A review of extended lactation in dairy cows managed in high-input and pasture-based farming systems

Abstract

Traditionally the lactation cycle of a dairy cow is based around a 12-month calving interval, allowing for 10 months of lactation, followed by a 2-month dry period. This means that the cow has to conceive within 2–3 months after calving, when she is also at peak lactation and metabolically is in a negative energy balance. Such challenging physiological conditions make it challenging for the cow to conceive at this time and many modern high-producing cows fail to get pregnant within the constraints of a 12-month calving interval. In addition, many cows still produce at a high level at drying-off time, increasing the risk of intramammary infections. Therefore, delaying conception past peak production and, as a result, extending the lactation beyond 10 months may increasingly be necessary. Additionally, extended lactation (EL) may offer other advantages such as fewer calves being born and thus fewer ‘surplus’ calves needing to be culled at a young age, fewer health and welfare issues and improved environmental outcomes (i.e. less greenhouse-gas emission; less antibiotic usage) during the lifetime of the animal. Extending lactation is a straight forward management practice in high-input dairy systems where a consistent supply of feed supplements is readily available, but may be more challenging to implement in low(er)-input pasture-based systems. The latter are much more seasonal, with a 12-month calving interval allowing pasture growth and quality to match the cow’s nutritional demands; cows calve in spring when high-quality pasture is abundant and are dried-off during winter when pasture growth is more limited. In this review, we explored the impact of EL in both high-input systems and pasture-based systems. It covers the effects of EL on milk production, composition and processing, as well as on reproductive performance, health and welfare, and environmental and economic outcomes.