374 Birth Season and Production System Effects on Post-Weaning Growth and Gastrointestinal Nematode Infection of Katahdin Lambs

Abstract The objective was to estimate birth season (F = fall, W = winter) and production system (C = conventional, O = organic) effects on Katahdin lamb growth and gastrointestinal nematode (GIN) infection. Lambs were weighed and weaned at ~60 d of age and body weight (BW, n = 5,089), fecal egg count (FEC, n = 4,772), and packed cell volume (PCV, n = 4,962) were recorded at ~90, 120, and 150 d of age. Records were analyzed from 599, 142, 1,113, and 126 F-C, F-O, W-C, and W-O lambs, respectively. Lambs were not dewormed and FEC was transformed as: LFEC = log10(FEC + 25). Traits were analyzed as repeated measures with fixed effects of time, sex, birth/rear type, and season-system, the linear covariate of actual age at 90 d, and random effects of sire, birth year, and contemporary group. The time x season-system interaction was significant for all traits (P < 0.001; Figure 1). At 90 d, BW was greater for W-C and W-O than F-C and F-O (P ≤ 0.05). By 150 d, W-O lambs had greater BW (P ≤ 0.04) than all other season-systems which weren’t different from each other. Lamb LFEC was greatest for F-C (P < 0.001), intermediate for F-O, and least for W-C and W-O at 90 d. All season-systems had similar LFEC at 120 d, but W-C lambs had greater LFEC at 150 d than all others (P < 0.01). At 90 d, PCV was less for F-C than all others (P ≤ 0.02) except W-O. Both F-C and F-O had greater PCV (P < 0.001) than W-C and W-O at 120 d and, at 150 d, PCV was less for W-C than all others (P < 0.001). To investigate production impacts of GIN infection in different season-systems, average daily gain from weaning to 90 d (ADG60-90, n = 1,842) was analyzed in a similar model with the additional linear covariate of LFEC at 90 d (LFEC90). The season-system x LFEC90 interaction was significant (P < 0.01), indicating differing impacts of GIN infection. A one-unit increase in LFEC90 (e.g., ~500 vs. 5000 eggs/g) had negative impacts on ADG60-90 for W-O, F-C, and W-C (-27.1, -22.4, and -12.5 g/d, respectively) and solutions were not different from one another. However, LFEC90 did not affect F-O ADG60-90 potentially due to decreased performance (158.4 g/d) than other season-systems (171.5 – 204.3 g/d). Overall, BW was less for F than W lambs and FEC decreased with age for F with the opposite for W lambs. Slight advantages in performance were observed for W-O compared with W-C lambs. Season-system differences could be attributed to varied forage quality and breeding objectives and results utilized in