Arkansas - Agricultural Communication Services - University of ...

Arkansas Animal Science Department Report 2001
mer of 1999, droughty growing conditions limited the availability
of forage; therefore cattle were supplemented for ad
libitum intake with bermudagrass hay and sudangrass
baleage. In mid-June, 1999, when the heifers averaged
approximately 12 mo of age, two blood samples were
obtained 10 d apart for each heifer for progesterone assay.
Heifers were considered cycling at 12 mo of age when
concentration of progesterone in one of the two samples
was ≥ 1 ng/ml.
In late August, heifers were split into two groups based
on chronological age, and heifers in the oldest of the two
groups were then synchronized and bred AI on observed
standing heat. Estrus detection was accomplished by 24 h
monitoring with the Heatwatch System. The second group of
younger heifers were synchronized 3 wk later and bred as
stated above. Artificial insemination was halted on November
15, 1999. Heifers were checked after 60 d of gestation for
pregnancy using ultrasonography.
Data were analyzed using the GLM procedure of SAS
(SAS Inst. Inc., Cary, NC). Sources of variation in the
dependent variables of weight, hip height, chest depth, and
body condition score were partitioned using a mathematical
model that included terms for an overall mean, breed type,
age, breed type x age interaction, and residual error. Least
squares means were separated using repeated t-tests in the
LSmeans option of PROC GLM of SAS. The distributions of
heifers cycling and pregnancy rate were tested using Chisquare
statistics.
Results and Discussion
Body Weight. The interaction of breed type x age was
not significant for BW at 6 mo, but it was an important source
of variation in BW at 14 mo for replacement dairy heifers in
this study. This may indicate that compensatory growth
occurred from 6 to 14 mo of age. These results are in agreement
with those of Ruvuna et al. (1986) who reported that
crosses of Holstein, Jersey, and Brown Swiss were superior to
purebreds for growth. At 6 mo of age the BSH heifers were
heavier (P < 0.05) than both H and JH heifers, but by 14 mo
of age mean BW for the H and BSH heifers were similar (P >
0.05, Figure 1). At 6 mo of age the H heifers in our study had
smaller mean BW than the industry standards reported by
Henrichs and Losinger (1998). However, by 14 mo of age the
H heifers in our study were approaching the industry standard
range for mean BW, probably due to compensatory growth
from 6 to 14 mo of age. It appears that the forage-based system
for heifer development was adequate for growth.
Henrichs and Losinger (1998) reported that H heifers developed
in the Southeast U.S. had lower BW when compared to
H heifers developed in other regions. Differences in photoperiod,
feeding strategies, and/or predominant forage types in
various regions of the U.S. could explain the differences in
growth (body weight:age) between the regions.
Hip Height. Skeletal size or frame development is often
emphasized as a key factor in replacement heifer rearing programs.
Traits that reflect long-bone growth may reflect true
size of replacement heifers better than BW because BW is
influenced by pregnancy and body condition. In our study the
interaction of breed type x age was significant for hip height
at 6 mo of age, but not at 14 mo of age (P > 0.05). The BSH
heifers were taller (P < 0.05) than the two other breed types
at 6 mo of age, but by 14 mo of age H and BSH heifers were
similar (P > 0.05) for mean hip height (Figure 2), and were
similar to the industry standards for range in mean wither
height (Heinrichs and Losinger, 1998). Because hip height
reaches maturity before wither weight in beef cattle (Brown
et al., 1983), it is expected that heifers in our study would be
taller at the hip than the industry standard for wither height at
6 mo of age. However the heifers in our study were shorter in
hip height than the industry standards for wither height
(Heinrichs and Losinger, 1998), indicating that our heifers
had not achieved sufficient long bone growth to 6 mo of age.
Mean hip heights for H, JH, and BSH heifers were 40, 38, and
39 inches, respectively, compared to an industry standard
range of 40 to 42 inches for H heifers measured at the withers
at 6 mo of age.
Depth of Chest. The interaction of breed type x age and
the main effect for breed type were not significant for depth
of chest at 6 or 14 mo of age. The three breed types ranked
highest to lowest for mean chest depth at 6 mo of age were
BSH, JH and H and at 14 mo of age were H, BSH, JH (Figure
3). Age was a significant (P < 0.05) source of variation for
depth of chest in this study.
Body Condition Score. Body condition scores are
important predictors of potential reproductive efficiencies of
dairy heifers. Mean body condition scores of the three breed
types of heifers were similar (P > 0.05, Figure 4). Mean body
condition score of heifers in our study exceeded (3.0 to 3.1
vs. 2.2 to 2.8) those of H heifers reported by Hoffman (1997).
Reproductive Performance. Reproductive performance
of heifers is important because of the proportion of heifers
culled for reproductive failure and because reproductive efficiency
determines how soon productive life begins. The
reproductive performance of three breed types of replacement
dairy heifers is presented in Figure 5. By 12 mo of age, the
JH breed group had the highest percentage (90%) of heifers
cycling, this was followed by BSH (75%), and then by H
(48%, P < 0.05). At about 15 mo of age, the percentage of
heifers pregnant was similar among the three breed types.
The breed types ranked from highest to lowest in percentage
pregnant were: BSH (96%), JH (87%) and H (77%). These
data are in agreement with results of previous studies indicating
that crossbreeding tends to improve reproductive efficiency
in dairy heifers. McDowell (1982) summarized dairy
cattle crossbreeding of the S-49 Southern Regional
Cooperative Research Project and concluded that crossbreeds
tended to surpass purebreds in overall breeding efficiency.
Wheat Pasture. Our data shows that wheat pasture may
be used for replacement dairy heifer development in
Arkansas. However, we acknowledge that these data represent
wheat production in one growing season; there are studies
with differing opinions about the consistency of production
patterns of wheat from season to season.
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