Genetic Analysis Of Somatic Cell Count In Early Lactation Of Holstein Cattle

The resistance to cow mastitis depends on genetic factors as well as physiological,nutritional and environmental factors,etc.It is difficult to directly use clinical mastitis as the resistance trait.In the world,the severity of mastitis is generally determined indirectly by the level of cow Somatic count cell(SCC).Since the 1990 s,developed countries in the dairy industry began to pay attention to the Somatic count cell in early lactation(SCCel).Studies have shown that the number of SCCel is affected by such factors as breed,parity,and length of dry milk period,and a high level of SCCel will lead to the permanent increase of cow’s milk Somatic cell number,which also has a negative impact on milk yield.However,there are few relevant studies on SCCEL in dairy cows in China,and the relationship between SCCEL and SCC and milk yield in each measurement day of lactation period is not clear in Holstein cattle in China.In addition,no specific genetic analysis of SCCEL has been reported in China.Therefore,this study collected SCC data and large-scale data of Holstein cattle accumulated in Beijing Dairy Cattle Center for a long time to explore the population pattern and influencing factors of the early lactation SCC of Holstein cattle.The relationship between SCCEL and SCC and milk yield during lactation was analyzed.Several early lactation SCS traits were defined,and the genetic parameters of these traits were estimated.The results of this study provided a new idea for the breeding of Holstein cattle mastitis resistance and provided a theoretical basis for the breeding of Holstein cattle somatic cell number traits in early lactation.Firstly,the SCC data collected and measured from 0-7 days postpartum of Holstein cattle in a scale ranch in Beijing were analyzed to analyze the population pattern of 0-7 days postpartum SCC of Holstein cattle,explore its influencing factors and preliminarily explore the regression relationship between it and the SCC measured at 4 months postpartum.The results showed that the SCC in early lactation(mean 860 ± 1 287 thousand /m L)was higher than the SCC measured at 8 to 133 days postpartum.Parity number,lactation days and ambient temperature and humidity had significant effects on SCC in early lactation(P <0.05).There was a significant regression relationship between early lactation somatic cell score(SCS)and measurement day SCS at 39 to 68 days postpartum(P < 0.05)with a regression coefficient of 0.4.The average value of SCC in early lactation(0-7 days postpartum)of Holstein cows was higher than that in other stages of lactation,and the SCC in early lactation of Holstein cows was sensitive to changes of different physiological states and environmental factors of cows.Secondly,the relationship between somatic cell number(SCCEL)in early lactation(6-35 days postpartum),somatic cell number(SCC)in different lactation stages and milk yield(MY)of Holstein cows was explored based on the long-term accumulated performance measurement records of Beijing Dairy Cattle Center.The results showed that the scale,season,number of parties and lactation days had significant effects on SCSEL(P <0.05).There was a significant positive regression relationship between SCSEL and SCS at each date of lactation(P < 0.01),and the regression coefficients ranged from 0.06 to0.19;There were significant negative regressions between SCSEL and lactation daily milk yield(P < 0.01)with regression coefficients ranging from-0.46 to-0.16.Therefore,a high level of SCCEL will affect the health and production performance of Holstein cattle during the whole lactation period,and pasture should pay attention to the management of SCCEL.Then,genetic analysis of SCCEL in Holstein cattle was performed using phenotypic data of SCC in Holstein cattle from 2008 to 2018 combined with pedigree data.The results showed that the heritability of SCSEL was 0.07 ± 0.03,0.04 ± 0.01,0.03 ± 0.00 and 0.05 ±0.00 in the first,second and third parities of Holstein cattle,respectively.SCSEL was a low heritability trait.There was a moderate to high genetic correlation between SCSELs of different parity,ranging from 0.54 to 0.87.There was a moderate or higher genetic correlation between SCSEL of different parity,and SCS of different parity in early lactation had different genetic basis.In view of the similar heritability between early lactation SCS and lactation SCS,as well as the predictive ability of lactation SCS,it is necessary to further study this trait in order to improve the selection method of mastitis resistance in Chinese dairy herds.In summary,this study analyzed the influencing factors and estimated the genetic parameters of somatic cell number traits in early lactation of Chinese Holstein cattle.The relationship between somatic cell number in early lactation,somatic cell number in lactation and daily milk yield of Chinese Holstein cattle was analyzed for the first time.The results of this study provide a theoretical basis for differentiated fine management according to SCC level in early lactation in postpartum cows,and lay a foundation for exploring the genetic mechanism of somatic cell number difference in early lactation in Holstein cattle.