Does the length of the dry period affect milk quality?

We have a renewed interest in the optimal dry period length for dairy cows. The average dry period length in the US is 57 days but variation exists. DHIA data say that on average, 13% of cows are dry less than 40 days and 26% are dry more than 70 days.

The dry period allows the udder to go through a period of involution and ensures that the numbers of mammary cells continue to increase normally during early lactation. The number of mammary cells is a major factor determining milk yield.

The length of the dry period has consequences for milk production, feed intake, labor needs, animal health, and milk quality in the current and subsequent lactation. Many studies have looked at various biological aspects of changing the dry period length. For example, a dry period length less than 60 days increases milk production in the current lactation but may reduce milk production in the subsequent lactation, especially in first parity cows. For older cows, shorter dry periods down to 40 days has a negligible effect on milk yield in the subsequent lactation.

The economics of dry period length are a balance between profit in the current lactation and profit in the subsequent lactation. Another aspect of dry period length economics is the opportunity cost when parlor capacity or robot capacity is limited. By drying off a low producing cow earlier, a dairy farmer creates an opportunity to milk a higher producing cow instead. Milk yield per minute of milking time is then increased, which may increase profitability.

One factor to consider is the relationship between the dry period length and mastitis and milk quality (somatic cell count, SCC) in the current and subsequent lactation. We reviewed some of the literature. In these studies, all cows received standard dry cow therapy. There is little information about udder health of cows managed for shortened or eliminated dry periods, however.

Therapy at the beginning of the dry period is important for udder health, with the goal to cure existing udder infections but also to prevent new infections. For management reasons, the dry period is an ideal period to treat udder infections because the cow is not being milked. Dry period length has no effect on the likelihood of curing existing infections. Still, the majority of new subclinical and clinical mastitis cases occur during the dry period and persist into the subsequent lactation. The incidence of new udder infections is increased shortly after dry off and again just prior to calving. Dry period length has no effect on the risk of new udder infections.

Increased milk yield at dry off has been linked to an increased risk of new udder infections in the dry period and at calving, mainly because of increased risk of leaking milk and pressure in the udder. This link supports a short dry period. However, SCC is generally substantially increased during the last 2 months of gestation if the cow continues to be milked. This is probably due to a concentration effect because cows produce less milk later in gestation.

There are no reports that shorter dry periods increase mastitis incidence in the subsequent lactation. In addition, shortening or eliminating the dry period has no effect on SCC levels in the subsequent lactation. Presence of antibiotic residues in milk of cows managed for short dry periods is rare and not different from cows that are managed for traditional dry period lengths.

Collectively, the scientific studies reviewed show that dairy farmers can change the dry period length with little or no effects on mastitis incidence and SCC.