For decades, dairy research and conventional management have pointed to 60 days as the optimal standard for dry period length between lactations.
But Barry Bradford, Professor of Dairy Management and Nutrition at Michigan State University, sought to challenge this traditional standard, armed with two arguments regarding the research data upon which that recommendation was formulated:
- The short dry periods in many of these studies were unplanned and likely mostly composed of cows that calved early for various reasons; and
- The suggestion that long dry periods result in lower milk production could be based on a false correlation between the two factors. Low milk production is often the cause of early dry-off. So, the data from older studies on long dry periods was likely based on cows that were lower producers in general.
At the Western Canadian Dairy Seminar, Bradford presented findings from a study he lead to examine these issues with a different lens.
Bradford and his team sought to conduct a fresh data analysis on gestation length and dry-period length that removed these biases. Added to the mix were variables including previous lactation length, and milk and component production across lactations. They evaluated a data set from 16 U.S. dairy herds totaling 32,182 lactation records.
They hypothesized that cows with a dry period deviating from their intended dry-period length due to biologically shorter or longer gestation lengths have more severe impacts on cow productivity compared to cows with management-intentional variations in dry-period length.
Their data mining from these cow records resulted in the following conclusions:
- Shorter gestation length is the main contributor to poor performance in the following lactation. In other words, calving early – which creates a shortened dry period – is associated with worse outcomes than the dry period length itself.
- There is little to no evidence of negative impacts from moderately short (40-50-day) dry periods, when gestation length is normal.
- Cows with greater production potential (e.g. those with higher previous-lactation milk yield) were most negatively impacted by short gestation.
- Managing for shorter dry periods (e.g. 45 days vs. 60) appears to be feasible, with a few caveats. Cows that dried off with high milk yield seemed to benefit the most from a “full” dry period. And cows with short dry periods and an average gestation length had, on average, greater somatic cell counts at first test.
- Cows with long lactations followed by long dry periods are at greater risk of removal after calving, likely due to poor metabolic health. These cows had a 24% increased removal rate compared with the reference population.
- The long-dry-period group also showed greater fat:protein ratios in early lactation, and these relationships were even more extreme for cows with both a long previous lactation and long dry period. Bradford said this data points clearly to a subset of cows that become over-conditioned prior to calving, resulting in excessive body fat mobilization, poor fertility, and greater culling rate.
Bradford said that because high-producing cows tend to benefit the most from a full-lengthy dry period, “delaying dry-off because of concerns about cows giving too much milk may be counter-productive.”
Finally, a rather compelling conclusion was that modern Holstein cows have an average gestation length of around 276 days, several days shorter than current references typically suggest.
