The mammary gland of the dairy cow needs the dry period to optimise milk production. This is not just to maintain cow condition. The udder needs time to rest and recharge.
In New Zealand having all (or at least a large proportion of) cows dry at once gives us a significant opportunity to improve milk quality. We can 'reset the clock' with the treatment or culling of infected cows.
The whole herd is at a very high risk of new mastitis infection, so there is also pressure to prevent new infections over the dry period to minimise the impact of mastitis in the following season.
The importance of the dry period to mastitis is why we use Dry Cow Therapy (DCT). While DCT is one of the most significant advances in mastitis control, getting all aspects of management right, throughout the dry period is crucial.
From the final milking until about 30 days into the dry period, many changes occur in the udder. The mammary gland continues to make milk for two to three days post dry off. After that, milk producing cells break down and the secretion of milk fat and proteins decreases markedly.
The amount of fluid in the udder decreases until day 16 - 30 of the dry period. The breakdown of the udder cells allows blood proteins to leak into the gland (this protein is a major part of the secretion found in the udder of the dry cow).
The incidence of mastitis in the middle of the dry period tends to be low. This is due to the high levels of antibacterial factors (self cure is often seen in the dry udder) and a reduced rate of access of bacteria into the teat due to the ‘plug’ of keratin formed in the teat canal. However, there is still opportunity for significant outbreaks of mastitis in the dry period – especially caused by Strep. uberis which is less affected by the antibacterial factors.
Bacteria picked up in the dry period can also be carried through and cause clinical mastitis after calving.
While the cow is ‘bagging up’ there is a great deal of change with the regeneration of milk producing cells, and formation of colostrum.
The concentration of the major components of milk (fat, casein and lactose) increases from two weeks prior to calving.
The volume of fluid in the gland increases, slowly in the last two weeks of the dry period then dramatically one to three days before calving.