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Hormones are chemical messengers produced by glands and tissues that coordinate nearly every biological process in cattle.

They travel through the bloodstream, binding to target cells to trigger specific physiological responses.

In cows, hormones regulate growth, reproduction, metabolism, lactation, and stress responses—processes essential for both survival and productivity. 1.

Growth Hormone (Bovine Somatotropin – BST) Source: Anterior pituitary gland Type: Protein hormone Bovine somatotropin (BST) is central to metabolism and growth.

It enhances the utilization of nutrients by promoting protein synthesis and mobilizing fat reserves, directing energy toward milk production in lactating cows.

BST acts indirectly through insulin-like growth factor-1 (IGF-1), which stimulates cell division, bone growth, and tissue development. When naturally secreted, BST fluctuates throughout the day, influenced by feed intake, stress, and lactation stage.

Recombinant BST (rbST) has been developed for agricultural use, though its application varies globally due to regulatory and ethical considerations. 2.

Estrogen (Estradiol, Estrone, Estriol) Source: Ovarian follicles, placenta (during pregnancy) Type: Steroid hormone Estrogen regulates the female reproductive cycle and secondary sexual characteristics.

It prepares the reproductive tract for breeding by promoting uterine lining growth, cervical mucus secretion, and heat behavior (estrus). During the follicular phase, rising estrogen levels signal the brain to release luteinizing hormone (LH), leading to ovulation.

Estrogen also influences mammary gland development and plays a minor role in regulating bone density and metabolism. 3.

Progesterone Source: Corpus luteum on the ovary and the placenta during pregnancy Type: Steroid hormone Progesterone maintains pregnancy by preventing uterine contractions and supporting the uterine lining for embryo implantation.

It suppresses further estrus cycles during gestation. In dairy reproduction management, progesterone levels are often monitored to determine fertility cycles or detect early pregnancy.

Artificial synchronization protocols (like Ovsynch) use progesterone analogs to regulate ovulation timing in breeding programs. 4.

Prolactin Source: Anterior pituitary gland Type: Peptide hormone Prolactin stimulates and maintains lactation by promoting the synthesis of milk proteins, fats, and lactose.

Its release increases during late gestation and after calving. Although prolactin levels are influenced by suckling and milking, stress can inhibit its secretion.

Prolactin also plays roles beyond milk production, including immune regulation and maternal behavior. 5.

Oxytocin Source: Produced by the hypothalamus; stored and secreted by the posterior pituitary gland Type: Peptide hormone Oxytocin is responsible for milk let-down—the release of milk from alveoli in the udder into the teats during milking or nursing.

The hormone is rapidly released in response to tactile stimulation of the udder or calf suckling. It also triggers uterine contractions during calving, assisting in the delivery of the calf and expulsion of the placenta.

Oxytocin secretion can be inhibited by stress or discomfort, making calm handling during milking crucial. 6.

Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH) Source: Anterior pituitary gland Type: Glycoprotein hormones LH and FSH work together to control the estrous cycle and ovulation. FSH promotes the development of ovarian follicles and stimulates estrogen production. LH triggers ovulation and supports corpus luteum formation, which produces progesterone. In bulls, these hormones regulate testosterone production and spermatogenesis. 7.

Cortisol Source: Adrenal cortex Type: Steroid hormone Cortisol, the primary stress hormone, helps cows adapt to physical or environmental stress.

It increases blood glucose levels, enhances metabolism, and suppresses inflammation. Short-term cortisol release helps during calving or illness, but chronic stress can weaken immunity, reduce fertility, and lower milk yield.

Managing handling, environment, and nutrition helps maintain balanced cortisol levels. 8.

Insulin Source: Pancreas (Islets of Langerhans) Type: Peptide hormone Insulin regulates glucose metabolism, allowing cells to absorb and store energy.

In dairy cows, insulin plays a major role during early lactation when energy demand is high. It works in opposition to BST and cortisol to balance nutrient use—directing glucose to vital organs and the mammary glands.

Metabolic disorders like ketosis often occur when this balance is disrupted. 9.

Insulin-Like Growth Factor-1 (IGF-1) Source: Liver and other tissues (in response to BST) Type: Protein hormone IGF-1 mediates many of growth hormone’s effects, promoting tissue growth and development.

It supports muscle and bone maintenance and influences milk synthesis by increasing nutrient uptake in mammary cells. 10.

Thyroid Hormones (T3 and T4) Source: Thyroid gland Type: Iodine-containing amino acid hormones Thyroid hormones regulate basal metabolism, energy balance, and thermoregulation.

They are vital for growth, lactation, and reproduction.

Imbalances can result in reduced milk yield, poor weight gain, or fertility issues. 11.

Relaxin Source: Ovaries and placenta Type: Peptide hormone Relaxin softens pelvic ligaments and the cervix before calving, facilitating birth.

It also helps the uterus return to normal size postpartum. 12.

Melatonin Source: Pineal gland Type: Indoleamine hormone Melatonin helps regulate reproductive cycles by responding to day length (photoperiod).

In seasonal breeders, it influences timing of estrus cycles, but in cattle (non-seasonal breeders), it plays a smaller role.

It also contributes to antioxidant defense and immune modulation. Summary Together, these hormones form a complex and interconnected system that determines every aspect of a cow’s physiology—from how she digests feed and produces milk to how she reproduces and responds to stress.

Understanding these hormones allows veterinarians and farmers to optimize nutrition, breeding, and herd health for better welfare and productivity.