Despite the fact that oestrus in the Merino ewe has been the subject of active research in Australia and South Africa for over three decades, and that the effects of several exteroceptive factors have been adequately demonstrated, the reasons for differences in oestrous activity between different environments and for annual variations within the same environment remain obscure.
Whilst in many environments the Merino ewe is anoestrous during the spring and early summer months, a variable degree of oestrous activity is maintained during this period within the pastoral areas of New South Wales, Queensland, tropical Western Australia, South Africa and Rhodesia. This difference cannot be explained entirely in terms of photoperiodism.
Investigations by the author into causes of reproductive wastage in Merino flocks in tropical western Queensland indicated that failure of oestrous activity was a major factor, and that the plane of nutrition might be of importance in controlling oestrous activity, particularly during the spring and early summer months (Smith,1960). Many flocks in this area are mated during the late spring and early summer months so that lambing will occur during the autumn when seasonal conditions are most favourable, and thus the problem of anoestrus during the former period is of considerable economic importance.
An attempt has therefore been made to define the effects of plane of nutrition and bodyweight upon oestrous activity in Peppin Merino ewes in western Queensland. Several other factors (time of parturition, lactation and parasitism) which appear to be of importance have also been investigated.
The first part of this dissertation consists of a review of the literature pertinent to the periodicity of oestrus in the Merino ewe, with particular reference to the effect of environmental factors. There follow the results of observations on oestrous activity in Merino flocks in several different parts of semi-arid tropical Queensland.
The third part of the thesis is a description of experimental work designed to:-
(a) determine the effects of plane of nutrition and bodyweight upon current and subsequent oestrous activity, with particular reference to the effect of seasonal variations in the nutritive value of natural pastures.
(b) investigate the effects of helminthoses and cutaneous myiasis during the spring and early summer months upon subsequent oestrous activity.
(c) investigate the effects of parturition and lactation at different times of the year upon the duration of subsequent anoestrus and particularly upon the onset of oestrus in ewes mated in the late spring and early summer.
Due to the conditions under which this work had to be performed, it was not possible to obtain all of the data which might be considered desirable in such an investigation; in several instances, the proposed experimental method had to be abandoned due to unfavourable weather conditions. Severe flooding during March, 1963 and heavy rains during February and August, 1963 impeded this work considerably.