LAUSR

Little is known about U.S. organic production of ruminant livestock. Objective was to examine growth, gastrointestinal nematode (GIN) infection, and eligibility of organic certification between conventional (CON) and organic (ORG) management (Mgt) up to 150 d of age. Katahdin lambs (ORG, n = 240; CON, n = 690) were born in fall (Oct–Dec) or winter (Jan–Mar) between 2014 and 2019 in Booneville, AR. Intact male and female lambs were fence line weaned ~90 d of age, grazed tall fescue and/or mixed forages and were supplemented (corn/soybean meal) when forage quality became limiting. Organic sheep and pastures were certified by NICS. Body weight was determined at birth and approximately 60, 90, 120, and 150 d of age. Blood and feces were collected for packed cell volume (PCV) and fecal egg counts (FEC) at 90, 105, 120, 135, and 150 d of age. Lambs were selectively dewormed based on Five Point Check and incidence recorded. Data were analyzed by Proc GLM or Mixed with repeated measures (SAS) and included Mgt, sex, season, yr, d of measurement and interactions; age of lamb was included as covariate if needed. Birth weight was usually greater in fall born ORG and less in winter born than CON (mgt x season x yr, P < 0.001), and varied between ORG and CON over time (mgt x season x yr x d, P < 0.001). A similar 4-way interaction occurred for FEC and PCV (P < 0.001). Incidence of deworming was similar between ORG and CON (ranged between 0 and 3 times/lamb, and 3.2% to 151% over season and yr). Because copper oxide wire particles were used for most ORG deworming, between 50 and 92% of lambs were eligible for organic certification by 150 d of age (mgt x season x yr, P = 0.03). Comparisons between farming systems are complex and should consider factors such as varietal and seasonal changes in forages.