EFFECT OF SELECTION FOR LIFETIME PRODUCTION OF LAMB WEANED ON HORMONAL FACTORS THAT AFFECT GROWTH IN TARGHEE EWES AND LAMBS

Targhee ewes (n = 22, average age 5 yr) rearing twin lambs were used t o investigate serum growth hormone (GH), IGF-I, triiodothyronine (T-3) , thyroxine (T-4), and prolactin (PRL; ewes only) concentrations assoc iated with selection pressure for lifetime production of kilograms of lamb weaned per ewe. Period 1 started on d 4 after birth and was condu cted in confinement. Periods 2, 3, and 4 were conducted on fenced inte rmountain sagebrush-bunch-grass range starting at an average of 49, 84 , and 112 d after birth, respectively. Blood samples were collected ho urly for 6 h on d 4, 11, 18, 25, 49, 84, and 112 after lambing. Ewe mo dels included the fixed effect for line, with ewe age, lambing date, a nd starting weight nested within line as potential covariables. Lamb m odels included fixed effects for line, sex, and sibling sex, with lamb age and birth weight nested within line tested as possible covariable s. Growth hormone concentrations were greater (P = .06) for selected t han for control ewes, but lamb GH concentrations did not differ (P = . 90) between selected and control lambs. Ewe prolactin concentration te nded (P = .13) to be greater for control than for selected ewes. Ewe a nd lamb IGF-I did not differ (P > .55) between selected and control ew es and lambs. Ewe T-3 and T-4 concentrations did not differ (P > .19) between selected and control ewes; however, lamb T-3 and T-4 concentra tions were greater (P < .01) for control than for selected lambs. Incr eased GH concentration in selected ewes seemed to be associated with g reater milk production. Differences in selected and control lamb T-3 a nd T-4 concentrations could be an indicator of receptor sensitivity, m etabolic activity, or seasonal reproductive transitions in ewe lambs.