Friday, February 2, 2018

Selective dry cow therapy


It has been a while (long while) since I made an entry in this blog. All a result of a little too much other work. However, yesterday I heard that a recently published paper by Christian Scherpenzeel, co-authored by Theo Lam, Luuk Maas (at that time an MSc student at Wageningen University, who created the first model) and myself, was chosen to be the Editor’s choice of the Journal of Dairy Science. That is great to hear of course and a reason to write another entry for this blog. Maybe a start of new activity.

Optimizing dry cow therapy

The paper aimed to develop a mathematical model to look at optimal selection rules within a selective dry cow treatment situation.  The linear programming model could minimize the costs of mastitis associated with the dry period. This is done because there is a tradeoff between the costs of dry cow treatment and the costs of clinical mastitis in early lactation. We added an extra constraint: amount of allowed daily doses of AB. The model did show which cows (based upon somatic cell count at the end of lactation) should be treated with dry cow antibiotics given the constraints that we brought in.
Costs of clinical and subclinical mastitis as well as antimicrobial use were quantified. Based on data from a large field trial, a linear programming model was built with the goal to minimize the costs associated with antimicrobial use at drying off. The basic calculations were done on an ‘average’ example herd with different levels of bulk tank somatic cell count.
Economically optimal use of antimicrobials was determined while restricting the maximum percentage of cows dried of with antimicrobials from 100% to 0%. This restriction reveals the relationship between the maximum percentage of cows dried off with antibiotics and the economic consequences. 

Blanket dry cow therapy is not economically optimal

From an economic perspective, blanket dry cow treatment seems not to be the optimal approach of dry cow therapy, although differences between approaches were small. A thing we already knew because of the publications of Huijps et al. and Berry et al. in the early 2000’s.
With lower bulk tank somatic cell counts, more dry cow antimicrobials can be omitted without economic consequences.  The economic impact of reducing the percentage of clinical mastitis was found to be much larger than reducing the bulk tank somatic cell count. The optimal percentage of cows to be dried off with antimicrobials depends on the udder health situation, expressed as the bulk tank somatic cell count and the incidence of clinical mastitis. So we concluded that economics is not an argument against reduction of dry cow antimicrobials by applying selective dry cow treatment.

Consequences of these economic results

The economic differences between the optimum situation and blanket dry cow therapy are, however, small. As are the economic differences if we reduce the use of dry cow antibiotics further than the economic optimum. When we first identified this (and presented at an NMC meeting already in 2003 when NMC organized a special session on selective dry cow therapy during the annual meeting in Fort Worth, TX), the general choice in the veterinary world was to go for better udder health and accept the small economic difference in performance. Given societal discussions about the use of antibiotics in animal (and dairy) farming, in some countries, such as the Netherlands, selective dry cow therapy is embraced to reduce the use of antibiotics. 

The debate continues......

The discussion about selective dry cow therapy is being held in many countries and at many meetings, as well as in the NMC annual meeting that is held right now, in Tucson AZ. At that meeting a few different viewpoints were selected. Daryl Nydam (Cornell University) pleaded for the introduction of selective dry cow therapy, underbuilding that with an experiment where they found no differences in clinical mastitis and milk production in selected cows dried off only with teat sealant versus cows dried of with teat sealant in combination with antibiotics.  Larry Fox (Washington State University), on the other hand, pleaded that we have to be very selective with selective dry cow therapy. With an overview of literature he concludes that  blanket dry cow therapy is, although not with much difference, still better in guaranteeing good udder health after calving than selective dry cow therapy. Tine van Werven (Utrecht University) showed the results of the Dutch experience (country wide experiment) after the forced reduction of the use of antibiotics of 70 %. Not only the use of dry cow treatment reduced, also lactational treatment reduced. One could expect an decrease in udder health, but the bulk milk somatic cell count in 2017 is at an all time low (on average for the whole country). 17 % of the farms seem to not use any dry cow treatments anymore. These farms also had the lowest lactational use of antibiotics. All in all, selective dry cow therapy seems to be very well possible. Scott McDougal (Cognosco) presented an outlook from the New Zealand perspective. In New Zealand the veterinary association has stated that in 2020 dry cow therapy will only be used in the treatment of existing intramammary infections. There has been emphasis on selective dry cow therapy earlier, with (at least partly) a negative effect on bulk milk somatic cell count. Practical issues (very high numbers of cows to dry off in one day) make the application of selective dry cow therapy also difficult. It seems taking the last cow SCC before drying off is as good as other selection algorithms using cow records and cow SCC. 

My opinion

If you want to be on the safe side and maximize udder health, go for blanket dry cow therapy it is proven effective. But, if you want to reduce the use of antibiotics on you dairy farms (for instance to meet societal expectations and/or for the principle of prudent use of antibiotics), the introduction of selective dry cow therapy is an easy start, and you save some money as well. And finally, blanket dry cow therapy may never be a tool to compensate lousy dry cow management and housing. 

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