Effect of sire fecal egg count estimated breeding value on parasite resistance traits in Haemonchus contortus infected Katahdin lambs

In the midst of anthelmintic resistance, genetic resistance to gastrointestinal nematodes (GIN) in lambs could reduce anthelmintic requirements; therefore, the fecal egg count (FEC) estimated breeding value (EBV) was developed as a measure of genetic merit for parasite burden. One of the first breeds to effectively implement the FEC EBV was Katahdin. To better understand the relationship between the FEC EBV and Haemonchus contortus (Hc) challenge infection, a divergent mating scheme was established with extremely high (High FEC, n = 5) or extremely low (Low FEC, n = 5) FEC EBV Katahdin rams over 2 years. Purebred lambs sired by these rams were born beginning in mid-March and managed on pasture until approximately 120 days of age. A primary infection was established based on FEC during this period. At this point, lambs (n = 109 in Year 1, n = 114 in Year 2) were removed from pasture, treated with an anthelmintic to reduce FEC and transported to the Animal Sciences Farm at West Virginia University. After a rest period, lambs were given 10,000 (Year 1) or 5000 (Year 2) Hc L3. Body weights, FEC, and packed cell volume were collected weekly. Lambs were harvested (n = 60/year) at 5 weeks post-infection. Abomasum worm counts were determined, and worm length was measured using Image J. Sta-tistical analysis was performed by year using the Mixed Model procedure of SAS (SAS Institute, Cary, NC) with fixed effects of sire EBV type. Change in FEC after the prepatent period was greater in Year 1 High FEC-sired lambs compared to Low FEC-sired lambs (210 vs. 34 eggs/g, respectively; P < 0.05). At harvest in Year 2, a greater proportion of Low FEC-than High FEC-sired lambs had worm counts of zero (P < 0.05). Worm fecundity was lower in lambs sired by Low FEC rams (P < 0.05). Taken together, sire selection for low FEC EBV will lower FEC and worm count and improve GIN resistance in progeny.