FLORIDA AGRICULTURAL EXPERIMENT STATION
Dairy Science Mimeo Report 64-1
September 18, 1963
Effect of Several Environmental Factors Upon Birth
Weights and Gestation Lengths of Dairy Calves
C. J. Wilcoxi. and J. A. Staffa2.
Knowledge of gestation lengths is important to the dairyman, as it permits
him to predict freshening dates and give his cows dry periods of proper
length. Birth weights are associated with livability of calves, diffi-
culties at parturition of dams, and early weight gains. Averages have
been known for each of the dairy breeds for some time, but there occurs
considerable variation in both characteristics. Although the literature
has been carefully searched by the authors, no attempt has been made to
list all of the references. Readers are referred to research and review
in 1952 by Brakel et al. (3), and several recent publications (8, 11, 12).
The right and left ovaries function at different frequencies. Clark (4)
reported that 42% of 704 cases of single pregnancy occurred in the left
uterine horn, and 58% in the right. Reece and Turner (23) found that the
right ovary of dairy cattle was functionally more active than the left,
and this accounted for the higher percentage of right uterine horn
pregnancies. Foote et al. (11) found that Holstein calves gestated in the
right uterine horn averaged 1.78 lb heavier (P <, 0.05). Horn pregnant
did not significantly affect gestation length. On the other hand, Foote
et al. (12) were unable to detect any effect of horn on birth weight or
gestation lengths of Angus or Shorthorns.
Fitch et al. (1) reported that male calves were 4 to 11 lb heavier than
females. Subsequent work has been in agreement (5, 11, 12, 18, 20, 27).
Most researchers have shown that male calves were gestated significantly
longer than females, the difference usually being about 1 day (1, 11, 12,
13, 15, 21, 22, 26), with a few exceptions (9, 17, 24, 25).
The size and age of the cow has generally been found to influence birth
weights (2, 10, 11, 12, 18, 20, 27). Immature cows produced smaller
calves, and gestation lengths have generally been shorter (1, 2, 10, 11,
13, 14, 15, 22, 26). For example, Laben et al. (16) found that first
gestations were 1.3 days shorter than second, and second, 1.2 days shorter
than third. Some (6, 17, 19, 21, 24, 25), however, were unable to detect
Season and year of calving may affect gestation lengths under certain
circumstances, since Brakel et al. (3) reported that the gestation periods
of cows calving in the spring averaged 2.07 days longer than those calving
Assistant Dairy Husbandman.
Working under the auspices of the National Science Foundation Summer
Science Research Participation Program for Secondary Students.
in the fall. Herman et al. (13) reported that calves born in fall and
winter were carried 1 to 3 days longer than those born in spring and
summer. Stallcup et al. (26) found that gestations of Jerseys calving
in spring were 3.09 days longer than those calving in autumn. They did
not find a season effect with Holsteins, however. A number of other
researchers have not found that birth weights and gestation lengths were
affected by year and season.
The relationships of gestation length and birth weights to each other have
been studied. For example, Davis et al. (7) showed them to be positively
correlated. The regression of birth weight on gestation length was found
to be 0.36 lb per day by Rollins et al. (22). DeFries et al. (8) suggested
that calves which were gestated 1 day less than average were about 1 lb
lighter than average. Regression of gestation length on birth weight
averaged 0.16 days. These researchers correctly pointed out that it was
difficult to establish a definite cause and effect relationship between
these two variables.
The objectives of this investigation were to measure and evaluate the
influence of several environmental factors upon the gestation lengths and
birth weights of dairy calves born to first-calf heifers. Factors
considered were sex, age of dam in months, horn of pregnancy, and year and
season of parturition.
Estimates of effects were obtained by analysis of multiple covariance.
Variances and covariances were computed on a within breed-year-season
basis, which should substantially free the estimates of the remaining
effects from possible confounding with breed, year, and season. The year
was considered to be made up of four seasons: 1, December, January,
February, etc. Birth weight was included as an independent variable in
the study of gestation lengths; in the study of birth weights, gestation
length was assumed to be an independent variable.
The data were gathered from the records of breeding and parturitions of
first-calf heifers in the Florida Agricultural Experiment Station for
the years 1960-62. Guernseys, Holsteins, and Jerseys were used; 114 cases
were available for analysis. Calves dead at birth, twins, and gestation
periods of less than 250 days were not included.
RESULTS AND DISCUSSION
The mean gestation lengths and birth weights for the three breeds of
cattle are presented in Table 1. Gestation lengths were surprisingly
close to recognized values. Birth weights were somewhat lower than
accepted values; this perhaps can be attributed to the small sample
available for analysis and the fact that the cows were first-calf heifers.
Table 1. Mean Gestation Lengths and Birth Weights
Breed Cases Gestation Length Birth Weight
(No.) (Days) (s)a. (Lb) (s)a.
Jersey 54 279.3 4.3 47.5 5.8
Guernsey 27 284.4 4.7 69.2 7.4
Holstein 33 278.4 4.8 79.9 10.3
a. Standard deviations.
Table 2 presents estimates of effects on gestation length for each breed,
and for breeds combined (pooled). The only significant individual effect
was sex in Holsteins; females were gestated about 1 day less than males.
Effects of Various Factors Upon Gestation Length
RZ = coefficient
* P < 0.05
of multiple determination.
** P < 0.01
The analysis of the pooled within-breed variation revealed that the linear
birth weight effect was highly significant. An appreciable amount of
variation, 29-52%, was accounted for by the effects measured. Values in
the table are partial regression coefficients, thus estimates of each
effect have been freed of the influence of the other effects.
Results of the individual and pooled analyses on birth weight are shown
in Table 3. No individual effect was found to be significant. However,
Effects of Various Factors Upon Birth Weight
of multiple determination.
R = coefficient
** P < 0.01
the pooled estimates showed several highly significant values. Females
weighed about 4 lb less than males. The effect of age of dam was markedly
curvilinear. A curvilinear effect of gestation length on birth weight
was apparent also. Individual and pooled effects accounted for 37 to 52%
of the variation in birth weight.
Records of 114 Guernsey, Holstein, and Jersey calves born in the Florida
Agricultural Experiment Station herd from 1960-62 were used to study the
effects of several environmental factors upon birth weights and gestation
lengths. Mean gestation lengths and birth weights were in general agreement
with published values.
Because of the limited number of cases for the individual breeds, few
significant effects were detected. When estimates were pooled, however,
an appreciable amount of the variation in gestation length was shown to be
associated with variation in birth weights. Sex, horn, and age effects
were not significant, however. With birth weights, all estimates except
horn were significant. Females weighed 4 lb less than males; age and
gestation length effects were curvilinear.
In investigations concerning birth weights and gestation lengths, this
research suggests that consideration be given to the factors shown to
have influenced them. The curvilinearity of several of these effects has
frequently been ignored in past research.
The authors wish to acknowledge the partial support of the National Science
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