The objective was to examine GIN infection and BW of lambs born in fall (Oct–Nov; n = 20-158/yr; n = 716 total) or winter (Jan–Feb; n = 65-208/yr; n = 1717 total). Katahdin lambs born between 2008 and 2020 were weaned between 90–120 d of age and supplemented with a 15% CP grain mix when forage quality was low. Dams were supplemented up to 30 d before to 60 d post-lambing. BW and FAMACHA scores were determined at approximately 60, 90, 120, and 150 d of age; FEC and PCV were determined at approximately 90, 120, and 150 d of age. Lambs were dewormed if FAMACHA score was > 3. Data were analyzed using mixed models with a repeated statement for day of sample collection, and age within season-yr used as a covariate. Season × yr × day influenced all traits (P < 0.001); litter number, sex, age of dam influenced BW (P < 0.001). FEC were higher in fall than winter born lambs only on d 90, and similar on d 120 and 150 (P < 0.001). PCV was lower in winter than fall born lambs on d 120 and 150 (P < 0.001), and FAMACHA lower on all days measured in fall than winter lambs (P < 0.001) making the winter born more susceptible to anemia with a greater need for deworming. Haemonchus contortus was the predominant GIN on most days of culture for both seasons. Fall compared with winter lambs were lighter in BW at all time points (P < 0.001) possibly due to poor forage quality available to dams. Rainfall data will be presented. GIN control was more manageable in fall than winter born lambs, but they weighed less.
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