# From milk to maturity: The potential for lactocrine programming of heifer reproduction

## TL;DR

This paper explores how hormones in cow's milk may influence the reproductive development of young dairy heifers.

## Contribution

The paper introduces the lactocrine hypothesis as a novel framework for understanding how milk hormones may program heifer reproduction.

## Key findings

- Milk from cycling or pregnant cows contains higher steroid hormones than fresh cows.
- Early maternal milk has higher insulin and IGF-1, which may affect heifer physiology.
- Lactocrine signaling may optimize heifer reproductive development through hormone exposure.

## Abstract

Summary: A dairy calf is generally fed a liquid diet for 60 days, which is often sourced from all herd cows or a commercial milk replacer product. The milk of cycling or pregnant herd cows contains greater concentrations of steroid hormones (progesterone [P4] and estradiol [E2]) compared with that of a fresh cow (the calf's dam). On the contrary, the early maternal milk (transition milk) of a cow is greater in insulin and insulin-like growth factor-1 (IGF-1) content. Each of these hormones has the potential to regulate prepubertal heifer reproductive and neuroendocrine physiology, which is consistent with the “lactocrine hypothesis.” The effects on circulating luteinizing hormone (LH) and anti-Müllerian hormone (AMH), ovarian antral follicle count (AFC), and timing and maintenance of puberty are all of interest but are not well studied in the existing literature. Leveraging the lactocrine effects of bulk milk or maternal milk consumption may drive female reproductive programming and optimize current management practices.

Summary: A dairy calf is generally fed a liquid diet for 60 days, which is often sourced from all herd cows or a commercial milk replacer product. The milk of cycling or pregnant herd cows contains greater concentrations of steroid hormones (progesterone [P4] and estradiol [E2]) compared with that of a fresh cow (the calf's dam). On the contrary, the early maternal milk (transition milk) of a cow is greater in insulin and insulin-like growth factor-1 (IGF-1) content. Each of these hormones has the potential to regulate prepubertal heifer reproductive and neuroendocrine physiology, which is consistent with the “lactocrine hypothesis.” The effects on circulating luteinizing hormone (LH) and anti-Müllerian hormone (AMH), ovarian antral follicle count (AFC), and timing and maintenance of puberty are all of interest but are not well studied in the existing literature. Leveraging the lactocrine effects of bulk milk or maternal milk consumption may drive female reproductive programming and optimize current management practices.

•A multitude of preweaning dairy calf feeding strategies exist.•Bulk whole milk and early maternal milk are not similar in hormone content.•The prepubertal ovary, uterus, and brain are responsive to hormone exposure.•The effects of early exposure to maternal hormones in milk are plausible, but unknown.•The lactocrine contributions of milk may drive optimal heifer reproductive development.

A multitude of preweaning dairy calf feeding strategies exist.

Bulk whole milk and early maternal milk are not similar in hormone content.

The prepubertal ovary, uterus, and brain are responsive to hormone exposure.

The effects of early exposure to maternal hormones in milk are plausible, but unknown.

The lactocrine contributions of milk may drive optimal heifer reproductive development.

Standard dairy calf management sees that calves are fed a consistent liquid diet until weaning at ∼60 d of age, which is generally provided as whole milk from the bulk tank (a composite of all lactating cows) or a commercial milk replacer product. Although these diets aim to satisfy the nutritional needs of the calf, their composition is not similar to the transient profile of hormones and nonnutritive bioactive factors in the milk the dam produces at this stage. Inclusion or exclusion of hormones in milk during this time may have substantial effects on reproductive programming via lactocrine signaling. This review proposes 4 potential lactocrine factors (IGF-1, insulin, estradiol, and progesterone) relevant to current dairy calf feeding strategies and highlights known interactions within female reproductive development, wide knowledge gaps, and opportunities for further investigation.