Oestrous activity and lamb production in the New Zealand Romney ewe

Abstract

An experiment is described in which the effects of time of mating within the breeding season, farms, and ewe age on patterns of oestrous activity and other reproductive parameters in the New Zealand Romney ewe were examined. The following results were obtained: (i) The patterns of oestrus on each of five farms situated at varying altitudes, over nine 17 day periods of mating from January to June, followed that of a sigmoid curve (P < 0.001). The patterns of oestrous activity on farms 4 and 5 (situated at 1500 and 2000 ft), when compared with the three lower properties, were characterized by a later onset, a shorter period of peak activity, and an earlier decline in the proportion of ewes in oestrus (P < 0.001). The proportion of ewes 1.5 years old which exhibited oestrus was less than in the other three age groups studied: 1.5 years 53.2%, 2.5 years 57.7%, 3.5 years 56.3%, 4.5 years 58.1% (P < 0.01). (ii) The proportion of ewes which returned to service was lower the later the mating (P < 0.001). The incidence of barrenness in ewes which had failed to return to a single service and were therefore considered pregnant increased linearly with time of mating (P < 0.001). (iii) Of the ewes in oestrus, 78.8 % lambed to a single service. There was a significant linear increase in the proportion of ewes lambing to those in oestrus with time of mating (P < 0.001). Further, less ewes lambed to a single service on farms 4 and 5 than on farms 1, 2, and 3 (P < 0.01). (iv) There were significant differences in total and live lamb production with time of mating (0.01 > P > 0.001), between farms (P < 0,001), and between ewe ages (P < 0.001). Multiple births reached a peak at mid-season. Interactions of time of mating with ewe age for multiple births, and with farms for live lamb production, were also significant (P < 0.01). (v) The incidence of perinatal losses to 3 days after birth showed a highly significant difference between farms (P < 0.001). There was also a mid-season rise in lamb losses (P < 0.001) and this accounted for a non-seasonal peak period in live lamb production measured as a ratio either to ewes lambing or to ewes in oestrus. (vi) The performance of maiden ewes (1.5 years) was lower than that of the other three age groups for proportions in oestrus, multiple births, and the ratios of live lambs to both ewes lambing and ewes in oestrus. No significant differences were demonstrated in returns to service, in barren ewes, or in the proportion of dead to total lambs born for this age group.