Progesterone concentrations, however, did not differ between the groups. Both baseline levels Ž
. Ž
. P s 0.25 and posttreatment production P s 0.45 were of the same magnitude in RBH and VH.
In conclusion, the study could not confirm that suprabasal progesterone concentrations during oestrus in RBH derive from the adrenal glands. Still, apparent differences were found in
adrenocortical activity when ovariectomised heifers, VH and RBH, were subjected to an ACTH- challenge. It is suggested that a sustained adrenal stimulation associated with environmental or
social stress could be one factor in the repeat breeding syndrome. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Cattle endocrinology; Repeat breeding; Stress; ACTH; Progesterone; Cortisol
1. Introduction
The influence of stress on reproductive performance of dairy cattle has been shown to Ž
be of importance where intensive livestock management is applied Nanda et al., 1990; .
Dobson and Smith, 1995; Wilson et al., 1998a,b . The peri-oestral period is very delicate with its precise schedule for the final development of a pre-ovulatory follicle towards
Ž .
ovulation Dieleman et al., 1983; Hyttel et al., 1989 and is dependent on the interplay Ž
and regulatory actions of the hypothalamic–pituitary–gonadal axis Rivier and Rivest, .
1991 . Another neuroendocrine connection, the hypothalamic–pituitary–adrenal axis Ž
. HPA exerts profound multilevel inhibitory effects on reproduction. In order to study
the pituitary–adrenal response to stress, evaluating stress effects on, e.g. reproduction, or Ž
. simply to test adrenal function, the administration of adrenocorticotropin ACTH has
Ž been used in several studies Gwazdauskas et al., 1972; Wagner et al., 1972; Alam et al.,
. 1986; Verkerk et al., 1994; Dobson and Smith, 1995 . Concerning stress and reproduc-
tion, different hypotheses have been proposed regarding which mechanisms are respon- sible for the negative effects, e.g. inhibiting the pre-ovulatory LH-surge or delaying
ovulation. It may be an effect of glucocorticoids andror progesterone released from the
Ž adrenal cortex or ACTH from the pituitary affecting the reproductive function for
. reviews, see Moberg, 1991; Rivier and Rivest, 1991; Rivest and Rivier, 1995 . The
secretion of pituitary–adrenal hormones in response to stress is controlled by hypothala- Ž
. mic corticotropin-releasing hormone CRH and arginine vasopressin, and even CRH
itself has been shown to directly affect reproduction by inhibiting gonadotropin-releas- Ž
. ing hormone secretion Chrousos et al., 1998 .
The events that bring about ovulation are highly dependent and integrated. Slight changes may disturb the maturation of the oocyte and the timing of ovulation and render
oocytes that may be able to fertilise but have impaired developmental competence, thus leading to early embryonic death. With a delayed ovulation, the chances of pursuing
normal timing during AI are impaired, resulting in fertilisation failure or early embry- onic death and, consequently, in repeat breeding. Repeat breeding is regarded to be one
of the most costly reproductive dysfunctions in cattle. During a number of years, strictly
Ž .
defined repeat breeder heifers RBH have been subjected to different studies at the Swedish University of Agricultural Sciences, SLU, with the objective to find possible
causes for the syndrome. Findings from these various studies have identified asyn- chronous and deviating hormone patterns in connection with disturbed oestrous events
Ž as being an important factor in the repeat breeding syndrome Gustafsson et al., 1986;
. Albihn 1991; Bage et al., 1997 . Marginally elevated, so-called suprabasal, plasma
˚
Ž
y1 y1
. progesterone concentrations 0.5–1.0 nmol l
vs. - 0.5 nmol l as normality were
measured during oestrus and could either be a consequence of an incomplete pre-ovula- tory luteolysis or be caused by an increased progesterone release from the adrenal
glands. In different ways, increased levels of progesterone are known to delay the onset of oestrus: inhibition of the pre-ovulatory LH-surge by negative feedback on the
Ž .
hypothalamus Stoebel and Moberg, 1982; Duchens et al., 1994, 1995b , inhibition of Ž
endometrial oxytocin receptor formation McCracken et al., 1984; Lamming and Mann, .
Ž 1995 and subsequent release of uterine PGF
Porter and Behrman, 1971; Beard et al.,
2 a
. 1994 . Prolonged exposure to progesterone leads to a variety of uterine responses all of
Ž which may be due to an inhibitory effect on the synthesis of receptors for oxytocin for
. reviews, see Silvia et al., 1991; McCracken et al., 1999 .
The present study was designed to measure plasma progesterone and cortisol response to ACTH-treatment in ovariectomised RBH, thereby investigating the relation-
ship between adrenocortical function and reproductive physiology and, eventually, to evaluate the potential influence of stress as a component of the repeat breeding
syndrome.
2. Materials and methods
