مجله علوم دامی ایران
سال چهل و پنجم شماره 1 (بهار 1393)

A data file containing 607662 (317608) milk (open days) records belonging 294417 (177838) Holstein cows that were calved from 1983 to 2008 was used. The bivariate repeatability model was used for data analyzing so that herd، calving year، calving month، days in milk (only for milk trait) and calving age for each parity، were assumed as fixed effects and additive genetics and permanent environmental effects were considered as random. Phenotypic (genotypic) trends were estimate based on mean of phenotypic values (breeding values) on birth year by simple (segmented) regression. Heritability and repeatability for milk yield (open days) were obtained 0. 20 (0. 054) and 0. 42 (0. 120)، respectively. Phenotypic trend of milk yield was 116. 65 Kg per year (P<0. 05) but open days wasn’t followed any trend (P>0. 05). Genetic trend of milk yield before 1996 was estimated -4. 15 per year (P>0. 05)، but after 1996 milk yield has uptrend and was observed 43. 74 Kg per year (P<0. 05). Genetic trend of open days before 1996 wasn’t significant، but after 1996 genetic trend of this trait was estimated 0. 36 day per year (P<0. 05). Phonotypic correlation between these two traits was low but positive (0. 041) and genetic correlation estimated 0. 46 that shows more open days for high productive cows. Genetic correlation compared to the phenotypic correlation explains that the strong relationship between these traits was covered by environmental effects. According to the results of this study، it is suggested to pay more attention to the reproductive traits in addition to productive traits in breeding programs.

The data used in this research were according to survival rate records of 9700 lambs from 735 rams and 8082 ewes، which collected at Moghani Flock Breeding Research Station during the year 1365 to 1390. The frequency distribution causes of culling and survival distribution function of lambs to yearling age were estimated by SAS (2000). Results revealed that 49. 81 percent of total lambs were culled from the flock until the first year of life because of reasons including illness، lack of breed purity، etc. The mortality rate over this period was estimated 20. 81 percent which occurred 8. 03، 7. 5، 4. 43 and 0. 85 percent in first three months، second three months، third three months and fourth three months of lambs life، respectively. According to percentage of mortality rate، the accumulative survival of lambs from birth to yearling age was calculated 79. 19 percent. The regression coefficient of lambs survival from birth up to yearling age showed a monthly 1. 77 percent decrease in lambs survival of this breed. The most mortality rate occurred in first، fifth، second، fourth، sixth and ninth months of lambs life، respectively. In comparison with third and fourth three months of lambs life، the fatality in first and second three months were higher because of heavy rainfall and frigid weather in winter quarters.

Description of lactation curve were investigated using sine function، compare it with two functions (Dijekstra and Wood). The data contained 32233 dairy milk yield records from 93 first parity Iranian Holstein cows that had calving between Aug. 2008 and Jul. 2009. Actual milk production per day and subsequently the actual production of each animal were available in the first lactation. Lactation records of milk production per cow from 7 until the end of lactation was used for the study. The data was fitted by the Trust-Region method in MATLAB software. The models were tested for goodness of fit using sum of squares error (SSE)، root mean square error (RMSE) and coefficient of determination (R-square). Also expression for the feautures of the lactation curve were calculated by using mathematical’s formulaes. Goodness of fit and lactation features of models was subjected to analysis using the MIXED procedure of SAS (2001) that the random effect was effect of cow. The results showed that Sine function can be fitted to daily milk production records. Statistical analysis showed that significant differences exist between the goodness of models (P< 0. 01) and the best fitting was related to the Sine and Wood function. The random effect of cow was also significant (P< 0. 01) that reflects the variable response of each animal to each model. Numerically، all the estimated features of the lactation curve given by the Sine function were similar to Dijkstra and Wood functions. The results showed simple Sine function has high competence and ability to fit the daily records of milk production. Also، Sine function could describe the persistency of lactation much better than the other models. Biologically، the parameters of Sin function were acceptable and showed some part of lactation curve but all three functions couldn’t fit the first 30 days of lactation in the best way.