Some Factors Affecting Sex Ratio of Dairy Herds in East Azarbijan, Iran
Sh. Golzar Adabi,
The objective of this retrospective study was to investigate
the potential factors affecting sex ratio on dairy herds in East Azarbaijan
province of Iran. The data used in this experiment were collected from
186 registered dairy herds are routinely compiled on a monthly basis (from
June 1990 to July 2007). For statistical analyses, sex ratios were compared
with expected value using goodness-of-fit Chi-square ( χ2)
analysis. The effect of some factors such as herd size, artifical insemination
year and kind of frozen semen (internal and external ) on sex ratio was
determined. Statistical analysis of data shown that herd size had significant
effect on sex ratio (χ2 = 5.989, df = 4, p < 0.05).
The χ2-test for year variables were significant (χ2
= 52.496, df = 17, p < 0.01), Also the effect of frozen semen (internal
and external) on sex ratio was not significant (χ2 = 5.893,
df = 7, p > 0.05). Results suggested a positive effect of herd size
and year on sex ratio therefor it is possible using this result as a practical
and inexpensive method of sex ratio modification.
Sex ratio theory is one of the more developed areas
of evolutionary ecology, yet adaptive explanations for sex ratio variation
among vertebrates are still the subject of much debate and uncertainty,
partly because it can be difficult to infer process from pattern and adaptive
and non-adaptive explanations may be confounding (Ian and Hardy, 1997).
In animal production systems, the possibility of modify sex ratio can
result in a substantial increase of the production in intensive cattle
farms. Also, sex ratio manipulation can sensibly enhance the effectiveness
of selection and genetic improvement programs, through the differential
increment of males or females born after AI (Seidel, 2003).
Gutiérrez-Adán et al. (1999) suggested
that the differential ability of X-or Y-bearing spermatozoa to fertilise
oocytes depending either on time of insemination or oocyte maturation
state, may be due, at least partially, to intrinsic differences in the
physiological activity of X-or Y-bearing spermatozoa before fertilisation.
Trivers and Willard (1973) hypothesized that in species in which reproductive
success varies more among one sex than the other, mothers in better physiological
conditions would gain an advantage by investing more heavily in the more
variable sex. Similarly, mothers with limited resources would gain an
advantage by investing in the more reproductively stable sex, thereby
ensuring a continuation of the genetic line. Although many studies have
reported support for this hypothesis (Berry and Cromie, 2006; Sheldon
and West, 2004; Roche et al., 2006a). Recent results by Roche et
al. (2006b) confirmed positive effect of maternal Body Condition Score
(BCS) on the Secondary Sex Ratio (SSR) in dairy cows, indicating that
despite domestication of the species, they conform to the Trivers-Willard
hypothesis. One of the critical parameters for reproduction is the energy
availability of the mother (Wade and Schneider, 1992). Because of this,
it is possible that variation in maternal energy storage is associated
with a modification of the secondary sex ratio.
Fisher`s (1930) theory dictated that maternal investment
in male and female offspring is similar and that secondary sex ratio (SSR,
the proportion of males to females at birth) should be 50:50 if one sex
does not require greater maternal investment than the other. Nevertheless,
there is compelling evidence to conclude that, under certain conditions,
natural selection favors systematic deviations from this expected 50:50
sex ratio. Furthermore, factors as diverse as latitude of residence, ethnicity,
dominant weather patterns, timing and frequency of coitus relative to
ovulation, diet; paternal age and alcohol consumption, parental age gap,
maternal blood type, BCS, vaginal pH and the systemizing and empathizing
skills of the dam have all been statistically associated with altered
SSR in mammals (Roche et al., 2006a).
The prediction that females in better body condition
would produce more male than female progeny has been observed in red deer,
roe deer, mature ewes, reindeer, Barbary sheep, domestic pigs and a number
of other species, although there are exceptions (Cheryl et al.,
2004). The data on roe deer (Wauters et al., 1995) were obtained
with farmed animals on a diet controlled for low-and high-energy intake
by varying the oil content. In that study, 75% of the calves born to the
high-energy does were male, while the low-energy group produced only 46%
males. Some recent studies, however, indicate that, within species, the
sex ratio varies with the costs or benefits of producing male or female
The aim of this retrospective study was investigate the
potential some factors affecting sex ratio on dairy herds in East Azarbaijan
province of Iran.
MATERIALS AND METHODS
The data used in this experiment were collected
from 186 registered dairy herds are routinely compiled on a monthly basis
(from June 1990 to July 2007 ) by the Jahad Agricultural Organization
(JAO) of East Azarbaijan. Data were recorded by the farm staff on a prepared
sheets. The validity and reliability of records were checked regularly
by JAO personnel. Although the herds have different management practices,
all of them were crossbred or Holsteins and non-seasonally calving dairy
herds that were artificially insemination as routine.
The animals were inseminated at different times from
the detection of the onset of estrus (between 8 and 44 h). The insemination
was carried out using standard methodology for cattle, using frozen/thawed
semen, which was deposited in the uterus. Estrus onset was detected and
noted down by an experienced person, who carried out continue observations
throughout the day, watching behavior and clinical and gynecological symptoms
characteristic of the estrus in females of this species (Hafez and Hafez,
Sex ratio was defined as the ratio of male to female
live births multiplied by 100. the expected value for sex ratio was assumed
to be equal numbers of males and females. For statistical analyses, sex
ratios were compared with expected value using goodness-of-fit Chi-square
(χ2) analysis. Also, χ2-test for trend,
pearson χ2 and analysis of variance were used. All analysis
were done using SPSS soft ware (version 11.5). A p < 0.01 was considered
to be statistically significant.
RESULTS AND DISCUSSION
Over the study years, 25812 single live births were recorded, which
comprised 13138 male and 12674 female pregnancies. Statistical analyses
were done on all regardless of the number of calves, considering that
herd size has no significant effect on sex ratio. The frequency of the
number of live
|| Percent of live births by sex and herd size in dairy cattle in
East Azarbijan, Iran
|χ2 = 5.989, df = 4, p < 0.05
|| Percent of male and female by birth year in dairy cattle in East
|| Percent of male and female by internal and external sperm in dairy
cattle in East Azarbijan, Iran
births by sex and herd size has been shown in Table 1.
Deviation from the expected ratio was significant in herds (p < 0.05).
Also, the χ2-test for trend analysis was significant (χ2
= 5.989, df = 4, p < 0.05).
The effect of year variables during the study is summarized in Table
2. The χ2-test for trend analysis were significant
(χ2 = 52.496, df = 17, p < 0.01). One of the most important
changes during year is climatic factors (p < 0.05, data not show). The
odds ratios indicated positive associations between minimum, maximum air
temperature and evaporation rate and the likelihood of male calf subsequently
being born. Also the effect of kind of frozen semen on sex ratio is not
significant (χ2 = 5.893, df = 7, p > 0.05). The effect
of internal and external frozen semen on sex ratio are shown in Table
The control of sex ratio in farms conveys many advantages,
since it allows to sensibly improve the yield of the operations based
on the type of production (milk or meat). At this moment, there are many
techniques of proven effectiveness that allow to alter the normal sex
ratio, such as flow cytometry stored sperm. However, they are of little
practical use, because the cost of preparation of purified samples of
X or Y sperm is still excessively high (Martinez et al., 2004).
Furthermore, mares in better condition produced more
male foals than expected from an assumption of parity. Where sex ratios
differ significantly from parity in both directions, it is unlikely that
the difference is due solely to differential loss of less viable or more
costly fetuses-usually males in mammalian species (Ansari-Lari, 2006).
Alternatively, mechanisms that favor the differential conception of males
or females have been postulated (Krackow, 1995).
Hilder et al. (1944) said that there seems to
be no indication that the age of the parents has any particular effect
on the sex ratio of the offspring.
In Suadi Arabia, Ryan and Boland (1991) reported that
the sex of single calves from primiparous cows revealed no significant
deviation from the expected ratio. However, the sex ratio was biased towards
males offspring in multiparous cows (Foote, 1977). Owing to the small
sample size, assessment of sex ratio was not done separately in higher-parity
In present study, the linear trend revealed that the odds of male birthes
increases in parallel with herd size (Table 1). This
is in agreement with Ansari-Lari (2006) findings. It is apparent that
herd size per se cannot for this difference and some other factors closely
associated with herd size may be responsible. With respect to factors
such as condition of dam and nutritional status, it can be presumed that
animals would be kept in better circumstances in large dairies than in
smaller ones (Ansari-Lari, 2006). It is reported that dairy cows, but
not heifers, on a high plane of nutrition give birth to proportionately
more bull than female calves than cows on a poorer diet (Skjervold and
Repeat breeder cows, i.e., ones that have problems becoming
pregnant by artificial insemination, also tend to produce more males (King
et al., 1985).
In particular, a low pre-pregnancy weight is associated
with a deficit of males. The results are consistent with a recent reports
showing that in rural Ethiopia, well-fed women were more likely to conceive
males (Cagnacci et al., 2004), that the relationship between caloric
availability and sex ratio is observed across several countries (Williams
and Gloster, 1992) and that there is a higher energy demand for pregnacies
with male embryos (Tamimi et al., 2003). These report support the
theory of Trivers and Willard (1973), suggesting that a higher proportion
of females are produced in non-optimal maternal conditions. The effect
of pre-pregnacy weight on sex ratio can be exerted through a selection
of gametes as well as through an increase in the rate of male embryo abortions.
There is no obvious reason for the significant effect
of year on sex. Of course Roche et al. (2006a) revealed that one
contributing factor to the annual effect on sex ratio may be climate and
its associated effects on sex ratio.
In agreement with the present study Roche et al.
(2006b), using 34 years data on New Zealand dairy cows, reported a significant
effect of time of the year at conception on secondary sex ratio. Nonetheless,
Foote (1977) failed to report any significant effect of time of the year
on sex ratio in dairy cattle while Skjervold and James (1979) reported
a significant effect of calving season on sex ratio in parity 1 animals
The physiology underpinning an effect of climate on the
SSR adjustment is unknown. A number of physiological and endocrine traits
can be influenced by environmental factors. Similarly, some psychological
disorders have a pronounced annual rhythm (Lerchl, 1998), indicating climatic
Xu et al. (2000) reported a heritability of sex
ratio of 2%, suggesting that the permanent environmental effect of the
dam is the main contributor to the repeatability of sex ratio. Vandenbergh
and Huggett (1995) in an experiment of rodents reported a greater probability
of male offspring in dams born between two male siblings. They attributed
this effect to a possibly higher level of testosterone in the dam acquired
during development in the womb.
Research in humans has revealed a higher sex ratio in
mothers with greater testosterone levels (Grant, 1996). Nonetheless, not
all studies have documented an effect of previous neonate sex on the sex
of the subsequent birth (Berry and Cromie, 2006).
Skjervold and James (1979) reported a significant effect
of breed of sire on sex ratio in Norway. Although not statistically tested
in the study, Chi-square analysis of the data provided by Foote (1977)
identified a significantiy (p < 0.05) lower proportion (50.5%) of males
born from Holstein sires than from Jersey sires (52.8%).
Some researchers have studied the variation of the sex
ratio depending on the time of the mating relative to ovulation (Seidel,
2003; Rorie, 1999). Numerous efforts have been made to alter the sex of
calves by varying time of insemination (Martinez et al., 2004).
It has been suggested that early inseminations (i.e., far before ovulation)
would result in more female calves whereas late inseminations (i.e., close
to ovulation) would result in more male calves, due to different timing
of capacitation and survival time of the X-and Y-chromosome bearing spermatozoa
in the female reproductive tract (Martinez et al., 2004). However,
several other studies offer contradicting explanations for potential effects
of varying insemination time on sex ratio (Rorie et al., 1999).
Although the mechanisms underlying sex selection are
still unclear, the data seem to indicate that males are more fragile (Naeye
et al., 1971) and that a greater attrition is exerted on them when
reproductive/metabolic conditions are non-optimal (Crawford et al.,
1998). Cagnacci et al. (2004) indicated that maternal weight and
probably maternal metabolism, as important determinants of secondary sex
ratio in human offspring. In another study, 75% of the calves born to
a high energy group were male, while a low energy group produced only
46% males (Cheryl et al., 2004).
A study on New Zealand Holstein-Friesien dariy cattle
(Roche et al., 2006b) revealed a higher probability of male calves
in dams that lost less body condition score and body weight from calving
to conception. Although primiparous animals do not loose as much body
condition post-calving as multiparous cows (Berry et al., 2006).
The present study has a limitation. The cows BCS was
not recorded in farm charts and it was not possible to calculate the effect
of this factor on sex ratio. Morever there is some practical and inexpensive
method of sex ratio modification such as: time of artificial insemination,
dietary manipulation of dairy cattle and Body Condition Score (BSC) between
calving and conception.
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