Reproductive potential and endocrinological responses of sheep kept under controlled lighting. I. Comparative reproductive performance of four breed types of ewe

Five ewes of each of four breed types were kept in each of two environmentally-controlled rooms over a period of 2.5 years. The daylength varied between 20 and 9 h on a 6-monthly cycle, including a period of 22 weeks during which daylength was decreased by 0.5 h per week, 2 weeks in which it increased from 9 to 20 h, and 2 weeks at 20 h; each room operated 3 months out of phase with the other. Towards the end of the period of decreasing daylength each ewe was synchronised with a progestagen sponge for 12 days, given an injection (i.m.) of 750 I.U. pregnant mares' serum gonadotrophin (PMSG) on withdrawal, and inseminated 48 and 58 h later (500 million fresh undiluted sperm per insemination). Thus, insemination occurred at a single synchronised oestrus every 6 months. Progesterone pregnancy diagnosis was performed on blood samples taken 18 days later and parturition was induced by an injection (i.m.) of 16 mg dexamethasone on day 143 of gestation. Lambs were weaned at birth, allowing the ewes approximately 5.5 weeks to recover before the next insemination and a ‘successive’ conception.The performance of the four types of ewe, starting as maidens, is tabulated.

Conceptions	Merino	Dorset Horn × Merino	Border Leic. × Merino	South Suffolk × Merino	$\text{P}$
Successive (%)	20.0	12.5	11.1	47.6	$\text{< 0.01}$
Non-successive (%)	76.0	76.9	66.7	95.0	$\text{< 0.05}$
Overall (%)	51.1	52.4	44.4	70.7	$\text{< 0.05}$
Mean litter size	1.7	1.5	1.5	2.2	$\text{< 0.05}$
Mean lambs per ewe per year	1.8	1.6	1.3	3.1	$\text{< 0.01}$
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