An economic evaluation of the optimal ewe replacement policy for typical Canterbury light land and irrigated properties

Abstract

Every year in New Zealand large numbers of ewes which are suitable for another full year of production are killed at the freezing works. This is the result of the breeding ewe's productive life being traditionally determined by the state of its incisor teeth. The basis for this culling policy is that ewes with worn or missing teeth are considered unable to graze and therefore to produce satisfactorily.

The study takes the following form; (1) review of the relevant literature, (2) survey of farmers running 'gummy' ewes, (3) trial work analysing production and management characteristics of 'broken mouth' and 'gummy' ewes, and (4) development of two linear programming models (representing typical Canterbury light land and irrigated farms) utilising information gained in the above studies.

The survey and trial work showed that a higher lambing percentage can be expected from aged (‘gummy') ewes. However, this was the case only when the ewes were given preference, over younger ewes, for good feed especially in the six week period before parturition. The results also show that the average wool weights from the aged ('gummy') ewes was lower than that expected from mixed aged ewes.

The linear programming model solutions indicate that ewes should be culled at five and six years of age, under irrigated and dryland conditions respectively. The flock structures obtained were stable. Large variations in the cull ewe. prime lamb and wool prices were needed to alter the flock structures significantly.

The results show that the traditional culling practice leads to unnecessary wastage of productive stock from the national ewe flock. Retention of 'broken' and 'gummy' mouth ewes is found to be physically possible and economically desirable, both to the individual farmer and to the nation.