A retrospective study on the association between different lengths of the dry period and subclinical mastitis, milk yield, reproductive performance, and culling in Chilean dairy cows.

The objective of this study was to determine the association between different dry period lengths and somatic cell counts, milk yield, reproductive performance, and risk of early culling during the subsequent lactation of Chilean dairy cows. The length of the dry period was classified into 5 categories: 0 to 30 d, 31 to 52 d, 53 to 76 d, 77 to 142 d, and 143 to 250 d. Generalized mixed models were used and included herd as random effect. Time-to-event analyses were performed for evaluation of reproductive performance and culling risk. The odds of subclinical mastitis (log linear score, LNSCC≥4.5) during early lactation increased with extended dry periods (143 to 250 d) during first, second, and third test day compared with the reference dry period of 53 to 76 d [odds ratio (OR)=1.27, 1.16, and 1.31, respectively]. Short (0 to 30 d) and extended dry periods had a detrimental effect on early lactation and 305-d milk yield compared with the reference dry period. Longer dry periods were associated with increased number of days for calving-to-first service interval and calving to conception interval. Average calving-to-first service interval for short and extended dry period were 83 d and 89.4 d, respectively. Average days to conception were 127.8 d and 131.4 d for a dry period of 31 to 52 d and extended dry period, respectively. Similarly, the number of services per conception increased with length of previous dry period from 1.62 (31 to 52 d) to 2.44 (143 to 250 d). Cows with previous short and extended dry period had higher odds of culling when compared with cows in the reference group (OR=2.20 and 1.57, respectively). Compared with the reference group, cows in the dry period category 77 to 142 d had the highest odds of death followed by the category 143 to 250 d (OR=1.27 and 1.18, respectively).When death and live culling were combined, the highest odds of combined death and culling were for cows in the categories 0 to 30 d and 143 to 250 d (OR=1.63 and 1.44, respectively). We conclude that extended dry periods (143 to 250 d) increase the odds of subclinical mastitis occurrence during early lactation and have a negative association with reproductive performance. Short and extended dry periods were negatively associated with early lactation and 305-d milk yield and were related to increased overall culling when compared with the reference dry period.