Álvaro García
Estrus refers to the period when a cow is sexually receptive and ready for mating. In dairy cows, estrus typically lasts for approximately 18 hours, although it can vary from 7 to 12 hours. Its duration is influenced by factors such as climate, nutrition, animal age, and temperature. Understanding these effects is crucial for dairy farmers aiming to optimize milk yield and manage herd health effectively. This article discusses the hormonal changes, feed intake variations, and overall impact of estrus on milk production in dairy cows.
Hormonal Changes
Estrus in dairy cows is characterized by a surge in reproductive hormones, primarily estrogen, luteinizing hormone (LH), and follicle-stimulating hormone (FSH). The estrus cycle can be divided into four main phases: proestrus, estrus, metestrus, and diestrus. During proestrus, estrogen levels begin to rise, preparing the reproductive system for ovulation. The estrus phase is marked by the peak of estrogen, leading to behavioral changes that signal the cow is in heat. This period also sees a surge in LH, which triggers ovulation. Following ovulation, metestrus begins, during which estrogen levels drop and progesterone levels start to rise as the corpus luteum forms. Finally, diestrus is dominated by high progesterone levels, maintaining pregnancy if fertilization occurs. These hormonal fluctuations affect various physiological aspects, including feed intake and metabolic processes, which directly impact milk production.
Behavioral Changes
During estrus, dairy cows exhibit several behavioral changes that indicate they are in heat and ready for breeding. These behaviors are primarily driven by the peak in estrogen levels and include:
- Increased Activity: During estrus, cows exhibit increased activity levels, signaling readiness for breeding. Accelerometers and pedometers, monitoring movement patterns and step counts, are utilized by farmers for accurate detection, enhancing reproductive efficiency in dairy farming.
- Mounting and Being Mounted: One of the most distinctive signs is standing still to be mounted by other cows, known as standing heat. Cows may also attempt to mount others.
- Bellowing and Vocalization: Cows in heat often bellow more frequently and louder than usual.
- Sniffing and Licking: Increased sniffing and licking of other cows, particularly around the head and genital areas, is common.
- Tail Raising and Vulva Swelling: Cows may raise their tails more frequently, and the vulva often becomes swollen and reddened.
- Decreased Feed Intake: Some cows may eat less due to the increased activity and focus on mating behavior.
- Mucous Discharge: There may be a clear, mucous discharge from the vulva, which can be seen on the tail or hindquarters.
- Social Behavior: Cows may show more interest in other cows and form temporary bonds, spending more time near others in estrus.
Feed Intake Changes and Impact on Milk Production
During estrus, cows typically experience reduced feed intake, which can negatively impact milk production as nutrient availability diminishes. However, the immediate decrease in milk yield is more directly linked to hormonal changes and increased physical activity. The surge in estrogen and LH during estrus directly impacts the mammary gland’s function, altering milk synthesis and secretion. Additionally, increased physical activity, such as mounting and restlessness, elevates energy expenditure, diverting energy away from milk production. Stress induced by these behaviors further affects milk synthesis. While reduced feed intake contributes to the decline in milk production, the sudden drop observed is primarily due to the interplay of hormonal changes, increased physical activity, and stress. This complex interaction underscores the multifactorial nature of milk production regulation during estrus.
Actual farm case
Cow 140, a 4-year-old second lactation Holstein, faced challenges in conception, undergoing six heats and corresponding inseminations before finally being diagnosed as pregnant at 300 days in milk. It is worth noting this cow is consistently performing above expectations. This prolonged conception period reflects the complexity of reproductive management in dairy farming and the importance of careful monitoring and intervention. The recorded reductions in kilograms of milk production during each estrus cycle shed light on the significant impact of reproductive activity on milk yield. These reductions, ranging from approximately 24.9 kg to 47.4 kg per heat, underscore the physiological and metabolic shifts occurring in the cow’s body during estrus. The average percent reduction in milk production across all five estrus episodes was 32%, with her production level consistently returning to normal within a couple of days.
Her body condition score (BCS) remained stable until 190 days in milk, after which it began to steadily increase. This increase in BCS coincided with three consecutive heats occurring roughly 20 days apart, ultimately leading to successful breeding at the sixth insemination, when her BCS had risen by half a point to 4.0. Her milk production started to drop at 190 days in milk, precisely when her BCS began to increase. This suggests that the improvement in her energy status corresponded with the subsequent heats and ultimately led to successful breeding at 300 days in milk.
Implications
The effects of estrus on milk production in dairy cows involve hormonal changes, behavioral shifts, and metabolic adjustments. The surge in reproductive hormones, coupled with heightened physical activity and stress during estrus, significantly impacts milk synthesis and secretion. While reduced feed intake contributes to the decline in milk production, the immediate drop observed is primarily attributed to hormonal fluctuations and increased energy expenditure. By understanding and managing these fluctuations, dairy farmers can strive towards achieving optimal herd health and productivity.
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