عنوان مقاله [English]
Introduction Secondary sex ratio (SSR) is the proportion of males to females at birth. It has been shown in many different mammalian species, many factors are associated with SSR. Changes in secondary sex ratio in dairy cows is considered economically important and the ability to change it could affect the revenues and profitability of a dairy farm. Thus, sperm or embryo sexing techniques in recent years has attracted more attention. Most breed of dairy cattle are more likely to have female calf is born to use them as replacement heifers and in order to maintain their productive herd number. On the contrary, when the goal is the production of meat, bull calves due to higher growth rates and production efficiency, are more convenient and more economically efficient. The aim of present study was to investigate some key factors affecting SSR in Iranian Holstein cows.
According to Fisher, the sex ratio in the population under the control of natural selection is not always the same. There is overwhelming evidence to support the theory that shows Fisher Primary and secondary sex ratio sex ratio can deviate from this balance and natural selection caused a change in this ratio can be in certain circumstances. For example, the secondary sex ratio of 52:48 has been reported in dairy cows. Studies on mammalian species suggest that several factors, including latitude of the location, the dominant regional climate model, time and frequency of mating to ovulation, diet, age of parents, physical score, breed and produced eggs from ovarian left or right can have a significant effect on the secondary sex ratio. Weather conditions may modify the internal environment and the effect on physiological mechanisms or through the impact on the frequency and type of foods available to parents, the secondary sex ratio is impressive. The impact on the quantity and quality of parent's access to food sources in many species of mammals, the sex ratio has been fixed. Previous reports have shown that high environmental temperature and higher rates of evaporation from a week to a month before conception secondary sex ratio was increased.
Materials and method The demographic, production and reproduction data of six large dairy farms between years 1375 and 1389 were used. The rolling average of 305 d fat corrected milk yield was 8145 kg with a range of 7578 to 8670 Kg. Breeding Center of Iran and the herds covered by the registration and recording them on a regular basis carried out by experts dairy cooperatives. The flocks were in the area northeast of the country. Production data (date corrected milk production and the cumulative production by 60 and 305 days of lactation), reproductive data (such as calving year, calving season, type of birth, sex of calf, the latest information on AI, days open) , and details the possible removal of the flock or death were recorded. Data were collected from flocks before the analysis was re-verification. Used by sperm, sperm into four groups: Iranian, Canadian, American and other countries (including sperm taken from European countries) groups. The data were analyzed using multivariate logistic regression model.
Statistics 0/05 was used to ensure meaningful results.
Results and discussion Results showed that the ratio of males to females was 53 to 47. Origin of the sires (including Iranian, American, or Canadian) had no impact on secondary sex ratio. Type of calving (eutocia vs. non-eutocia calving including dystocia and still birth) had significant impact on SSR (P