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Arkansas Animal Science Department Report 2001
mental feed can be reduced.
Calves were weaned in October at an average of 204 d
of age. After weaning, calves were penned for 2 wk and fed
BG hay and 2.2 lb/d of commercial mixed feed.
Subsequently, calves were moved back to their preweaning
forage environment (i.e. BG or E+). Calves were managed
postweaning for moderate gains consistent with a common
backgrounding program by supplementing with cottonseed
meal, corn, and (or) a commercial mixed feed. Tall fescue or
BG hay was provided free choice for calves on E+ or BG,
respectively. Supplemental feed (same as above) was provided
from weaning to late October and was continued to April
of the following spring. Amounts of supplemental feed were
adjusted based on visual estimates of forage availability and
ambient temperatures
Growth parameters of A and k were estimated on 177
Angus (AA), Brahman (BB), Angus x Brahman (AB) and
Brahman X Angus (BA) heifers born from 1988 to 1991
using the three-parameter growth curve model as described
by Brody (1945). Data were collected every 28 d until
approximately 18 mo and then at prebreeding, postcalving,
and weaning of calf. In a preliminary analysis, year was not a
significant source of variation, therefore data were pooled
over year and analyzed by the general linear model (GLM)
procedure of SAS (SAS Inst. Inc., Cary, NC). Included in the
models for A and k were the independent variables of breedtype,
forage and breed-type x forage interaction. Sire of calf
was not included in the model due to the fact that sires were
rotated among breeding pastures in both forage treatments to
prevent confounding of sire and forage effects.
Results and Discussion
There was a significant (P < 0.01) breed-type x forage
interaction for mature weight in this study. Presented in Table
1 are the least squares means and standard errors for estimated
mature weight by breed-type and forage environment.
There was no difference (P > 0.05) in mature weight of
straightbred Angus cows on either forage with means of
1,298 and 1,344 lb, respectively, for BG and E+ forages.
These estimated values for Angus cows are higher than those
reported by Stewart and Martin (1983) and Brown et al.
(1972) who reported mean A values of 1,067 and 970 lb,
respectively. Angus x Brahman cows grazing E+ were heavier
(P < 0.05) at 1,283 lb than were their counterparts grazing
BG at 1201 lb. It is not entirely clear why these Angus x
Brahman cows which grazed BG had smaller A values than
the mean of their parental breed-types, however, this could be
due to an interaction between the direct breed effects of the
Angus cattle and the maternal breed effects on the Brahman
cows grazing this BG forage. There were no differences (P >
0.05) in mean A values for the Brahman x Angus cows on
either forage. The Brahman cows seemed to have a difficult
time coping with the negative effects of the E+ forage as they
had a mean A value of only 1,120 lb which is smaller (P <
0.05) than all other breed-type x forage combinations with the
exception of the Angus x Brahman crosses on BG at 1,201 lb.
Determining the ideal weight for maximum animal production
is an important question that needs an answer. Stewart
and Martin (1983) reported that in Angus cows, their optimum
estimated mature weight in order to achieve maximum
maternal performance was 1,045 lb. This weight was considerably
lower than our estimated mature weights of 1,298 to
1,344 lb, however, Kapps et al. (1999) reported similar values
for mature weight in Angus cows of 1,320 lb.
Shown in Table 2 are the least squares means and standard
errors for k by breed-type. There was no breed-type x
forage interaction for k in this study. Angus x Brahman crosses
had the earliest (P < 0.05) rate of maturing of all breedtypes
maturing at a rate of 0.053. There were no differences
(P > 0.05) between the k values between straightbred breedtypes
with Angus at 0.039 and Brahman at 0.042. These values
of k are considerably lower than values reported in the literature.
There was however a difference between the two
reciprocal crosses with Brahman x Angus cows maturing at a
slower (P < 0.05) rate than did the Angus x Brahman (0.049
vs 0.053). This increase in rate of maturing over the purebred
cattle can be expected due to the effect that heterosis has on
this trait. Nelson et al. (1982) reported a percentage heterosis
increase in maturing rate of 3.5 % in Brahman x Angus cross
cattle thus supporting our results. In a growth curve study by
Tawah and Franke (1985), they used 574 straightbred and
crossbred cows and reported results stating that generation
one crossbred cows had a 0.034 greater k value than did the
straightbred cattle.
Implications
These results suggest that the growth parameters of A
and k differ by breed-type. These differences are of importance
to the biological and economical efficiency of beef production
and need to be carefully considered when attempting
to correctly match breed-type to available production
resources. Further research is needed in this area, particularly
in the field of crossbreeding to help producers deal with
different biological types of animals in a wide variety of production
environments, including those with limited resources.
Literature Cited
Brody, S. 1945. Bioenergetics and Growth. Reinhold
Publishing, New York.
Brown, J.E, et al., 1972. J. Anim. Sci. 34:525-537.
Kapps, M., et al., 1999. J. Anim. Sci. 77: 569-574.
Nelson, T.C. et al., 1982. J. Anim. Sci. 55:280-292.
Stewart, T.S and T.G. Martin. 1983. J. Anim. Prod. 37:
179-182.
Tawah, L.C. and D.E. Franke. 1985. J. Anim. Sci. 61
(Suppl.) 8.
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