M. F. McDonald

Reproduction in ewe lambs and its effect on 2-year-old performance

Abstract Reproductive performance was studied in 1- and 2-year-old Romney and Border Leicester x Romney ewes. Control lambs were joined with vasectomised rams, and the remainder were joined with entire rams for 5 weeks. Fertility, growth rate, and weaning rate were higher in crossbred ewe lambs, and birth rank effects were small. Conception rate to first service was negatively related to liveweight gain over joining. Weaning rate (57% lambs weaned/ewe joined) was lower than normally expected from adult ewes. Lamb survival was improved in previously lambed 2-year-old ewes, whereas oestrous and ovarian activity, fertility, fecundity, and lamb growth were unaffected. Ewe liveweight and fleece weights were temporarily reduced. It is concluded that lambing in l-year-old ewes can enhance their reproductive performance as 2-year-olds.

Seasonal effects on birth weight in sheep are associated with changes in placental development

Abstract Lambs born in the autumn or winter have substantially lower birth weights than those born in the spring, but the physiological basis of this difference is unknown. This study examined the effects of season on foetal growth and placental development in ewes managed under controlled grazing to minimise the confounding effects of maternal liveweight change. Mature Romney ewes pregnant to matings in December (n = 13) or March (n ‐ 13), with similar liveweights at mating, were managed to achieve similar liveweights at Day 140 of gestation. At that time, measures of foetal growth and placental development, adjusted for litter size, were (December‐ versus March‐mated): foetal weight (4.15 ± 0.16 versus 5.07 ± 0.16 kg, P 0.10); placentome number (89.4 ± 4.2 versus 106.9 ± 4.3, P < 0.01); number of placentomes per number of caruncles (0.79 + 0.03 versus 0.88 ± 0.03, P < ...

Resistance to cold stress in the newborn lamb 1. Responses of Romney, Drysdale × Romney, and Merino lambs to components of the thermal environment

Abstract Newborn lambs aged 1-30 h were exposed to a range of environmental conditions (5-30°C ambient temperature|0.0-1.9 mls air speed|wet or dry) and their metabolic rates, rectal temperatures, and hind-limb skin temperatures examined. Wet lambs exhibited substantially greater metabolic rates (by 3.8-7.0 W/kg) than those tested when dry. The results indicate that wet Romney lambs may attain their summit metabolic rate of 12-22 W Ikg and become hypothermic at ambient temperatures as high as 15°C. Conversely, dry lambs are unlikely to become hypothermic unless air temperatures are well below freezing point. Lambs exposed to air movement (1.0-1.9 m/s) had elevated metabolic rates (by 0.8-1.5 W/kg) compared with those in still air. Drysdale x Romney lambs were better able to conserve body heat than Romneys. Wet Romney and Drysdale x Romney lambs exposed to a series of declining ambient temperatures were able to minimise the hind-limb skin temperature-air temperature gradient but dry lambs generally failed ...

Resistance to cold stress in the newborn lamb 2. Role of body weight, birth rank, and some birth coat characters as determinants of resistance to cold stress

Abstract The effects of body weight and birth rank on resistance to cold stress were examined in Romneytype and Nd-type lambs. Twins exhibited lower coat depths than singles and they also appeared to have a lower summit metabolic rate per kg body weight. These effects were, however, small and the greater rate of body cooling experienced by twin lambs was principally a function of their low body weights. Sire of .the lamb exerted a small influence on its body weight and coat depth but these differences were not translated into a corresponding effect on the rate of decline in rectal temperature. Nd-type lambs had significantly greater coat depths and midside wool weights (per unit area of skin) than Romney-type lambs. The metabolic rate required by the lamb to maintain deep-body temperature was significantly related to coat depth (b = - 0.14 W Ikglmm) but not to midside wool weight. Rate of decline in rectal temperature was also low in deep-coated lambs. Most of the difference between Romney-type and Nd-typ...

Induction of oestrus and ovulation in Romney ewe hoggets with a progestagen

Abstract Ninety-six Romney ewe hoggets from a group of 236 hoggets of known history were treated with intra-vaginal sponges containing 20 mg or 60 mg methyl-acetoxy-progesterone (MAP) for 14 days near the onset of the breeding season. Observations were made for oestrus in all hoggets and for ovulations in a sample of animals from each group. After treatment the amount of MAP “absorbed” from each sponge was chemically determined. The effects of sire, birth rank of hoggets, dose level of MAP, and occurrence of oestrus on the amount of MAP “absorbed” were estimated. The incidence of oestrus within seven days of sponge withdrawal was 62%, and its occurrence was similar for both dose levels of MAP. The numbers of ovulations in treated hoggets were normal.

Embryo survival in mature Romney ewes relative to live weight and face cover

Abstract In Experiment 1 fertilised eggs were transplanted at the rate of 1, 2, or 4 per recipient ewe and these animals were autopsied in early pregnancy. In Experiment 2 ewes were mated to fertile rams and autopsied 18–30 days later. Some of these ewes had not conceived to egg transfer (Expt 1). Transfer of 1, 2, or 4 eggs did not affect the percentage of ewes which became pregnant. An ovum had a greater chance of surviving if transferred singly rather than as one of a multiple number of eggs. The number of corpora lutea (lor 2) did not affect the rate of survival of transferred ova or survival of ova in mated ewes. No significant relationship existed between live weight of the ewe and survival of ova. Face cover was related (P < 0.01) to rate of survival of ova among ewes transplanted with 2 eggs each, but no clear effect was apparent in the other ewes after egg transfer or in the mated ewes.

Maintenance of oestrus synchronisation in two-year-old ewes at second heat after progestagen sponge treatment

Abstract Romney and Border-Romney maiden 2-year-old ewes were treated for 14 days with intravaginal progestagen sponges at the start of the breeding season. After sponge withdrawal and at the next cycle 80% and 90% respectively of all ewes were in oestrus, and of these 77% and 80% were marked over 2 and 3 days respectively. Pregnant mares serum gonadotrophin (PMSG) (600, 1200 i.u.) injected on day 12 or 13 after first heat did not affect synchronisation of oestrus, although the mean onset of oestrus was earlier than in control ewes. Improved nutrition, treatment with PMSG, and injection on day 12 reduced mean cycle length.

A note on oestrous response to progesterone treatment in New Zealand romney ewes

Abstract Daily injections of 10mg of progesterone into 319 Romney ewes for 13-15 days just before the onset of the normal breeding season induced ovulation in all animals. Oestrus also occurred in 65% of the sheep. Most ewes studied returned to oestrus about 16 days after the induced ovulation and regular cycles were initiated.