Evaluation of Two Sources of Angus Cattle Under Florida Conditions
Dr. John Arthington-
UF/IFAS Range Cattle Research & Education Center
This summary was prepared from a larger data set that was recently published in the Journal of Animal Science. The research was conducted by Dr. John Arthington at the Range Cattle Research and Education Center in Ona and Drs. David Riley, Chad Chase, and Sam Coleman (Subtropical Agricultural Research Station, Brooksville), and Dr. Owen Rae (College of Veterinary Medicine, Gainesville). This research collaboration was supported by a generous donation of Angus embryos by Galen and Lori Fink of Manhattan, KS.
In Florida, replacement heifers must cope with a subtropical environment and regularly reproduce throughout their lifetime, thus they need to be adapted to the conditions of the area. Multiple generations of resident cattle in an area to which they were originally not adapted may result in acquisition of adaptation. Angus cattle have been raised and maintained in Florida for many years, but not in great numbers, perhaps because they are not well adapted to regional conditions.
The Angus herd at the Subtropical Agricultural Research Station in Brooksville, FL has existed since the 1950s with minimal introduction of outside bloodlines. Angus cows and bulls were received into this facility from the University of Florida and a Virginia research facility in the early 1950s. Cows were bred to bulls from a seedstock producer in Maryland until the late 1970s. Bulls produced within the herd were used since that time, except for a short period in the 1990s when bulls from the original source were again used. This time length and isolation may have facilitated some adaptation to the local conditions. The objective of this study was to compare the performance and aspects of adaptability of this Florida Angus bloodline to cattle that are representative of modern Angus bloodlines under U.S. subtropical conditions.
Outline of Research Methods
All cattle in this study were produced by embryo transfer. Embryos were from two sources. Embryos from 5 bulls and 15 cows of the Angus herd at the Subtropical Agricultural Research Station near Brooksville, Florida comprised the local source. Embryos from 5 bulls and 11 cows comprised the outside source and were representative of modern popular Angus bloodlines; these were provided by a seedstock producer from Kansas. Embryos were transferred into Brahman x British crossbred cows on multiple dates in 2003, 2004, and 2005 at the University of Florida, Range Cattle Research and Education Center. Calves (n = 82) were born from mid-October through early January and were weaned in August at an average age of 265 days. Following weaning, bulls and heifers were managed for approximately 9 months in separate groups on bahiagrass pastures.
Approximately 1 month after weaning (September), heifers were exposed for 9 months to an unrelated Angus bull. Heifer body weight was measured at weaning, and 100 and 150 days later. Average daily gain was calculated for each interval and total increase in hip height from weaning through 17 months age. Age at conception was calculated by subtracting 280 days from the day of calving. Pregnancy rate, calf crop born, and calf crop weaned were evaluated. For bulls, body weight and scrotal circumference were recorded at, on average, 320, 363, 426, 462, and 511days of age, respectively. At each of these times, a semen sample from each bull was obtained and evaluated for % individual motility, % normal morphology, and sperm concentration. Average daily gain was calculated within and across these intervals. Total increase in scrotal size from 11 to 17 months age was calculated. Among the bulls that had satisfactory semen characteristics at the last evaluation, 4 bulls from each source group were placed in single-sire breeding herds with 20 multiparous Braford cows for a 90 day breeding season. Calving rate was determined for each source, and evaluated as a trait of the bull.
Bull calves were heavier than heifers at birth (64 vs. 60 lb) and at weaning (498 vs. 474 lb). Outside-source calves had greater hip height (42.8 vs. 41.2 inches) and weaning weight (496 vs. 474 lb) than local Angus.
Although no difference was detected between source groups as yearlings, outside-source heifers were heavier (681 vs. 642 lb) and had greater hip height at 17 months of age than local heifers (44.7 vs. 42.3 inches). Average daily gain of heifers was greater from weaning to yearling age for outside-source heifers (0.44 lb/day) relative to local-source heifers (0.22 lb/day). There was no source group difference for average daily gain from yearling to 17 months or in the overall period, or for the increase in hip height during the same period. Local-source heifers had a much greater age at first conception (454 vs. 500 days) and reduced pregnancy rates (as a proportion of heifers exposed to bulls during the 9 month period after weaning; 29 vs. 70%) compared to outside-source heifers.
The ADG of bulls for the entire interval from weaning through 17 months of age, tended to be greater for outside-source (1.8 lb/day) compared to that of local bulls (1.6 lb/day). By 462 and 511 days, scrotal circumference of outside-source bulls was greater than local-source bulls (34.3 vs. 32.3 and 35.2 vs. 33.0 cm for outside-source and local source bulls at 462 and 511 d, respectively).
When bulls were placed in single-sire herds with 20 Braford females per herd, the proportion of pregnant cows was 85% (170/200) for cows exposed to local bulls and 89% (248/280) for cows exposed to outside-source bulls. These did not differ from statistical expectation. No source differences were detected for birth weight of calves born to these Braford cows; however, weaning weights of calves sired by outside-source bulls tended to be greater than those sired by local-source bulls (560 vs. 549 lb).
There were no advantages of the local-source group relative to the outside group, as evidenced by the few differences detected in this study for traits related to performance or adaptation. The source difference in age at first conception was large, with local heifers conceiving almost 100 days later than outside-source heifers. This is also reflected in pregnancy and calving rates for heifers after their first year of bull exposure, with outside-source heifers having much greater rates than local heifers. This large difference in reproduction success as young cattle may be a consequence of the absence of selective pressure for early calving, since for many years the local-source herd was managed to calve for the first time as 3-year-olds. Alternatively, if the local group could be considered to have acquired some adaptation to Florida conditions, then one aspect of that adaptation may be later maturity, as is seen in Bos indicus and other types of tropically-adapted cattle. Evaluation of lifetime productivity of cows and bulls of both the local and outside sources would provide more clear information about adaptation differences of the two.