By Lorna MacPherson, dairy consultant, Scotland’s Rural College

There is increasing concern about antimicrobial resistance, both in livestock and in human medicine and farmers in all livestock sectors are being strongly encouraged to reduce their antibiotic use where possible.

It is common practice to administer antibiotics to dairy cows at drying off to help clear up existing mammary infections and prevent new infections developing during the dry period. However, this blanket antibiotic approach is now considered to be unacceptable and a three-year Scottish Government Funded project run by SAC Consulting – part of Scotland’s Rural College (SRUC) – is currently looking at monitoring dairy herds that are reducing their antibiotic use through the practice of selective dry cow therapy (SDCT).

With SDCT, antibiotics are only administered if there is evidence of infection, usually based on somatic cell count (SCC). Mastitis history and other factors such as teat end damage may also be taken into account. Cows which have no evidence of infection are given an internal teat sealant to act as a barrier to the entry of bacteria.

This project is a joint collaboration between SAC Consulting, Müller, Zoetis and the University of Edinburgh. It is due for completion in December 2019, with dairy farmers supplying Müller in Aberdeenshire and Ayrshire taking part.

The effect of SDCT is being monitored through SCC pre- and post-calving and mastitis cases of dry period origin (cases occurring within the first 30 days after calving). Many of the participating farmers have been drying off as many as 80% of their cows without antibiotics and achieving good results in terms of dry period performance.

The dry period protection rate is a key measure of success and looks at the percentage of cows ending their lactation with a low cell count (last three SCC recordings below 200,000 cells/ml) and calve in with a low cell count at first recording (SCC below 200,000 cells/ml). The target for the dry period protection rate is 90% and the best farms are achieving this, both for cows on SDCT and ADCT (antibiotic dry cow therapy).

The percentage of cows that are dried off with a high cell count (one out of the last three recordings over 200,000 cells/ml) and calve in with a low cell count (below 200,000 cells/ml) is known at the dry period cure rate. The target is 80% and average results are only slightly lower for cows on SDCT.

This target is also being achieved by the best herds both in the SDCT group and the ADCT group, with one of these high performing herds drying off up to 76% of cows without antibiotics. It must be noted that dry period performance varies greatly from farm to farm and even herds using more antibiotics at drying off are still not achieving the targets.

Results from 13 herds are summarised in the table below).

Dry period protection rate Dry period cure rate

Average Range Average Range

SDCT 79% 71-91% 76% 64-94%

ADCT 91% 72-100% 79% 68-90%

Clinical cases of mastitis varied between the herds from eight to 61 cases/100 cows/year. Interestingly, the herd with the highest number of cases hit the 90% target for dry period protection rate for both the SDCT and ADCT group.

The clinical mastitis of dry period origin rate varied from farm to farm but ranged from 2.6% to 36%. However, more than half of the farms are well within the target of less than one in 12 or below 8.3% of dry period cases per 30-day period.

On some farms there was a higher percentage of mastitis of dry period origin from cows on SDCT. On other farms, the percentage was greater in cows that received antibiotics at drying off and so the effect of SDCT on mastitis of dry period origin also varies greatly between farms.

In fact, research shows that administering antibiotics to cows with low cell counts at the end of lactation (more than 200 at last three recordings) can actually increase the risk of mastitis (see graph).

Ultimately it is hoped that this project will build confidence in SDCT, showing that this practice is not detrimental to cow health and performance, and increase farmer engagement in this crucial area of reducing antibiotic use.