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B. P. LOUW, A. W. LISHMAN, W. A. BOTHA and J. P. BAUMGARTNER

Prolactin is readily secreted before and during oestrus in ewes (Kann, 1971; Cumming, Brown, Goding, Bryant & Greenwood, 1972) and Denamur, Martinet & Short (1973) indicated that this hormone has a luteotrophic effect. Although these findings suggest that prolactin contributes to normal reproduction in ewes, the physiological rôle of the prolactin surge at oestrus has as yet not been elucidated.

In the rat and mouse, a surge of prolactin on the afternoon of pro-oestrus promotes luteolysis of corpora lutea (Malven & Sawyer, 1966; Grandison & Meites, 1972). Such an effect appears unlikely in sheep since luteal regression in this species commences several days before the onset of oestrus (Deane, Hay, Moor, Rowson & Short, 1966).

Prolactin is also rapidly released into the circulation in response to the milking stimulus in lactating ewes (Fell, Beck, Brown, Catt, Cumming & Goding, 1972) and it has been suggested that the secretion of large

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A. W. LISHMAN, W. J. STIELAU, I. E. DREOSTI, W. A. BOTHA, A. M. STEWART and C. E. SWART

Departments of Animal Science and Biochemistry, University of Natal, Pietermaritzburg, South Africa

The anoestrous condition which follows upon periods of underfeeding in ewes is apparently similar to the normal seasonal anoestrus and is usually manifested as a prolongation (Hunter, 1962) or precipitated onset (Smith, 1962) of the seasonal period of sexual rest. It has been suggested that the ovarian quiescence noted in underfed animals may be the result of pseudohypophysectomy (Mulinos & Pomerantz, 1940; Lamming, 1966; Leathem, 1966).

An acute release of LH at oestrus in ewes has been demonstrated by Niswender, Roche, Foster & Midgley, 1968; Goding, Catt, Brown, Kaltenbach,Cumming & Mole, 1969; Wheatley & Radford, 1969) and it is possible that changes in the pattern of LH release may indicate functional changes in the pituitary of ewes which become anoestrous after periods of inanition. This phenomenon was studied in an attempt to elucidate the mechanism involved in a