a1 Department of Physiology and Environmental Studies, University of Nottingham School of Agriculture, Sutton Bonington, Loughborough, Leciestershire, LE12 5RD
In countries with developed agricultural systems the proportion of dairy cattle inseminated artificially increased substantially following the Second World War, but a similar expansion in the use of artificial insemination (AI) has not been experienced for beef herds. The availability of deep frozen semen provided a further incentive to increase the percentage of dairy cattle artificially inseminated, for the more progressive herd owners soon realized that the technique offered the advantage of a long-term breeding programme with semen from progeny-tested sires available at a relatively modest cost. More recently, however, various developments in herd management have tended to militate against the use of AI in dairy herds, and during the past year there has been a substantial decrease in the total number of dairy cows inseminated which can only be partially explained by the smaller decrease in the size of the national herd. Amongst reasons advanced is the increase in average size of individual herds, resulting in more cows per man and therefore less time available for the observations of oestrus. Since accurate detection of the onset of oestrus is so important in obtaining high fertility following AI, some herd owners have returned to using natural mating. Evidence supporting this view can be derived from the fact that during 1973/74 there was an increase of over one thousand in the number of bull licences issued in England and Wales above the seventeen thousand issued in 1972/73. Of the 18.2 thousand bulls licensed, there were 3.2 thousand crossbred bulls, some obviously for use in dairy herds. The availability of synchronized and controlled ovulation would largely remove the difficulties which have been encountered, and would permit planned management techniques to be more widely applied within such herds.
p1 Present address: Department of Dairy Science, Michigan State University, East Lansing, Michigan 48824, USA.
p2 Present address: Department of Veterinary Physiological Sciences, College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Canada.