Revisiting age at first calving

The total associated cost of rearing replacement heifers is commonly reported to represent 15 to 20% of total operating expenses, turning it into one of the major expense items in a dairy enterprise. In growing herds or operations with high replacement rates, the rearing cost can be even higher. Justifiably, alternatives to decrease replacement costs should be investigated.

Keep in mind that the sustainable utilisation of farm resources has a maximum capacity. Ideally, most of these resources should be allocated to those animals that generate income, i.e. the milking cows. When replacement rates are high, more replacement heifers are required, and more resources are allocated to non-income generating activities.

In simple terms, lactating cows should stay in the herd for as long as possible to reduce the need for replacements, and replacement heifers should be managed to calve sustainably as early as possible to decrease the non-productive rearing period.

Proper replacement cows

Rearing proper replacement heifers is a costly exercise that has several objectives:

• a short non-productive life, i.e. getting pregnant and calving early;

• an extended productive life of optimal lifetime milk production;

• an efficient reproductive life with reduced days open after calving;

• adequate body size and weight at breeding and calving;

• ideal body conditioning at breeding and calving;

• produce replacement heifers of genetic merit; and

• low cost.

Reducing the non-productive days from birth to first calving could unlock cost savings for dairy producers. Many well-managed dairy operations already succeed in achieving age at first calving (AFC) of between 23–25 months, but can the AFC be further reduced to between 21–23 months without detrimental consequences to animal welfare, production, reproduction, and profitability? The growth potential of the modern dairy heifer can undoubtedly achieve the performance targets of 55% of mature body weight (MBW) at breeding, 94% of MBW pre-calving, and 85% of MBW post-calving at 21 months of age, but will this impact negatively on lifetime production or reproduction?

Negative impact on milk production

Earlier scientific literature reported that higher pre-pubertal growth rates in heifers are associated with lower milk production in the first lactation. Concerns were raised about the impact of fat deposition in the mammary gland and its negative effect on milk production. More recent research indicates that a lower milk yield in the first lactation is not correlated to high pre-pubertal growth rates, but rather to body condition.

Pregnant heifers can easily be overconditioned before calving, resulting in a higher propensity for the occurrence of metabolic disorders that will impact negatively on milk yield. A well-designed and properly implemented nutritional and genetic programme should be able to ensure that higher pre-pubertal growth rates materialise in lean muscle and skeletal growth instead of fat deposition. Once heifers have been confirmed pregnant, growth rates should be managed down slightly, with a keen eye on body condition to prevent overconditioning before calving.

First lactation

A published study from 2018 performed on more than 396 000 primiparous Holstein cows in the United Kingdom supported the generally accepted knowledge that an increased AFC will improve first lactation milk and solids yields. First lactation milk yield increased consistently from AFC at 21 months and reached a maximum at 36 months, and then plateaued up to 42 months. Importantly, in this study, lifetime daily milk yield peaked for the heifer group that calved at 22 months’ age, followed by the 21-month age group. From AFC at 23 months old, the lifetime daily milk yield decreased consistently for every older group to be > 20% lower for the 42-month-old group.

Although first lactation production was lower for the younger heifers, probably because more nutrients were directed towards growth instead of milk production, they compensated by achieving extended lactations. A study in the United States corroborates this finding by reporting that heifers calving at 21 and 22 months of age produced 510 kg to 632 kg more milk compared to heifers calving at 24 months old.

Calving interval

Age at first calving is also associated with calving interval. In the United Kingdom study from 2018, the intercalving period was stable between 400 and 405 days for heifers that calved from 21 to 26 months of age. For heifers with AFC from 27 months and older, the intercalving period increased consistently above 405 days. Also, an increase in AFC was associated with a higher probability of not calving for a second time. Another study reported that increasing AFC above 25 months of age reduced the number of cows that reached their third lactation. Heifers that calf for the first time beyond 26 months old should, thus, achieve higher milk yields in their first lactation but may struggle to become pregnant.

A large amount of scientific literature supports the optimum period for AFC to be between 22 and 24 months. As the genetic standard of dairy cows, nutritional programmes, and general management improves, critical thinking is encouraged to challenge the status quo. If a younger AFC is achievable, performance targets for heifers are achieved and lifetime production is improved; managing AFC down should be considered as an alternative to increasing the efficiency of dairy enterprises.

Dr Joubert Nolte