Recent study shows calving heifers at 22 to 26 months results in an increased lifetime milk yield and increased survivability. Heifer rearing represents a significant cost to any dairy farm. It is at least two years before a replacement starts milking and begins paying back the cost of rearing.

However, there is big variation in age of first calving (AFC) in the national dairy herd. With this in mind, we set about assessing the impact of AFC on milk production and survival.

The purpose of this research was to assess the association between age at first calving and milk production and survival in dairy heifers.

Data from the ICBF database was evaluated on three categories of AFC: 22 to 26 months, 27 to 32 months and >32 months. Only animals with an age of first calving between 20 and 40 months were used. Year of calving selected was 2014 to analyse data on five full lactations and only spring-calving herds were chosen. The total number of animals included in the study was 160,334.

Figure 1 shows that the 27- to 32-month category is surpassing the other two categories on milk production. This can be related to the fact that they are older when first calved and are carrying more weight, so therefore have the ability to produce more milk.

In contrast, the over-32-month animals do not surpass the 27- to 32-month category. Older heifers can develop fatty udders before they calve down and this may affect their milk production for the rest of their lives, resulting in suppressed yields.

But why was she late calving in the first place? Sixty-nine percent of heifers calve down between 22 and 26 months, so it’s a very achievable target. On farms where females are not meeting this target, it may well be as a result of management issues.

Figure 2 displays the percentage of animals surviving to parities two, three, four, five and six for the three age categories. The survival rate is highest in the 22- to 26-month category, with 30% of these animals surviving to parity six, in comparison with 11% of the 27- to 32-month animals. This results in higher lifetime milk production, despite slightly lower yields in the early lactations.

As the survival rate is highest in the 22- to 26-month category, this category produces more milk compared with the other two categories. Milk production is lower in the first lactation. However, lifetime milk yield is significantly increased.

Economic impact

To assess the economics of AFC, we compared parity one of the optimal age group with the group over 32 months of age at first calving.

At the end of parity one, the older 32-month category produced an additional 9kg of milk solids per cow than the optimal group.

However, at the end of parity four, the optimal 22 to 26 month category produced an additional 71kg of milk solids when compared to the older than 32 months category. Based on a milk price of 30.5c/l, the difference in net outcome between the optimal 22 to 26 month and the older than 32 month category is €307 per animal over its lifetime (average of four lactations).

The difference in net outcome equates to €76 per lactation, if this was to occur in a 100 cow herd, there would be a loss of €7,600 from calving at greater than 32 months compared to the optimum of 22 to 26 months.