Anestrus

During pregnancy, and for a short period following parturition, cows are normally acyclic. However, estrous behavior, not accompanied by ovulation, can occur in up to 7% of pregnant cows. Progesterone profiling suggests that 90% and 70% of dairy cows and beef cows, respectively, resume cyclicity by 50 days.

Throughout pregnancy, waves of follicles develop and become atretic in the ovaries; ovulation is inhibited by high progesterone concentrations during pregnancy effecting a negative feedback mechanism on the hypothalamus. Following parturition, follicle stimulating hormone (FSH)-induced waves of follicular growth are soon accompanied by luteinizing hormone (LH)-induced ovulation and return of regular cyclicity.

The first dominant follicle can normally be detected by 7–20 days post partum, with first ovulation normally by 20–30 and 20–60 days in dairy and beef cows, respectively. Due to the lack of prior progesterone priming, the first ovulation may not be accompanied by estrous behavior (silent ovulation). LH pulse frequency is the key to re-establishing post-partum cyclicity, as with insufficient LH pulse frequency, waves of follicles will continue to turn over without ovulation, leading to extended anestrus. Anestrus extending beyond 55–60 days post partum is likely to cause significant economic losses due to delay in the calving to first service and subsequent calving interval.


Aetiology of Extended Post-partum Anestrus

Any factor causing interference with gonadotropin releasing hormone (GnRH)/LH output during the early post-partum period can influence the post-partum anestrous period, including:

• Nutritional effects. Negative energy balance during late pregnancy/early lactation can cause extended anestrus due to suppression of LH pulse frequency. Poor body condition score (BCS) at calving will increase the duration of anestrus, especially in beef cows. Specific trace element deficiency may also be involved, especially sulphur and molybdenum induced copper deficiency.
• Suckling effects. Natural suckling has a greater influence than milking, therefore this effect is important only in beef cows. Temporary calf separation in beef cows may increase LH pulse frequency and significantly shorten the post-partum anestrous period.
• Delayed uterine involution. Assisted parturition, RFM and metritis can cause delayed involution, which may delay resumption of cyclicity.
• Cystic ovarian disease. Cysts form due to failure of ovulation of early dominant follicles and this can delay normal cyclicity.
• Persistent Corpus Luteum (CL). A persistent CL is usually found along with uterine infection/pyometra, as this can lead to failure of endometrial PGF2α release. Treatment with PGF2α injection will be effective, with estrus expected within 2–5 days.

Diagnosis of Nutritional Anestrus

Most cows presented for anestrus after 60 days post partum will be cycling and have simply not been observed in estrus due to poor heat detection. True anestrus can be confirmed by the finding of basal progesterone levels in milk or blood in samples taken 10–14 days apart. In practice, anestrus is normally diagnosed based on single rectal palpation/ultrasound findings of bilateral small/firm ovaries with no CL or large follicles detected, along with a history of poor body condition and negative energy balance.


Management of Nutritional Anestrus

Correcting any underlying nutritional problem(s) is essential.

Progesterone treatment for 7–10 days mimics the luteal phase, suppressing LH release from the anterior pituitary. An LH surge follows implant withdrawal and ovulation/estrus occurs in most cases 2–3 days later.

During the early post-partum period, 400–600 IU of pregnant mare serum gonadotropin (PMSG) can be injected at cessation of progesterone treatment to ensure ovulation and estrus, though this may lead to multiple ovulation in some cases.

An injection of a GnRH analogue, given more than 55 days post partum, will induce estrus in most acyclic cows within 23 days. LH released in immediate response to the GnRH injection causes silent ovulation and formation of a CL. The onset of estrus following treatment can be controlled by a PGF2α injection 7–14 days after the initial GnRH treatment. This regimen may suit herds having fortnightly fertility visits, as cows treated for anestrus with GnRH and still not observed in estrus would be re-examined after 14 days and injected with PGF2α if a CL was now present.

(Note: These treatments are unlikely to be effective in thin, high-yielding cows in severe negative energy balance. If ultrasound findings confirm small ovaries with negligible follicular activity present, treatment should be delayed until the cow is in a better nutritional state.)