Phases of the dry period
The dry period is a crucial period of rest for the udder. Its ultimate objective is to optimise milk production in the next period of lactation.
- It is a time for regeneration of the milk-secreting tissue and preparation of the udder for optimal performance in the next lactation.
- The dry period gives the cow time to restore some of her body reserves for the upcoming lactation.
- If the cow has been infected with a pathogen during lactation, it is the perfect time to treat her with better chances of cure and fewer issues with the milk withdrawal period.
Phases of the dry period
The dry period can be divided into three periods of interest:
- The involution phase: milk production decreases when the milk is no longer physically removed. The remaining milk will be reabsorbed by the tissue, which is a gradual process that takes about 2 weeks to complete. The udder will be visibly “filled up” for approximately one week, after which it will shrink rapidly to its dry state. During this process, the composition of the mammary secretion changes: milk components, such as casein and milk fat, decline, while immunoglobulins and lactoferrin increase in concentration, providing a gradually increased passive immunity.
- The steady state: during this phase, the udder is in a period of rest. Very little secretion is left in the udder, with a high concentration of protective elements such as lactoferrin and immunoglobulin. Leukocytes are more effective since less debris or milk components are disturbing their work. Most teats are sealed with a keratin plug, although approximately 20 % of teats never seal during the dry period (Dingwell et al., 2004. Prev Vet Med 63, 75–89).
- The transition phase: starting two weeks before calving, the non-lactating udder redevelops, with formation of new secretion tissue and production of colostrum. Milk accumulates, giving rise to an increased intramammary pressure with possible reopening of the streak canal and leaking of milk. Local immunity in the udder decreases, while the cow’s general immunity is also compromised due to hormonal changes at calving.
The impact of the dry period on clinical mastitis
The peak of clinical mastitis during early lactation can be mainly attributed to the dry period. Infections acquired during the dry period have the ability to remain quiescent within the udder, subsequently causing clinical mastitis early in the next lactation.
The probable reason why animals do not develop clinical mastitis during dry period, although infection is very common, is that the mammary environment is not fit for bacterial growth. High lactoferrin and leucocyte concentrations prevent rapid multiplication, but not infections from occurring.
References: Bradley and Green 2004. The importance of the nonlactating period in the epidemiology of intramammary infection and strategies for prevention. Vet Clin Food Anim 20:547-568