Follicle and CL development during the estrous cycle of dairy cows

Understanding the Estrus Cycle in Dairy Cows

Introduction to Fertility Programs

• Dr. Richard, a professor of animal science at Michigan State University, discusses fertility improvement methods for lactating dairy cows at Novus Dairy Farm in St. John's, Michigan. The farm applies research-based strategies to enhance cow fertility and profitability.

Follicle Development During the Estrus Cycle

• The discussion begins with an overview of how follicles grow during the estrus cycle, emphasizing the roles of hormones such as gene or H and prostaglandin F2 alpha. Understanding this growth is crucial for effective fertility programs.

Day 0: Standing Estrus

• On day zero of the estrus cycle, cows exhibit standing estrus with one or more large preovulatory follicles secreting estradiol; no corpus luteum (CL) is present yet. Ultrasound may reveal remnants of old CLs (corpus albicans).

Day 1: Hormonal Surge Initiation

• A surge in luteinizing hormone (LH) and follicle-stimulating hormone (FSH) occurs due to low progesterone levels, initiating ovulation processes within approximately 28 hours post-LH surge. New growing follicles emerge following this hormonal increase.

Day 2: Follicular Growth Continues

• Several new growing follicles persist on day two as FSH remains elevated but starts to decline slightly due to feedback from inhibin and estrogen produced by developing follicles. Some new follicles become atretic and are undetectable via ultrasound by this stage.

Day 3: Dominant Follicle Emergence

• By day three, most new wave follicles are no longer detectable; however, a cohort of four to six functional growing follicles exists alongside a developing CL that secretes progesterone. If aspirated, subordinate follicles can become dominant again due to hormonal changes affecting FSH levels.

Deviation Event in Follicular Development

Day 4: First Deviation Event

• Approximately 50% of lactating dairy cows experience a deviation event on day four where the largest follicle gains LH receptors and continues growth while subordinate ones undergo atresia; these dominant follicles can respond to induced LH surges for ovulation without starting a new cycle if ablated or aspirated later on.

Day 5: Increased Response Capability

• By day five, about 70% of cows have undergone deviation; these dominant follicles can respond effectively to exogenous LH surges for ovulation while other detected follicles remain atretic—most CL will not regress completely with PGF2 alpha treatment until later days in the cycle.

Final Stages of Estrus Cycle Management

Days 6 & 7: Dominant Follicles' Functionality

• On day six, nearly all cows have a responsive dominant follicle capable of responding to induced LH surges; typically requires two injections of PGF2 alpha for complete regression if needed before moving into subsequent cycles or treatments on day seven when only one functional follicle remains active alongside ongoing CL development and progesterone secretion.

Day 8: Ovulation Potential Diminishes

• By day eight, most cows still possess functional first-wave dominant follicles that should ideally respond to induced LH surges for ovulation; however, response rates begin declining as some may already be undergoing atresia leading into potential second-wave developments depending on individual cow conditions.

Estrus Cycle Dynamics in Cows

Corpus Luteum (CL) Functionality and Regression

• The CL continues to grow and secrete progesterone until natural regression around day 18 of the estrus cycle, influenced by PGF2α secretion from the uterine endometrium.

• By day 9, most cows experience complete CL regression due to endogenous PGF2α pulses. This marks a critical transition in the estrus cycle.

Follicular Development Stages

• On days 8 to 13, lactating dairy cows typically develop a first wave dominant follicle that may undergo atresia, initiating a new follicular wave. The size of this dominant follicle is crucial for its functionality.

• A functional dominant follicle measuring approximately 15.5 mm can respond to an LH surge; however, increased progesterone levels may inhibit this response, leading to potential non-ovulation events.

Atresia and New Follicular Waves

• Day 10 sees the first wave dominant follicle potentially undergoing atresia, allowing FSH levels to rise and stimulate a second wave of follicles. This dynamic is essential for maintaining reproductive cycles in cows.

• By day 12, several follicles from the second wave continue growing as FSH remains elevated but begins to decline due to inhibin production from developing follicles. This balance is vital for successful ovulation later on.

Dominance and Deviations in Follicles

• On day 14, deviation occurs where the largest follicle acquires more LH receptors than subordinate ones; about 50% of lactating cows exhibit this event by day four of the new wave, enabling them to respond effectively to LH surges for ovulation.

• If the newly established dominant follicle is ablated or aspirated post-deviation, it triggers another new follicular wave due to reduced inhibitory effects on FSH secretion from previous follicles.

Progesterone Levels and Estrus Cycle Progression

Understanding the Estrous Cycle in Lactating Dairy Cows

The Role of Follicular Waves

• Lactating dairy cows exhibit a significant increase in follicular activity, with the second wave dominant follicle remaining functional and growing by day 17. This could lead to the formation of a third follicular wave if it becomes atretic.

• On day 18, natural luteolysis of the corpus luteum (CL) begins. If the dominant follicle is still functional, it will continue to grow and secrete estradiol due to increased pulsatile release of LH as progesterone levels decrease.

Hormonal Interactions

• The decrease in progesterone triggers an increase in GnRH pulses from the hypothalamus, which travel through portal vessels to stimulate LH release from the anterior pituitary. This hormonal interplay is crucial for regulating reproductive cycles.

• By day 19, LH pulses drive growth and estradiol secretion from the dominant follicle. Inhibitory effects from estradiol on granulosa cells prevent new follicles from developing as CL regresses and progesterone levels approach baseline.

Estrus and Ovulation Dynamics

• On day 20, with complete regression of the CL, basal progesterone levels are reached while large amounts of estradiol are secreted by the preovulatory dominant follicle. Notably, estradiol concentrations are lower than those observed when cows were heifers due to increased steroid metabolism in lactating cows.