جستجوی مقالات مرتبط با کلیدواژه "رکورد روز آزمون" در نشریات گروه "علوم دام"

 In dairy cows, milk yield changes over the lactation period. Lactation curve modelling could be of importance from nutritional management and genetic selection point of view. The trajectory shape of the lactation curve has inclining and declining slopes which determine total amount of milk yield during the lactation. A great number of linear and nonlinear mathematical models have been so far utilized to describe the shape of the lactation curve in dairy cows. Among the models, mechanistic functions are considered to be more accurate than empirical models in terms of taking account of biological mechanisms undertaken in mammary gland to produce milk. To our knowledge, no research has been carried out to use a complex mechanistic model for describing the lactation curve of Iranian dairy cows. Based on this, the present research aimed to apply a five-parameter nonlinear mechanistic model for mathematical description of the shape of the lactation curve in Iranian first-parity dairy cows.

Animal Breeding Centre of Iran provided the data used in this research. Initial data in separate files were merged and edited by the SPSS software. Final data comprised 5596039 milk test day records from 821153 first-parity cows distributed in 579 herds of 26 provinces over the country. The cows were the progeny of 7957 sires and 530394 dams and calved between 1996 and 2020. The cows were categorized into two groups based on the age of first calving: <=25 or >25 months. Also, two groups of cows were created based on the proportion of Holstein gene inheritance: <100% (grade cows) or 100% (pure Holstein cows). A five-parameter nonlinear mechanistic model was applied to mathematically describe the shape of the lactation curve. The fitted mechanistic model had five parameters including MSmax (maximum milk secretion potential of the lactation), GR (relative proliferation rate of secretory cell number during early lactation), MSLmax (maximum secretion loss), NOD (proportion of parenchyma cell dead at parturition), and DR (relative decline rate in cell number) which their estimates were obtained for different seasons and ages of calving, as well as different genotypes (grade and pure Holstein) using NLIN procedure of the SAS software. Based on the estimated parameters of the model in each year of calving, phenotypic trends were calculated using the SPSS software.

The results indicated that Iranian first-parity dairy cows reach to peak of milk production during the third month of the lactation curve. Based on standard deviation as well as coefficient of variation (CV) of milk test day milk records, maximum variation was observed in the last month of the lactation period. The findings also indicated that cows calved in autumn had the greatest MSmax and GR while cows calved in spring and summer had the lowest magnitude for these parameters (P<0.05). The minimum magnitude of MSLmax and NOD were detected for the cows calved in summer (P<0.05). Regarding the DR parameter of the model, cows calved in spring were observed to have a minimum value. Maximum MSmax, Gr, and DR values ​​were determined for cows calving up to 25 months of age, while maximum MSLmax and NOD values ​​were observed for cows of later ages (P<0.05). Compared to pure Holstein cows, grade cows had higher MSLmax and NOD values, but other parameters were found to be greater in the pure Holstein cows (P<0.05), suggesting that the maximum milk secretion potential during lactation in pure Holstein cows is expected to be higher than that of grade cows. Annual phenotypic change trends were found to be 0.376 kg/year (R2=0.9), 0.00009869 (R2=071), -0.146 kg/year (R2=0.59), -0.011 (R2=0.85), and 0.001 (R2=0.85) for MSmax, GR, MSLmax, NOD, and DR, respectively (P<0.0001).     

This study found that the parameters of the mechanistic model fitted to the milk test day records of Iranian primiparous dairy cows are significantly influenced by the age and season of calving as well as the genotype of the cow. Cows calving in summer are expected to have more persistency. Moreover, pure Holsteins are more persistent than grade cows. Positive and negative annual trends have been detected for MSmax and MSLmax parameters, respectively, during 1996-2020 indicating a favorable increase in maximum milk secretion potential of the lactation and also a favorable decrease in maximum secretion loss over that period.

The objective of this study was to compare some nonlinear functions (Wood, Dhanoa, Wilmink, Ali-Schaeffer, Cappio Borlino, Cobby – Le Du, Dijkstra, Rook, Gous and Nelder) to describe the milk production curve of Iranian Holstein cattle. A dataset consisted of 6079976, 4879486 and 3312416 test-day milk yield records related to first, second and third three lactation periods, respectively, from 3550 herds collecting by the Animal Breeding Center of Iran from 1983 to 2017, were used. The average of test day milk records for three lactation was 31.17, 34.08 and 33.83 kg, respectively. The nlme package of R software (version 3.4.3) was used for fitting nonlinear functions. The nonlinear functions were compared using four goodness of fit criteria, including Akaike’s information criterion (AIC), Bayesian information criterion (BIC), Root mean square error (RMSE) and Durbin-Watson index (DW). The Rook function showed the best fit for the milk production curve shape for three lactations in Iranian Holstein cattle when compared to other functions. The Gous and Rook functions showed the highest accuracy in predicting peak time, peak yield and persistency of milk production parameters in different lactations, but in general, the Rook function has a high predictive value in estimating the milk curve parameter descriptors.Therefore, Rook function is recommended for describing the milk production curve of Iranian Holstein cattle.

In dairy cattle enterprise, mastitis is a common and economically important disease leading to milk loss, therapeutic cost, changing of milk quality, reproductive problems, animal death, and early culling. A wide range of cow-level and herd-level factors can influence on the number of somatic cell count (SCC) including udder infection, lactation stage, age, breed, short-term of days in milk, season, stress and level of production. This research aimed to estimate the effects of SCC on daily milk yield (DMY) at different lactation stages and months of production of first-parity cows (progeny of internal and external sperms). Variation of DMY in terms of Holstein gene inheritance and first calving age were special objectives.
Material and The data used in this research were provided by the Animal Breeding Centre, Karaj, Iran. Data set comprised a total number of 543,884 test day records collected from 64,634 first-parity dairy cows (progeny of 1,899 sires and 56,995 dams) distributed in 606 herds and calved between 2009-2013 (in ten provinces over the country). The trait under consideration (dependent variable) was DMY which is actually a quantitative variable with continuous distribution and measured based on ratio scale. The records were analysed using a linear fixed model. In the model, the effects of province, herd nested within province, year of calving, stage of lactation, sperm type of the sire, two-way interaction between stage of lactation and sperm type, as well as linear covariable of SCC within the two-way interaction, Holstein gene inheritance, and first calving age were taken into consideration. The model was fitted on the test day records of the cows for individual months of production using general linear model procedure of SAS software (Version 9.4) and the effect of SCC was estimated as the partial regression coefficient in 30 combined groups resulted from two-way interaction between stage of lactation and sperm type.
Most of the partial regression coefficients were statistically significant (P Daily milk yield of Iranian, American and Canadian sperms’ progeny decreases when somatic cell count of their milk increases and that it has no regular pattern at different lactation stages as well as different production months. Holstein gene inheritance and first calving age had positive effect on daily milk yield suggesting that these effects are needed to be taken into account as phenotypic and genetic analyses are carried out. Yearly milk production of daughters of the American sperms are apparently more suffered as compared to the other groups as a result of increasing SCC in DMY.