Short and sweet. Short dry periods examined by Chris Watson, vet at the Wood Vet Group, Gloucester

Consider this.  Experience in the US and on some farms in the UK demonstrates significant benefits from shortening the dry period and dispels many of the pre-conceptions around reduced performance/yield in subsequent lactations.  Interested?  Then read on…

There is always something rather challenging and exciting about questioning established principles.  That is exactly what is happening with the debate about dry cow management with many asking why we dry cows off and what is the ideal length for a dry period?

Mis conceptions

Mention shorter dry periods, and many vets will argue that subsequent lactation yields will be reduced.  However, much of the older information was biased by cows that had a short dry period due to problems such as abortion, twins, disease etc.  This subset of animals would have naturally had poorer subsequent lactations, not directly related to the length of the dry period but to the underlying problem producing it.  It was only when forward-looking studies were set up to look at this issue again that some good science replaced established agricultural ‘folklore’.

Studies set up to deliberately control the length of the dry period in cows before they were dried off showed that there were basic mis-conceptions about drying cows off. These can be logically considered under the following headings:

·        The physiology of the dry period – mammary gland and milk production

·        Dry cow feeding and managing groups

·        Mastitis control during the dry period

·        Fertility in the subsequent lactation

·        Health of the cow during the periparturient period

THE PHYSIOLOGY OF MILK PRODUCTION AND THE DRY PERIOD

Some textbooks still describe the initial part of the dry period as when the udder secretory cells (MEC - Mammary Epithelial Cells) are shed from the basement membrane by defoliation - the ‘dropping of leaves’ is the analogy being referred to. This is untrue in cattle.  The bovine udder largely stays fully competent with MECs during the dry period with a small amount of loss of older cells by apoptosis

(programmed cell death) and regeneration of a few new cells.  These small changes may be important as older cells perform less well during lactation and it is the new MECs that have the capability of replicating, replacing dead cells on the basement membrane, to keep up the quota of MEC for secretion.  This means that the cellular structure of the udder is on the whole fully maintained throughout the dry period and is capable of milk production with little change to the secretory cell structure.  If studies are designed to randomly allocate different dry periods to cows and then assess the subsequent lactation yield it looks as if there is a consistent picture emerging:

·     For dry periods down to 30 days there is little or no change in the subsequent milk production.  Some studies show a 3% decrease in yield, but others show no change.

·     For cows that have no dry period there is a subsequent reduction in milk yield of around 20-25%.This is a consistent and large effect. 

Why is the subsequent lactation being affected?  It seems likely that the small changes that are occurring to the MEC during the dry period could affect the future secretory capability of the gland.  Very short dry periods do not allow full replacement of the older cells by the gland and this may reduce functional secretion in the next lactation. Also there may not be time for new cells to be formed so there is less ability to replicate new MECs during the subsequent lactation as cells are lost. 

Short dry periods tend to produce improvements in the milk solids being produced so the reduced yield at very few days dry has less economic impact.  This means that even at 30 days dry there are potential economic net gains from milk production despite the appearance of small lactational losses after calving.  The net effect of a 30 day dry period can be a gain in yield of 6% overall due to the extra days in milk at the end of lactation.

MASTITIS

So what of the link between shortening the dry period and mastitis? As the length of time a cow is dry will determine the overall incidence of new infections, then shortening the dry period is likely to have a beneficial effect on mastitis status through decreasing the length of time for new IMI.  Recent studies look as if this is what is happening in the US herds that are starting to adopt short dry periods.

All the indications are that it should be a benefit to be dry for a shorter period of time.  The aim for dry cow mastitis control is to prevent new infection occurring and to remove existing infection from the gland if it is present. This means that if the dry period is shortened there are some issues we need to consider:

·     The withdrawal period of any antibiotic dry cow therapy (DCT) used.  The withdrawal period will determine the effective length that is possible for the dry period or we will have to choose products to suit the withdrawals we want to achieve.  What effect will the new, shorter dry period claim for Cephaguard® DC have on the market?

·     Enough time to establish a cure.  Both natural mechanisms and dry cow antibiotics must have long enough to act on any infection left from the previous lactation.

·     The risk of new infection at either end of the dry period. The initial part of the dry period is very reliant both on the udder’s natural defence systems being in place rapidly and the ability of DCT to protect.

Traditionally we have judged the success or otherwise of the dry period for controlling mastitis based on the cell count or bacteriology at drying off and again at the subsequent calving.  Although this is often presented as a rough estimate for dry cow performance based on easily recorded information (e.g. somatic cell counts) it cannot measure what is probably the most important factor - the dynamics of eliminating and gaining new infections (IMI - intramammary infections) during the dry period.  The udder is 10 times as likely to become infected with new organisms during the dry period, so there is the potential for a lot of new infection coming in as well as established infections curing.  In fact, the level of new IMIs during the dry period is much more likely to determine the overall balance of infection over the dry period, than is the cure rate from previous infections in the preceding lactation.

Work has shown that the cure rate in the dry period is likely to be very high except where established staphylococcal infection is present. This means that we are often wrongly assessing the dry period dynamics unless we have the ability to sample during the dry period to observe changes in infection which is obviously of no use in practice.

NUTRITION AND MANAGEMENT OF DRY COW FEEDING

Have we made life too complicated for the dry cow and ourselves?

The original aim of the dry period was nutritional - to increase body condition of the cow during this period. With modern feeding this is now not the case and we do not need to adjust condition score.

What we do need though is to:

·        Keep weight and condition constant

·        Prepare the cow for the next lactation

·        Minimise stressful events

Many units currently manage two dry cow groups with two rations being fed - a ‘near to’ ration often high in fibre to try and keep body condition constant, which is then changed.  To get the cow back ‘in gear’ for the next lactation usually means altering the ration prior to calving to shift them from high fibre diets over to the ‘transition’ ration which is more akin to the lactation requirement with higher energy levels. The principle is to try and get the rumen micro organisms and the rumen papillae ready for the next lactation.   Change is something cows don’t like, and can lead to problems.

The core issue of the dry period is the numerous changes that occur to the pregnant cow as she approaches parturition and the effect this has on her energy balance.  Poor energy balance can easily produce problems that are apparent after calving, with everything from metabolic disease to poor fertility and other health problems.  Negative energy balance usually starts to occur before the cow calves. This is in some small part due to the increasing demands of late pregnancy and udder development but the main problem is a decreased appetite. This is almost certainly due to hormonal changes associated with late pregnancy but changing diets and groups will make the issue far worse.

The aim must be to minimise the effects of the drop in DMI pre-calving and promote a rapid return to positive energy balance as soon as possible after

calving. Keeping the dry period short means that we do not need to worry about the dry cow getting too fat and the rumen papillae can be kept at the lactational state of readiness over the whole period without having to worry about the use of multiples transitional diets. Depending on the length of the dry period a single higher energy ration can be fed throughout which means there are no group or ration changes and thus less stress on the animals. Furthermore, there is just one dry cow group to manage.  Simplicity pays off.

Fertility

Many studies have shown the relationship between body condition score (BCS) and reproductive performance.  The magnitude of the BCS loss in the initial part of the lactation is positively related to many reproductive parameters. It is hardly surprising given the effect of short dry periods on BCS after calving that there may also be an effect on fertility.

There is a need to carry out more work to confirm this trend but the effect of shortening the dry period in one trial is shown in Graph 1 (see end).  Although the extremes of removing the dry period completely gave the best results there is still a beneficial effect at more moderate reductions.

CONCLUSIONS

There are significant potential benefits to be had from shortening the traditional dry period we apply to most dairy cows on our farms.  Most of these are due to the nutritional and management aspects of feeding and grouping the dry cow but the knock-on effect for fertility, periparturient health and possibly mastitis are significant.  It is unlikely that UK dairy farmers will move to such short dry periods as the US because we don’t have BST (bovine somatotrophin) and the perceived ‘ethics’ of not resting the cow after a lactation will have more impact here.  A more typical 35-40-day short dry period will be more likely, and interest seems to be gathering momentum.