Reproductive Efficiency of Cows of Different Sizes

The effect of cow size on reproduction has become of in, respectively. The hip heights of the small, medium greater concern in recent years as a result of marked and large frame-size bulls used were 53, 55 and 57.5 in, increases in cow size in both purebred and commercial respectively, at 2 yr of age. The bulls were selected from herds and the extremely large effect of fertility on the within the STARS herd or were loaned from private profitability of commercial cowherds. This increase in herds. All were of gray Brahman breeding. The cows mature size has come about as a result of the use of large were maintained on bahiagrass pastures, with hay and imported European breeds in crossbreeding programs and protein supplement being fed during the winter months. as a result of selection within the existing British and Bos Heifers were bred for the first time at 2 yr of age. A indicus breeds for mature size. Some of the increase in breeding season of 90 d was used from 1983 through mature size has likely been the result of a correlated 1986 but was lengthened to 120 d in 1987. response to selection for yearling weight, but unfortunately there has been considerable direct selection The weights, heights, reproductive rates and body for mature height and weight as evidenced by the condition scores of females born since 1984 are shown in advertising of the mature weights and heights in breed Table 1. The difference between the hip heights of the journals. Also, it is an unfortunate fact that for many small vs the large frame-size groups of females ranged years there has been a tendency for judges to use height from 2.4 to 3.5 in depending on age. This difference as a major criterion for placement in the showring. While would be slightly less than two frame-size scores based it is perhaps now generally assumed that increased cow on the American system, with the small frame size group size will lead to reduced reproductive efficiency, being about 5 on this system and the large cows being relatively few studies have examined the magnitude of about 7. Age at puberty data from heifers born during differences in cow size that exist in many breeds of cattle the early years of the study have been analyzed by in the United States today. The effect of cow size has Senseman (1989). Heifers from the small frame-size been confounded with breed composition in most of the group reached puberty somewhat earlier -588 d -and studies which have been conducted to date. The purpose at lighter weights -666 lb -than did the large-frame of this paper is to report on results of the effect of cow heifers that reached puberty at an average weight of 754 size on reproduction in the Brahman breed at the Sublb at 635 d of age. The heifers were not exposed to bulls tropical Agricultural Research Station (STARS) near until they were more than 2 yr of age, however, when the Brooksville, Florida, and to review other studies that have majority of the heifers of all frame sizes had reached investigated the effect of cow size within the same breed puberty. As a result, differences in pregnancy or breed cross on reproductive traits. percentages at 2 yr of age were not great -93.7, 89.7