required deworming in the LowFEC pasture group than the LowFEC dry-lot group ( P < 0.05) and 48% more lambs were dewormed in the HighFEC pasture group compared to the HighFEC dry-lot group ( P < 0.05). Improved survivability, health, and growth performance resulted in a greater number of dry-lot lambs (23% more) to market at a greater weight. Survivability in each housing group was measured by the percentage of lambs successfully surviving to the end of the trial. These results indicate the potential of management systems to help mitigate genetic susceptibility to parasitism allowing improved health status and increased growth during the post-weaning period. • Dry-lot lambs have improved production regardless of genetic parasite selection. • Higher conception rates in dry-lot ewes may be attributed to increased health. • Increased dry-lot input costs will likely be recovered with increased revenues.