Impacts of birth season and production system on gastrointestinal parasitism and growth in Katahdin lambs.

Gastrointestinal nematode (GIN) infection adversely affects the performance and well-being of forage-based sheep throughout the world. The study objectives were to estimate longitudinal differences between birth seasons and production systems for lamb postweaning growth and indicators of GIN infection. Data were collected on Katahdin lambs within a single flock from 2006 to 2022. Lambs were born in either the winter or fall and reared in conventional or certified organic production systems. Four sub-flocks were defined as the combination of birth season and production system (winter-conventional = W-C,  = 1766; winter-organic = W-O,  = 190; fall-conventional = F-C,  = 846; and fall-organic = F-O,  = 189). All lambs were naturally infected with GIN from previously grazed pastures, weaned at approximately 60 d of age, and remained on pasture throughout the postweaning phase. At approximately 90, 120, and 150 d of age lamb body weight (BW) was recorded, and fecal egg count and blood packed cell volume (PCV) were quantified. Fecal egg count was log-transformed (LFEC) prior to analyses. Traits were initially analyzed within collection timepoint to compare all 4 sub-flocks. Final BW at 150 d and LFEC and PCV throughout the postweaning phase were similar between conventional and organic lambs born in the same season. Repeated measures models were then used to analyze records from W-C and F-C lambs over time. The collection timepoint × deworming treatment × sub-flock interaction effect was significant for all traits (< 0.01). Within lambs that did not require deworming, no difference in BW between birth seasons was observed. However, LFEC trends for untreated F-C and W-C lambs were nearly exact opposite of one another, being greater for F-C at 90 d, (< 0.01) not different at 120 d, and greater for W-C at 150 d (< 0.01). Additionally, PCV of untreated F-C lambs was lower at 90 d but greater at 120 and 150 d than untreated W-C lambs (≤ 0.04). This was the first study conducted in the U.S. to compare longitudinal performance between lambs born in different seasons and reared in different production systems. Under the conditions of this study, organically managed lambs had similar performance to their conventionally managed counterparts. However, large differences in GIN infection risk over time existed between birth seasons and deworming regimens which have important implications for producer management decisions and genetic improvement programs.