Selective dry cow therapy effect on milk yield and somatic cell count: A retrospective cohort study.

Discipline: mastitis; Keywords: drying off, milk production, composite somatic cell count, time series analysis.

Mastitis is the most common disease in dairy cows, causing significant economic effects on milk production and compromising animal welfare. Antibiotic dry cow therapy (aDCT) is an effective way to control mastitis, as cows are susceptible to new within-udder infection at the time of drying-off and again during the immediate post-calving period. Selective dry cow therapy means that only infected or presumed infected cows are treated, instead of dry cow therapy being used as a treatment for all cows. Because antibiotic resistance poses a global threat, livestock production is under increasing pressure to reduce antibiotic use. However, changes in mastitis management should not impair animal welfare or cause significant economic losses. To investigate the pros and cons of these approaches, the objective of the study by Dr R.E. Niemi and colleagues was to compare milk yield and somatic cell count (SCC) between aDCT-treated and untreated cows in herds that used selective aDCT, taking into account risk factors for reduced yield and high SCC. Their results were published in the Journal of Dairy Science, Volume 105 of 2022, page 1387 to 1401; the title of the paper being: Selective dry cow therapy effect on milk yield and somatic cell count: A retrospective cohort study.

The information source was 2015 to 2017 Dairy Herd Improvement data, with 4720 multi-lactation cows from 172 Finnish dairy farms. The overall proportion of aDCT-treated cows was 25% (1176/4720). The response variables were test-day milk yield (kg per day) and naturally log-transformed composite SCC (× 1000 cells per mL) during the first 154 days in milk.

Due to interaction, SCC on the last test day prior to dry-off affected post-calving milk yield differently in aDCT-treated cows than in untreated cows. A higher SCC prior to dry-off correlated with a greater daily yield difference after calving between cows treated and untreated. The majority of cows had SCC of less than 200 000 cells per mL before dry-off, and as SCC before dry-off decreased, the difference in yield between aDCT-treated and untreated cows decreased. The post-calving SCC was lower for aDCT-treated cows compared with untreated cows. For instance, for cows with an SCC of 200 000 cells per mL before dry-off, compared with untreated cows, aDCT treated cows produced 0.97 kg per day more milk and, at 45 days in milk, had an SCC that was 20 000 cells per mL lower. Higher late-lactation SCC and within lactation mastitis treatments were associated with higher post-calving SCC. A dry period lasting more than 30 days was associated with higher yields but not with SCC.

The results indicate that a missed aDCT treatment for a high-SCC cow has a negative effect on the subsequent milk yield and SCC, which emphasizes that the selection of cows to be treated should be done carefully.