The induction of reproductive activity in lactating ewes

Abstract

In two experiments, Merino ewes that had lambed in either the autumn or spring were treated with various combinations of a progestagen (methylacetoxyprogesterone, MAP) and pregnant mare's serum gonadotrophin (PMSG), from 3 days post partum. Irrespective of the season of lambing, the treatments resulted in ovulation in nearly all ewes before day 15 post partum; oestrus was observed in all ewes that had lambed in the autumn but in only half the spring ewes. Regular oestrus activity was not resumed after the induced oestrus and/or ovulation. After further treatments with MAP and PMSG (days 15–26 post partum), 29% of the ewes showed oestrus (autumn and spring), with 47.1% and 58.3% of ewes ovulating in the autumn and spring respectively. Cystic and/or persistent corpora lutea (CP) were noted in 24% of the treated ewes. In spring-lambing ewes, 42% of ovulations before day 15 were judged to be abnormal. Fertilization rates were low in ewes that mated. The best results, 56% of ova fertilized, were from matings between 26 and 30 days post partum in spring-lambing ewes. In two further experiments, the fertility of ewes was examined at matings induced at various times post partum in crossbred ewes that had lambed in the spring. The fertility was low (=17%) up to 30 days post partum, but it improved in later matings (=30%). The incidence of a second oestrus succeeding the induced cycle was low (29%), but fertility was higher (50%). Approximately 50% of the untreated ewes lambed to matings between 30 and 60 days after lambing.

Effect of weaning pre‐ or post‐mating on performance of spring‐mated ewes and their lambs in New Zealand

“…While lactation is reported to lower fertility when ewes were bred within 30 days postpartum (Restall et al 1978) and increase the postpartum anoestrus interval (Mallampati et al 1954), Hulet & Foote (1967) found no effect of lactation on the number of ewes lambing when breeding occurred 30-90 days postpartum. In Australia, Dawe et al (1969) achieved 65% pregnancy rates in Border Leicester × Merino ewes that had been lactating for 9 weeks prior to the mating period, during the non-breeding season.…”

Strategies for rapid rebreeding of lactating ewes in the spring

“…These systems all recommend early weaning of lambs in order to improve fertility in spring matings, but early weaning is often not consistent with forage-based lamb production. Attempts to breed lactating ewes in spring have consistently been disappointing (Bourke, 1964;Whiteman et al, 1972;Cognie et al, 1975;Pope et al, 1989), and use of hormone or light manipulation to improve spring fertility has generally not been successful in lactating ewes (Restall et al, 1978;de Nicolo et al, 2006;Martemucci and D'Alessandro, 2010). Selection to improve ewe fertility in a crossbred (50% Dorset, 25% Rambouillet, 25% Finnish Landrace) sheep population mated in May and June at Virginia Tech (37°N latitude) resulted in significant selection response (Notter and Cockett, 2005) and correlated increase in duration of the breeding season (Vincent et al, 2000;Notter et al, 2011;Goff et al, 2013).…”

Does inadequate luteal function limit the establishment of pregnancy in the early post-partum ewe?

“…However, despite successful fertilization at an induced oestrus 28 days after parturition, no pregnancies were sustained and there was a high incidence of inadequate luteal function in lactating ewes induced to ovulate by using progestagen pessaries and pregnant mares' serum gonadotrophin (PMSG) at pessary withdrawal. Luteal function was classified as inadequate when corpora lutea either failed to secrete progesterone completely, as assessed by twice daily peripheral blood samples, or caused a transient rise in progesterone lasting only 12-36 h. Lactation generally prolongs the duration of post-partum anoestrus (Mauleon & Dauzier, 1965;Kann & Martinet, 1975;Shevah et al, 1975;Kann et al, 1977) and is associated with reduced fertility at an induced oestrus during breeding and non-breeding seasons (Restall et al, 1978; (Hunter el al, 1955), held at 37°C in ovum culture medium (Flow Laboratories, Irvine, UK) (Wenham & Robinson, 1972).…”