Economic and environmental impact of dry period length in dairy cows

Shortening or omitting the dry period of dairy cows can improve cow welfare through fewer management changes and improved metabolic health in early lactation, at the cost of milk production. Part of the milk losses are compensated by additional milk before calving. The aim of this study was to model how a transition to a dry period of 28 or 0 days vs a dry period of 56 days affects net financial flows at herd level and greenhouse gas emissions per unit of milk. The evaluation included financial flows from milk, meat, calves, feed, and youngstock, and greenhouse gas emissions from feed production, enteric fermentation, and manure storage. In the stochastic model in R, dry period length affected milk yield, calving interval, and the risk of culling for fertility. Values for these parameters were derived from data from commercial dairy farms that shorten or omit the dry period since 2010/2011. Other parameters were based on literature. Herds of 100 cows were simulated at cow level for 5 years following the change in dry period length (n=40 herds per scenario). Total herd milk yield in the first year was not different among different dry period lengths (ANOVA, P>0.05), whereas a dry period of 28 days on average reduced yield by 2.9% from the second year onwards (P<0.05); and no dry period reduced yield by 7.0% in the second year (P<0.05), and by 3.7% from the third year onwards (P<0.05). At herd level, dry periods of 28 and 0 days in comparison with 56 days reduced net financial flows by €949 and €1,680 per year and resulted in similar greenhouse gas emissions per unit milk.