Is stress really all that important?

There is growing concern in many parts of the world that fertility of dairy cattle is reducing as milk yields increase. Stress could be one important cause. As an example, fertility is lower after caesarian operations. Delayed uterine involution after dystocia is associated with abnormal ovarian cyclicity and prolonged intervals to the next pregnancy. There is a greater reduction in fertility as the clinical conditions of lameness, milk fever or mastitis worsen. Changes in social groupings greatly increase the number of inseminations required per pregnancy. Transport reduces the number of CL after superovulation, and can interfere with pregnancy rates after estrous synchronization. Embryos collected from heat-stressed donors are less viable and have delayed trophoblast function. Human-animal interactions influence stress-responses in cattle--the behavior of stockman and embryo transfer personnel could affect success. Putting aside financial aspects, exposure of an animal to avoidable stress compromises welfare, whether application of biotechnology is involved or not. The fact that stressors can be deleterious to such an important function as reproduction, emphasizes that stress is very important and should be minimized whenever possible.

Ultrasonography and hormone profiles of persistent ovarian follicles (cysts) induced with low doses of progesterone in cattle.

The aims of this study were to expose dominant ovarian follicles at the end of the oestrous cycle to low progesterone concentrations similar to those that occur during stress, and to examine the effect of a subsequent small increase in progesterone 10 days later. Half a progesterone releasing intravaginal device (0.5 PRID) was administered to 13 heifers from day 15 of the oestrous cycle. In group 1 (n = 7), one 0.5 PRID remained in place until day 40 or until each heifer ovulated. In group 2 (n = 6), the first 0.5 PRID was removed on day 28, and replaced immediately with a second 0.5 PRID. Ultra-sonography and blood collection (10 ml) were conducted each day for 26 days from day 14 and then on alternate days. The largest follicle that emerged during the first 5 days after insertion of the initial 0.5 PRID remained > 10 mm in diameter for 15.3 +/- 1.7 and 11.6 +/- 0.4 days in groups 1 and 2, respectively. This period of dominance, during which no other follicles emerged, was closely correlated with the duration of plasma oestradiol concentrations exceeding 10 pg ml(-1). In four heifers from group 1, the persistent follicle ovulated between days 30 and 37 (sub-group 1a; 0.5 PRID expelled). In three heifers from sub-group 1b (0.5 PRID retained), the dominant follicle secreted oestradiol for 17 +/- 5 days but remained detectable by ultrasonography for a total of 33 +/- 8 days (range 26-52 days). Monitoring continued beyond day 40 in these animals. In group 2, the new 0.5 PRID inserted on day 28 resulted in an increase in plasma progesterone concentration of 0.9 +/- 0.3 ng ml(-1). Simultaneously, oestradiol decreased by 10.1 +/- 3.3 pg ml(-1), and a new follicular wave emerged 5-7 days later. In conclusion, exposure to very low concentrations of progesterone produced persistent follicles that secreted oestradiol for 17 days. This oestradiol production could be disrupted by a second increase of 0.9 ng ml(-1) in peripheral progesterone concentration. In the absence of the second progesterone treatment, some of the persistent follicles remained detectable by ultrasonography for up to 52 days, despite cessation of oestradiol secretion.

Ultrasonography and hormone profiles of adrenocorticotrophic hormone (ACTH)-induced persistent ovarian follicles (cysts) in cattle.

The objective of this study was to develop a model for the study of abnormal ovarian follicles in cattle by treating heifers with adrenocorticotrophic hormone (ACTH) (100 iu at 12 h intervals for 7 days, beginning on day 15 of the oestrous cycle). Cortisol concentrations increased (P < 0.05) within 24 h after beginning ACTH treatment and cortisol and progesterone concentrations remained elevated after cessation of ACTH treatment for 8 and 4 days, respectively. The pulses and surges of LH decreased during ACTH treatment, but FSH profiles were similar to those in controls and persistent or prolonged follicles were eventually observed in all heifers. In five heifers, prolonged dominant follicles ovulated after 10 days, whereas in six heifers, persistent follicular structures were present for 20 days, but ceased to secrete oestradiol after approximately 12 days. In the heifers with persistent follicular structures, new follicles emerged when the persistent follicle became non-oestrogenic. During the last 2 days of normal follicular growth, the concentration of oestradiol was greater than it was during prolonged or persistent follicle development (P < 0.05). There were no differences in the growth rates or maximum diameters of abnormal follicles that had different outcomes, but oestradiol concentrations were greater in prolonged follicles that ovulated compared with those follicles that persisted (P = 0.06). In conclusion, stimulation with ACTH resulted in a marked deviance from normal follicular activity. The aberrations were probably caused by the interruption of pulsatile secretion of LH (but not FSH) leading to decreased but prolonged oestradiol secretion.

Effect of transport on pulsatile and surge secretion of LH in ewes in the breeding season.

The aim of this study was to elucidate the mechanism(s) involved in stress-induced subfertility by examining the effect of 4 h transport on surge and pulsatile LH secretion in intact ewes and ovariectomized ewes treated with steroids to induce an artificial follicular phase (model ewes). Transport caused a greater delay in the onset of the LH surge in nine intact ewes than it did in ten ovariectomized ewes (intact: 41.0 +/- 0.9 h versus 48.3 +/- 0.8 h, P < 0.02; ovariectomized model: 40.8 +/- 0.6 h versus 42.6 +/- 0.5 h, P < 0.02). Disruption of the hypothalamus-pituitary endocrine balance in intact ewes may have reduced gonadotrophin stimulation of follicular oestradiol production which had an additional effect on the LH surge mechanism. In the ovariectomized model ewes, this effect was masked by the exogenous supply of oestradiol. However, in these model ewes, there was a greater suppression of maximum LH surge concentrations (intact controls: 29 +/- 4 ng ml-1 versus intact transported 22 +/- 5 ng ml-1, P < 0.02; ovariectomized model controls: 35 +/- 7 ng ml-1 versus model transported 15 +/- 2 ng ml-1, P < 0.02). Subsequent exposure to progesterone for 12 days resulted in the resumption of a normal LH profile in the next follicular phase, indicating that acute stress leads to a temporary endocrine lesion. In four intact ewes transported in the mid-follicular phase, there was a suppression of LH pulse amplitude (0.9 +/- 0.3 versus 0.3 +/- 0.02 ng ml-1, P < 0.05) but a statistically significant effect on pulse frequency was not observed (2.0 +/- 0.4 versus 1.7 +/- 0.6 pulses per 2 h). In conclusion, activation of the hypothalamus-pituitary-adrenal axis by transport in the follicular phase of intact ewes interrupts surge secretion of LH, possibly by interference with LH pulsatility and, hence, follicular oestradiol production. This disruption of gonadotrophin secretion will have a major impact on fertility.

Effect of transport on pulsatile LH release in ovariectomized ewes with or without prior steroid exposure at different times of year.

The initial aim of the present study was to test whether the stress of transport suppresses LH pulsatile secretion in ewes. In a pilot experiment in the late breeding season, transport resulted in an unexpected response in three out of five transported, ovariectomized ewes pretreated with oestradiol and progesterone. Before transport, seasonal suppression of LH pulses had occurred earlier than anticipated, but LH pulsatility suddenly restarted for the period of transport. This finding was reminiscent of unexplained results obtained in ovariectomized ewes infused centrally with high doses of corticotrophin-releasing hormone after pretreatment with low doses of oestradiol with or without progesterone. Hence, an additional aim of the present study was to examine whether these latter results with corticotrophin-releasing hormone could be reproduced by increasing endogenous corticotrophin-releasing hormone secretion by transport. Subsequent experiments used groups of at least eight ovariectomized ewes at different times of the year with or without prior exposure to steroids to assess whether these unexpected observations were associated with season or the prevailing endocrine milieu. In the mid-breeding season, transport for 4 h in the absence of steroid pretreatment for 8 months reduced LH pulse frequency from 7.5 +/- 0.3 to 6.3 +/- 0.4 pulses per 4 h (P < 0.05) and LH pulse amplitude from 2.6 +/- 0.5 to 1.8 +/- 0.3 ng ml-1 (P < 0.05). Similarly, in the mid-breeding season, 34 h after the cessation of pretreatment with oestradiol and progesterone, transport suppressed LH pulse frequency from 6.1 +/- 0.4 to 5.5 +/- 0.3 pulses per 4 h (P < 0.05) with a tendency of effect on amplitude (6.2 +/- 2.7 to 2.61 +/- 0.6 ng ml-1; P = 0.07; note the large variance in the pretransport data). During mid-anoestrus, evidence of a suppressive effect of transport was only observed on LH pulse amplitude (4.7 +/- 0.6 versus 3.0 +/- 0.5 pulses per 4 h; P < 0.05) in ovariectomized ewes that had not been exposed to ovarian steroids for 4 months. Repetition of the pilot experiment with 12 ewes during the transition into anoestrus resulted in one ewe with LH pulses seasonally suppressed but increased by transport; 11 ewes had a distinct pulsatile LH pattern which was decreased by transport in six ewes. In anoestrus, there was no effect of transport on LH pulse frequency or amplitude in intact ewes, or those ovariectomized 2-3 weeks previously, with or without prior oestradiol and progesterone treatment. However, basal concentrations of cortisol were greater in anoestrus than in the breeding season, and the increment in cortisol during transport was similar in anoestrus and the breeding season but greater during the transition into anoestrus (P < 0.05). Progesterone concentrations increased from 0.31 +/- 0.02 ng ml-1 before transport to 0.48 +/- 0.05 ng ml-1 during the second hour of transport (P < 0.05). In conclusion, transport reduced LH pulse frequency and amplitude in ovariectomized ewes that had not been exposed to exogenous steroids for at least 4 months. In most animals, the previously observed increase in LH pulsatility induced by exogenous CRH was not reproduced by increasing endogenous CRH secretion by transport. However, in four ewes, transport did increase LH pulsatility, but only during the transition into anoestrus in ewes with seasonally suppressed LH profiles after withdrawal of steroid pretreatment.