Agricultural Research Center, Ona -
Mimeo Report- RC-1972-l11 J'f- 72
HUME LIBRARY
INFLUENCE OF BREEDING ON AGE AT CONCEPTION
OF HEIFERS BRED AS YEARLINGS JUN 29 1972

F. M. Peacock, E. M. Hodges, W. G. 1irk,
M. Koger and A. C. Warnick1/,FAS. ,-niv.of Forida

Increasing costs of capital resources for beef production
in Florida has created the need of higher beef cattle produc-
tion relative to increased costs. This economic pressure has
increased the interest of the cow-calf producer in breeding
heifers to calve at two years of age instead of three years.
2/
Research in Louisiana, Reynolds et al (1963) showed the Angus
breed attained puberty at 433 days of age, Brahman 816 and
Brahman x Angus crossbreds at 460 days of age. These wide
variations in ages make it necessary to select breeds with
the potential to conceive at an age relative to the established
management system on the particular ranch. Calving at two
years of age, if successful, should influence economic returns
and also genetic improvement of the breed.
The concept of genetic improvement of the herd resulted
in the initiation of a study at the Agricultural Research
Center, Ona, on breeding heifers as yearlings. The main
purpose was to evaluate reproductive behavior of various
European Fl crosses and backcrosses.

l/ Associate Animal Husbandman, Agronomist, Animal Scientist,
Emeritus, Agricultural Research Center, Ona; Animal Geneticist
Animal Physiologist, Animal Science Department, Gainesville.
2/ Reynolds, W. L., T. M. DeRoven and J. W. High, Jr. 1963.
The age and weight at puberty of Angus, Brahman and Zebu
cross heifers. J. An. Sci. 22:243.








Crossbred heifers, similar to those commonly used in
commercial production were exposed to Charolais bulls during

a 100 day breeding season. The breeding season began and

ended approximately two weeks before the regular breeding
season for older cows, conforming with commercial management
in maintaining a regular breeding season. Breed groups of
heifers studied included various combinations of the Angus,
Brahman, Charolais and Shorthorn breeds, with major emphasis

on Brahman European crosses. Age at conception was studied
by subtracting a standard gestation length of 286 days from
the date of calving.
Results

Data on ages at conception of various combinations of

European Brahman crosses have been presented. Average ages
of conception during the 100 day breeding season ranged from

418 days for the 1/2 Shorthorn 1/2 Brahman to 465 for the
Charolais mix group. Heifers breeding at the youngest ages

were the Fl Shorthorn x Brahman and Angus x Brahman and
followed by the Charolais Brahman and 3/4 Shorthorn 1/4
Brahman.
Variations in numbers of heifers conceiving within breed

groups except for the 1/2 Angus 1/2 Brahman, made it necessary
to make projections of age as if the breeding season had been
lengthened until all heifers were bred (Table 1). The projec-
tions magnified breed variations from 423 days at conception
for the 1/2 Shorthorn 1/2 Brahman to 528 days for the 3/4

Brahman 1/4 Shorthorn heifers. The average projected ages







at conception fits closely the results obtained by Franke
and England (1965)/ in Louisiana for Fl British Zebu and

3/4 Brahman 1/4 British for age at puberty. These data show
that on a population basis, heifers of predominately Brahman

breeding do not reach sexual maturity as early as the F1
European x Brahman and those with predominately European
breeding and should not be expected to perform on the same

relative basis when exposed as yearlings.

This study shows that the F1 Angus x Brahman and Short-
horn x Brahman heifers followed by the 3/4 Shorthorn 1/4

Brahman as well as the Charolais x Brahman have the potential

of breeding as yearlings within a limited breeding system.

Slower sexual maturity of the 3/4 Brahman 1/4 Shorthorn
limits their production at an early age. However, a short
breeding season, relative to the regular season would permit

those heifers that have reached puberty to breed, with the
remainder being held until 2 years of age.


3/ Franke, D., and N. England. 1965. 5th Livestock Producers
Day Report, p. 44. Louisiana State University. Baton
Rouge, La.







BRELEDNG


--- ,,t *-- ti- II I- I^ i i
Av. age Range in Average Projected
No. Calving at first age at age at Av. age at2/
Breeding Animals status-1 exposure conception conception conception"


h A % Br


$ Sh k Br


3/4 Sh Br


3/4 Br Sh


4 Ch k Mix


% A 3/8 Br 1/8 Sh


Ch Br crosses


Ch Sh crosses


Oth..er crosses


390


391
396

407


410
405

402
381
414
376


356 517


329 487


386 489


387 520


412 497


415 502


389 499


385 487


410 496


434


418


443


454


465


454


434


423


447


528


450


447


461


I/ C = Calved; 0 = open,


did not calve.


2/ Projected age at conception = Total No. within breed groupX (age at
-o. Cr.ceiving age
;To. Cn.iceiving age


conception age at first exposure) +
at first exposure.


X AGE ON CONCEPTION OF YEARLING HEIFERS EXPOSED TO CHAROLAIS BUILS FOR 100 DAYS.