Discipline: lactation; Keywords: high milk yield, simulation model, extended lactation, calving interval, profitability.

Because of high milk yield, some farmers of high producing TMR-based dairy herds  service cows later in the lactation period, i.e. after the live weight nadir point when live weight starts to increase because of a positive energy balance. Often maximum feed intake has been reached and milk yield is decreasing following peak milk yield. Some research has shown a positive response in conception when cows are inseminated at this stage of the lactation period.  However, extending the voluntary waiting period to first insemination also extends the interval between calving and conception (days open). The lactation period is also extended (past 300 days), especially when cows are dried up at 50-60 days before the next expected calving date. Extending the voluntary waiting period also results in a longer calving interval. In some case farmers put in no effort to service cows before at least 120 days in milk.

The reasoning for this is based on the perception that cows producing milk during two extended lactation periods would be more profitable than cows producing during three standard lactation periods. This assumption goes against the general consensus in the literature that calving intervals should not exceed 12-13 months while a considerable effort should be put into extending the lifetime and productive lifetime of dairy cows.

What are the facts? This partially was the topic of a simulation study by Dr Carel Muller which he published in Agrikultuur 2018, with the title: Poor reproduction results in financial losses.

He developed lactation curves to compare the effect of calving interval, i.e. 12, 13 and 14 months, on the total and average daily milk yield of Holstein cows. In the model, the lactation starting, peak and milk yield persistency for cows were kept similar, being 30 and 50 kg/day and milk yield decreasing at 5% per month, respectively. Following each lactation period, a dry period of 60 days was included for each cow. To make provision for age effects, the starting milk yield at the second and third lactation period was 12 and 14%, respectively, higher than at first lactation. The production performance of cows was estimated up to the end of the third lactation using standard (300-day) lactation periods as a control.

The simulation showed that the total days in milk were the same for the three scenarios. The average daily milk yield differed, being 43.2, 42.8 and 41.8 kg per day, for cows having calving intervals of 365, 395 and 425 days, respectively. At a milk price of R5.00 per litre, milk yield losses amounted to R1940 and R6350 per lactation. Increasing starting and peak milk yields resulted in higher milk income losses. Furthermore, in practice, because three standard lactation periods compared to two extended lactation periods would entail a calving interval of 12 vs. 17 months, the milk income would be even less as more days in milk are at the end of the lactation period when milk yield is low.

To conclude: By increasing the voluntary waiting period the interval between calving and conception will be increased as well as the lactation period and calving interval, which will reduce the lifetime milk yield and milk income. This is because the extension of the lactation period occurs during the late lactation stage when milk yield is lower. For a higher lifetime milk yield cows should be in milk more times during the early lactation stage when milk yield and feed efficiency is higher.