Retrospective analysis of dry period length in Italian Holstein cows

This research studied the relevance of potential sources of variation for dry period length in Italian Holstein cows reared in northern Italy and investigated the effect of days dry (DD) on milk production and calving interval in the subsequent lactation. Field data of individual cow DD were merged: a) with information from the previous lactation (6832 lactations, 87 herds) and analysed to investigate factors influencing DD; b) with milk yield (MY) and calving interval (CI) of the subsequent lactation (≈4000 lactations, >80 herds) and analysed to investigate the effect of DD on subsequent lactation performance. Individual cow DD averaged 67±27d, with a median of 62d; nearly 20% of DD were greater than 80 d. Herd had the greatest impact on DD; average herd DD was 71 d and >50% of herds had a mean dry period length >70d. Longer DD were associated with second or later parity cows, lower daily MY at dryoff, and extremes in the length of the previous lactation, either short or long. After an adjustment for MY genetic merit, dry periods <35 d resulted in losses in subsequent lactation yield of ≈500kg and DD ranging between 35 and 55d were associated with a 200kg decrease when compared to DD that averaged 65d. DD>80d resulted in a 100kg increase in subsequent MY. However, yield lost in the subsequent lactation caused by average DD<65 d might be offset completely by the higher yield obtained in the previous lactation due to its longer length. Conversely, dry period length did not significantly affect ensuing CI. Therefore, data from this study and literature analysis suggest that a decrease in the duration of DD could be profitable for most herds considered. A general recommendation towards dry periods between 45 to 60d could be advised.