Physiology of Reproduction

Maintenance of a high level of fertility is important if the beef enterprise is to be profitable. A study of the breeding and calving records at the United States Range Livestock Experiment Station at Miles City, Mont., was made by Baker and Quesenberry on 4,753 cow-years over an 18-year period.

The average calf crop was 83.1 percent. The effect of yearly variations due chiefly to environmental causes was statistically significant. The age of the cow had no significant effect on fertility. More than half of the shy-breeding cows could be identified by the time they reached 4 years, and approximately 80 percent by 6 years of age. There was a highly significant difference, between bulls in percentage of calf crop; a variation of 45.5 to 94 percent was observed. The age of the bull did not have a significant effect on the percentage of calf crop, but the older bulls lost increasingly more weight during the breeding season. In a study comparing single and multiple bull-breeding units, herds with one bull had 6 percent more calves than herds with more than one.

On the basis of this study, the following recommendations should aid the rancher in raising the production level from the standpoint of calf-crop percentage:

Cows should be identified by proper brands, or marks, so that shy breeders can be eliminated at an early age. Good producing cows may be kept until at least 10 years of age without loss of fertility.

Cows that fail to produce calves in two successive years should be culled from the herd. This is especially true in the 4- and 5-year-old groups.

Where practical, it is advisable to test bulls for quality of semen before the breeding season, so that poor breeders can be eliminated from the herd.

There is an advantage in using bulls 4 years old or over of proved breeding ability. This is not only true for proved fertility, but by the time one calf crop has grown out the rancher has a chance to judge the value of the bull's calves as meat producers.

The establishment of small breeding pastures capable of maintaining herds of about 30 cows each is recommended as a means of increasing the calf crop.

Several researchers learned also that both beef and dairy cows ovulate about 14 hours after the end of estrus—a fact of importance in breeding practice, when considered together with information on the speed with which sperm travel. J. E. Brewster, Ralph May, and C. L. Cole found that spermatozoa require 6 to 9 hours in mature cows, and 4 to 7 hours in heifers, to reach the upper ends of the fallopian tubes. Results obtained by a number of workers have shown that a mating is more apt to be successful if it occurs during the latter part of estrus. The physiological basis for these results is indicated above, since spermatozoa deposited in the female tract near the end of heat reach the upper end of the tract, where fertilization takes place, a few hours before ovulation. Thus, fertilization can take place before the spermatozoa begin to degenerate.

THE AUTHOR

Ralph W. Phillips was formerly in charge of genetics investigations in the Bureau of Animal Industry. He joined the Food and Agriculture Organization of the United Nations in 1946.