Active immunization of ewes against ovine placental lactogen increases birth weight of lambs and milk production with no adverse effect on conceptionrate

In two experiments, 16 Booroola-Assaf and 35 Assaf ewe-lambs were actively immunized at 5 months of age against recombinant ovine placental lactogen ( oPL). At 9 months of age, the ewe-lambs were mated for the first time and t hen introduced into a frequent mating-system. Anti-oPL antibody titers, rep roductive performance, maternal serum levels of oPL during pregnancy, lamb birth weight and milk production of the ewes were followed in the immunized ewes and in their non-immunized control counterparts. All the immunized ew es developed anti-oPL antibodies, which interfered with oPL bioactivity in an in vitro cell proliferation assay. Conception rates did not differ (P > 0.05) between immunized and non-immunized ewes. Abundant antibody-bound non -active oPL detected in sera of immunized ewes by western blotting indicate d enhanced oPL production by the placenta following immunization. An increa se (P ( 0.02) in serum oPL bioactivity, but not immunoreactivity, was obser ved in the immunized ewes in late gestation relative to control ewes. The a verage litter size was 1.83 and 1.32 lambs born per ewe lambing in the firs t and second experiments, respectively. Average birth weights of lambs born to the immunized ewes were higher (P < 0.01) than for lambs born to contro l ewes by 10, 17 and 39% for those born as singles, twins and triplets, res pectively. Immunized ewes produced 19 and 33% more milk(P < 0.02) than the control ewes in the first 3.5 months of the first and second lactations, re spectively. These findings do not suggest a role for oPL in maternal recogn ition of pregnancy, but they strongly suggest important roles for oPL in fe tal growth and mammogenesis. Immunization of ewes against oPL may thus repr esent a novel practical technique for enhancing birth weights of lambs born to prolific sheep, as well as milk production by both dairy and mutton ewe s. (C) 2000 Elsevier Science B.V. All rights reserved.