Associations of somatic cell count with milk yield and reproductive performance in grazing dairy cows
Description
Poor udder health status can have a detrimental effect on milk yield and reproductive performance leading to reductions in the dairy farm profit. The objective of this retrospective longitudinal study was to assess the associations of somatic cell count (SCC) with daily milk yield and reproductive performance. A database having 1,930,376 lactations from 867 Argentinean grazing dairy herds records collected for 15 years was used. The association of the evolution of SCC (healthy vs. new case vs. cured vs. chronic; with 150,000 SCC/mL as threshold) and of the severity of SCC [mild (150,000 to 400,000 SCC/mL) vs. moderate (400,000 to 1,000,000 SCC/mL) vs. severe (>1,000,000 SCC/mL)] with the odds for conception were estimated. Also, the insemination rate by 200 DIM was estimated depending on SCC category (<150K or >150K), included as a time-dependent variable in Cox regression analysis. Finally, the associations of the linear score of SCC (LS-SCC) with daily milk yield were estimated depending on parity and milk production quartile
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Production, reproduction, and health records were periodically gathered by the official dairy herd improvement association (Asociación de la Regional Pampeana de Entidades de Control Lechero [ARPECOL]) following the gubernamental normative (ACHA, 2018) complying the rules of ICAR (International Committee for Animal Recording). Data for all the lactations started between January 1, 2000, and December 31, 2014, that included periodic measures (30 or 40 days apart) of milk yield and fat %, protein %, and SCC were extracted from commercial software (DIRSA S.A., Gonnet, Argentina) and centralized into a unique database (1,930,376 lactations from 867 herds). The official Reproductive data included the dates of service, positive pregnancy diagnosis, and calving (ACHA, 1981). Reproductive data fulfilling the requirements of “Calf Registry” that is a working traceability system having the purpose of individual identification, registration and official certification of the offspring from the different dairy breeds used in Argentina following the guidelines of the international livestock organizations (WHFF, ICAR, INTERBUL). Lactations should meet all the following reproductive criteria to be included in the study: having at least one AI recorded; cows with a positive pregnancy diagnostic must have a subsequent calving record; and, if new calving took place, the interval between the last insemination and the new calving must be between 260 and 290 d. To be included in the insemination rate analysis , lactations must have at least one SCC record every 40 days until 200 DIM; and, finally, to be included in the conception risk analysis, lactations must have at least one SCC record 43 d before service and a second one 30 d after the service. Also, a herd-year effect was included in the analysis if at least 50% of their lactations fulfilled the previous requirements. In short, a set having 432,432 lactations from 769 herds were included in the analysis of insemination rate, and 802,066 services from 580 herds were included in conception risk analysis ; for milk yield analysis lactations must have on average one SCC measure every 40 days to be included in the study, and the herd-year was included if at least 50% of their lactations fulfilled such requirement. There were 4,061,230 daily measures of milk and SCC, belonging to 448,041 lactations from 699 herds, that could be included in the analysis. Because of operational reasons for each lactation group, a subsample of 29.830 lactations (208,810 milk and SCC daily measures) belonging to 159 herds were selected for each parity group(1, 2>=3)