182 S. Landau et al. Animal Reproduction Science 64 2000 181–197
0.614 versus 0.386 gl, P 0.002, were found in FF from preovulatory follicles. The insulin concentration in the FF of cows fed the CG diet was 26 higher than in their counterparts fed CGM
P 0.04, SBM being intermediate. Dietary effects did not reach significance in subordinate follicles.
The finding that preovulatory follicular status is associated with increased intrafollicular insulin and glucose suggests that insulin is involved in follicular maturation. The nutritional effect on
intrafollicular glucose and insulin may have practical implications to optimise feeding in dairy cows during phases of the oestrus cycle. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Cattle; Ovary; Feeding and nutrition
1. Introduction
The selection of dairy cows for milk yield has been accompanied by a continuous decrease in reproductive performance Nebel and Mc Gilliard, 1993; Bagnato and Oltenacu, 1994
and in blood insulin concentration Bonczek et al., 1988; Snijders et al., 1998. Ovulation failures during the first follicular wave in cows were associated with lower plasma concen-
trations of insulin in the study of Beam and Butler 1997, suggesting that low fertility may be associated with low concentrations of this hormone.
Insulin receptors are widely distributed throughout all ovarian compartments Poretsky et al., 1999 and there is in vitro evidence and indirect in vivo evidence for a stimulatory
effect of insulin on ovarian steroidogenesis, mainly mediated by the insulin receptor. In- sulin and IGF-I are potent stimulators of progesterone secretion by bovine corpus luteum
McArdle and Holtorf, 1989; Sauerwein et al., 1992; Spicer and Echternkamp, 1995; Mam- luk et al., 1999. Both hormones act synergistically with gonadotrophins to increase cell
proliferation Spicer and Echternkamp, 1995 and enhance steroidogenesis in bovine granu- losa Spicer et al., 1993 and theca cells Spicer and Echternkamp, 1995; Spicer and Stewart,
1996.
Plasma concentrations of insulin and IGF-I decrease as nutrient-deficient cows become anoestrous Richards et al., 1989; Schrick et al., 1992 but, in contrast with insulin, which
is systemic and is supplied to the ovary by the blood, IGF-I is also locally produced in the ovary Spicer et al., 1992, and its concentration in follicular fluid FF is not affected
by nutritional status Spicer et al., 1992. Therefore, insulin concentration in blood is a more likely candidate to serve as a nutritional signal to the brain centres that regulate
GnRH secretion Miller et al., 1995. The concentration of insulin in porcine Spicer and Echternkamp, 1995 and human follicular fluid Poretsky et al., 1999 has been established,
but this does not appear to be the case for cattle. Follicular dominance is associated with high IGF-I concentration in the FF of cattle Webb et al., 1999 but such a relationship has
not been reported for insulin, to our knowledge.
There are several ways to increase circulating insulin levels. Diets that promote ruminal propionate production will elicit insulin secretion Landau et al., 1995 but decrease appetite
Farningham and Whyte, 1993. Feeding fat to Brahman cows resulted in higher circulating insulin concentration Beam and Butler, 1997; Thomas et al., 1997, but boosted milk
production in Holstein cows, thus impairing energy balance, and delaying resumption of
S. Landau et al. Animal Reproduction Science 64 2000 181–197 183
oestrus post-partum Moallem et al., 1997. Increased supply of dietary protein is associated with increased circulating insulin levels in ruminants in many studies Bassett et al., 1971.
Infusions of branched chain amino-acids such as leucine Kuhara et al., 1991; Downing et al., 1995 or the inclusion of leucine-rich components in the diet Landau et al., 1996;
Blache et al., 1996 increase plasma insulin. Protein in excess affects folliculogenesis in sheep in as few as 5 days Landau et al., 1996.
Schrick et al. 1992 have shown in beef cows that insulin concentration follow an oestrus-like rhythm, with a peak on the day of oestrus but no information relating insulin
to the oestrus cycle in dairy cows has been published to date. The objectives of the present study in high-yielding dairy cows were to assess: 1 whether
peripheral insulin concentration is affected by phase of the oestrus cycle; 2 whether insulin and glucose concentration in peripheral blood and in FF are affected by nutrition and; 3
possible relationships between insulin and preovulatory follicular status.
2. Material and methods
