Effect of dry period on reproduction of dairy cows during subsequent lactation

Abstract

Possible advantages of reducing length of dry period includeincreased income from milk production, simplified dry cow management,decreased metabolic disorders, and alleviation of overcrowdeddry cow facilities. There is abundant data to support a 60-ddry period to maximize milk yield the next lactation. The greatmajority of data is from studies using farm records. In these data sets, cows with less than 6- to 8-wk dryperiods probably were not intended to have short dry periodsand consequently were not managed for short dry periods. Additionally,recommendations from observational data may be biased due tointeractions, e.g., between milk yield and length of dry period.Some experiments specifically designed to compare 30- and 60-ddry periods indicate that shorter dry periods are possible withoutsacrificing milk production the next lactation. There is a paucityof data to determine the effects of shortening the dry periodon milk composition, metabolic disorders, and reproductive performance.The limited amount of information that is available indicatesthat shortening the dry period will probably not have negativeeffects on these parameters and may have slight beneficial effects.In some cases, the beneficial effects may have resulted, inpart, due to negative effects on milk production. Cows withlonger calving intervals and older cows may be more likely toavoid negative effects of shortening dry periods on subsequentmilk yield. Shortening the dry period by initiating milkingafter a dry period but prior to parturition does not increasemilk yield or improve animal health sufficiently to warrantconsideration. Several studies indicated complete eliminationof the dry period results in a 20 to 25% decrease in milk yieldthe following lactation. However, these studies have utilizedlow cow numbers and cows with extremely low milk production.As milk yield and persistency of lactation increases, eitherthrough genetic selection or administration of exogenous agentssuch as bST, the likelihood of successfully shortening or eliminatingthe dry period should increase. Future studies to increase ourknowledge of factors affecting mammary epithelial cell proliferationand apoptosis should yield strategies to reduce the length orcompletely eliminate the dry period.