The reproductive performance of ewe lambs in a controlled environment

Abstract

The effects of strain, season, type of birth, age and body weight at breeding on fertility and fecundity of ewe lambs raised in a controlled environment were evaluated. Data were derived from 2776 ewe lambs of a synthetic sire strain, two synthetic dam strains and two control strains (Suffolk and Finnish Landrace breeds) mated to 719 rams over 15 breeding periods during January, May or September over 9 years. The ewe lambs were exposed to rams at a synchronized and follow-up oestrus at 6-5 to 7-5 months of age. There were significant differences among strains. Fertility and fecundity were highest for lambs of the Finnish Landrace breed, intermediate for the two synthetic dam strains and lowest for the synthetic sire strain and the Suffolk breed. Although period of breeding and season had an effect on reproductive traits, the strain differences were not consistent. Type of birth did not affect performance of lambs at the synchronized oestrus. However, overall fertility and fecundity increased progressively as the type of birth of the lambs increased from singles to triplets and greater. Increased age and body weight at the synchronized oestrus had a significant and positive effect on fertility and fecundity.

Effect of stimulation with a gonadotropin mixture on reproductive outcome in nulliparous ewes bred during seasonal anestrus and early breeding season

“…However, ram-induced gonadotropin secretion might not stimulate production of sufficient quantities of estrogen to induce estrus in most ewe lambs during seasonal anestrus. Supplementation with estrogen directly (Knights et al, 2002) or with gonadotropins (Ainsworth and Shrestha, 1987;Ainsworth et al, 1991 and current study) improved reproductive performance in fall-born ewe lambs exposed to rams during mid-anestrous. Gonadotropic supplementation of yearlings bred during seasonal anestrus may be necessary (Robinson and Smith, 1967;Burfening and Van Horn, 1970) or of little benefit (current study) depending on the extent to which the ram effect response is maximized.…”

“…Treatment with equine chorionic gonadotropin (eCG) after progestogen priming induced fertile estrus in fallborn ewe lambs (Ainsworth and Shrestha, 1987;Ainsworth et al, 1991;Stellflug et al, 1993) and yearlings (Robinson and Smith, 1967;Burfening and Van Horn, 1970) bred during seasonal anestrus. Additionally, a small dose of estrogen increased the proportion of progesterone-primed fall-born ewe lambs expressing estrus during anestrus (Knights et al, 2002).…”

Effects of a Gonadotropin Mixture on Reproductive Success in Progesterone- Treated Non-Lactating Anestrous Ewes

“…Attempts to breed fall-born ewe lambs out-of-season have not been successful. Approaches used to evaluate the response have included ram introduction alone (Lopez-Sebastian et al, 1985), manipulation of the photoperiod (Ainsworth and Shrestha, 1987;Ainsworth et al, 1991), and use of gonadotropins in combination with progesterone and estradiol (Foote and Bennett, 1968). With ram introduction alone, fall-born ewe lambs exhibit "silent ovulations," where they are observed to ovulate, but often times do not exhibit estrus until the second or even third ovulation.…”

Induction of fertile estrus during seasonal anestrus in ewes and fall born ewe lambs

“…In order for the age at first lambing in fall and spring lambing systems to be comparable, fall born ewe lambs must be bred during the period of seasonal anestrous. Treatment with long days (from 5 to 20 weeks) followed by a period of short days (20-32 weeks) with progestogen-eCG treatments commencing towards the end of the short day photoperiod induced fertile estrus in fall born lambs during seasonal anestrous (Ainsworth and Shrestha, 1987;Ainsworth et al, 1991). The percentage of ewe lambs lambing and the litter size to two service periods after synchronization treatments were 58% and 1.6, respectively.…”

“…Photoperiodic manipulations (long day followed by short days) have also been used to induce puberty during the anoestrous period in fall born ewe lambs (Foster and Ryan, 1981;Ainsworth and Shrestha, 1987;Ainsworth et al, 1991).…”