Effect of subclinical mastitis detected in the first month of lactation on somatic cell count linear scores, milk yield, fertility, and culling of dairy cows in certified organic herds

It is well established that subclinical mastitis (SCM), characterized by somatic cell count (SCC) >200,000 cells/mL, has a negative effect on the productivity, reproductive performance, and survivability of cows from conventional dairy herds. However, in organic herds, where the use of antimicrobial drugs is restricted for the treatment and control of intramammary infections (IMI) in dairy cows, little is known about the effect of SCM on performance and survivability. The objective of this study was to evaluate whether SCM diagnosed during the first month of lactation was associated with SCC linear score dynamics, milk production, fertility, and culling of dairy cows in USDA-certified organic herds. We collected data from 2 organic herds in New Mexico and Texas. A total of 1,511 cows that calved between June 2018 and May 2019 were included in the study and were followed until month 10 of the current lactation. Cows with SCC >200,000 cells/mL in the first month of lactation were considered to have SCM. We used mixed linear regression models accounting for repeated measures to assess the effect of SCM on monthly milk production and SCC linear scores. We used Cox proportional hazards models to evaluate the effect of SCM on the risk of pregnancy and culling. We considered parity, farm, previous gestation length, stillbirth, twinning, dystocia, and 2- and 3-way interactions as potential confounders. Cows diagnosed with SCM during the first month of lactation produced less milk than cows without SCM. Cows with SCM had elevated SCC linear scores during their previous lactation and throughout the subsequent months of lactation compared to cows without SCM. The effect of SCM on SCC linear scores was more pronounced in multiparous than primiparous cows. Subclinical mastitis during the first month of lactation did not affect the likelihood of pregnancy during the first 300 d in milk. Cows with SCM in the first month were more likely to die or be culled during the 300 d of lactation than cows without SCM. We observed that elevated SCC in the first month of lactation had detrimental effects on the milk yield and survivability of dairy cows in USDA organic herds, but it did not affect reproductive performance. We demonstrated that cows with SCM diagnosed in the first month of lactation continued to have elevated SCC linear scores throughout their entire lactation, and that elevated SCC was carried over from the previous lactation.