When dairy farmers extend the calving interval of high-productive heifers and cows until a maximum of 531 days, they can still achieve equal milk production per day to cows in shorter lactations in the same herd, according to research led by the Wageningen University & Research (WUR) animal nutrition group in the Netherlands. Fertility was the same when insemination was delayed.

The researchers recently published their findings in the Journal of Dairy Science as part of the "Customized Lactation Length" project.

In this study, researchers analyzed the insemination and milk data of 13 dairy farmers who voluntarily delay insemination for a portion of their herd. Ten of the 13 dairy farmers selected individual cows for an extended interval after calving until first insemination; the other three dairy farmers used a fixed voluntary waiting period until first insemination ranging from 80 to 110 days, WUR said.

The researchers found that animals with a high peak production and high 305-day yield most often ended up in longer lactations, the announcement said. This is probably partly due to the farmers' applied strategy and partly the result of lower fertility in highly productive animals, making it more difficult for them to conceive. Highly productive cows had not only longer lactations but also more days after calving until first insemination compared with relatively low-producing cows in the same herds, the researchers said.

If these high-producing cows were selected for extended lactations, they were capable of achieving milk yield per day similar to or greater than cows in shorter lactations in the same herd, WUR said. However, there was an optimum level: While the 305-day yield was greatest for the group with the longest calving interval (more than 531 days) on most farms, production per day was most often the greatest in a calving interval between 364 and 531 days, the researchers explained.

The optimal calving interval also seemed to be different per herd and per parity, the researchers noted. On most farms, for heifers, an interval of at least 196 days from calving until first insemination resulted in the greatest milk yield per day of calving interval when high-producing heifers were selected (greater than 7,500-11,000 kg per 305 days, depending on the farm), the researchers said. However, on most farms, for cows, an interval of minimal 140 days from calving until first insemination resulted in the greatest milk yield per day of calving interval when high-producing cows were selected (greater than 9,500-12,000 kg per 305 days, depending on the farm).

The researchers said it remains unclear what these high-producing animals would have achieved in a shorter interval until first insemination.

Fertility

According to the researchers, the expectation was that pregnancy rate would be increased with a delay in first insemination post-calving, because cows would have more time to recover from calving and the negative energy balance.

However, they noted that the percentage of pregnant animals after first insemination was not higher when this first insemination was delayed. A possible explanation is that cows on these farms were selected for delayed insemination based on a high milk peak or high lactation production, the researchers said.

By selecting these highly productive cows for an extended lactation and giving them more time after calving, it's possible that cows are inseminated at equal daily milk yield and an equal metabolic state, resulting in the same chance for pregnancy as cows with lower milk peak or lower lactation production, the researchers suggested.

Dairy Campus experiment

In addition to this observational study, the researchers are conducting an experiment at the WUR Dairy Campus in which they divide 150 cows over three voluntary waiting periods until insemination: 50, 125 and 200 days. The preliminary results of this study were published earlier this year.

WUR said the intention is that in 2021, the results of the observational study and results of the experiment will contribute to a decision support model for an optimal lactation length per individual cow.

The "Customized Lactation Length" research project is financed by DairyNL and the Netherlands Ministry of Agriculture, Nature & Food Quality. CRV contributed to the supply of data.