Ž .
Fig. 1. The inverse relationship between conception rate CR and annual milk production of Holstein dairy Ž
Ž .
. cows in New York Butler, 1998, by permission of J. Dairy Sci. 81 1998 2533–2539 .
the first 3–4 weeks postpartum is highly correlated with the days to first ovulation. Because a shorter delay to first ovulation is positively associated with conception rate
later during the breeding period, the length of the postpartum interval to first ovulation represents an important interaction of energy status on reproductive performance.
High milk yield in cows is dependent on high levels of dietary protein as well as energy. Depending upon protein quantity and composition, serum concentrations of
progesterone may be lowered, the uterine environment altered, and fertility decreased Ž
. Butler, 1998 . Since metabolism and utilization of dietary protein depends on energy
availability, the effects of feeding high dietary protein superimposed on the preceding effects of NEBAL represents another important interaction of nutrition on reproductive
performance in dairy cows. A comprehensive review of nutritional effects on ovulation Ž
. and fertility in ruminants is recently available O’Callaghan and Boland, 1999 .
2. Nutrition and first ovulation in lactating dairy cows
Parturition results in an abrupt shift in the metabolic demands from nutrient accrual Ž
. body reserves and fetal mass to rapid mobilization of lipid and protein stores in
Ž .
support of the sudden onset of high milk production Bauman and Currie, 1980 . In dairy cows the rapid increase in energy requirements at the onset of lactation results in
NEBAL that begins a few days before calving and usually reaches its most negative Ž
. Ž
. level nadir about 2 weeks later Butler and Smith, 1989; Bell, 1995 . The timing of the
Ž NEBAL nadir has been implicated in the timing of first ovulation Beam and Butler,
. Ž
1999; Butler, 2000 that occurs about 30 days postpartum range of 17–42 days; Butler .
and Smith, 1989; Staples et al., 1990 . From a number of studies, NEBAL during the first 3 weeks of lactation is highly correlated with the interval to first ovulation. The
severity and duration of NEBAL is primarily related to dry matter intake and rate of
Ž .
increase during early lactation Villa-Godoy et al., 1988; Staples et al., 1990 . It is well known that cows overconditioned at calving will exhibit decreased appetite and develop
Ž more severe NEBAL than cows of moderate conditioning Garnsworthy and Topps,
. 1982 . As a result, overconditioned cows undergo increased mobilization of body fat and
Ž .
accumulate more triacylglycerols in the liver Rukkwamsuk et al., 1999
that are Ž
associated with a longer interval to first ovulation and reduced fertility Butler and .
Smith, 1989; Rukkwamsuk et al., 1999 . The first ovulation postpartum reflects the resumption and completion of preovulatory
ovarian follicular development and recovery from the hormonal conditions of late pregnancy. The effects of NEBAL on follicular development and first ovulation in
Ž .
postpartum dairy cows has been reviewed recently Beam and Butler, 1999 . Following parturition, a wave of follicular development occurs in 5–7 days regardless of NEBAL
and in response to an elevation in plasma FSH concentrations. The initiation of a follicular wave and formation of a large dominant follicle during NEBAL does not
appear to be a limitation for first ovulation. However, three outcomes of follicular
Ž . Ž .
development have been described Beam and Butler, 1997 : 1 Ovulation of the first Ž
. Ž . dominant follicle 16–20 days postpartum ; 2 Non-ovulation of the first dominant
Ž . follicle followed by turnover and a new follicular wave. 3 The dominant follicle fails
to ovulate and becomes cystic. The development of non-ovulatory dominant or cystic follicles prolongs the interval for first ovulation to 40 or 50 days postpartum.
Ovulation of a dominant follicle during early lactation is dependent on the re-estab- lishment of pulsatile LH secretion conducive to preovulatory follicular growth and
Ž .
estradiol production Butler, 2000 . The NEBAL that develops spontaneously in dairy cows represents a physiological state of undernutrition which impairs LH secretion and
Ž .
deters ovulation as detailed in a previous review Jolly et al., 1995 . From another recent Ž
. review Beam and Butler, 1999 , the frequency of LH pulses was significantly lower
during the first follicular wave postpartum in cows that developed a non-ovulatory dominant follicle as compared to those in which the dominant follicle continued
development to ovulation. This is consistent with the observation that follicles emerging after the NEBAL nadir, rather than before, exhibited greater growth and diameter,
Ž enhanced estradiol production, and were more likely to ovulate Beam and Butler,
. 1997 .
Low energy availability during NEBAL not only suppresses pulsatile LH secretion, Ž
. but also reduces ovarian responsiveness to LH stimulation Butler, 2000 . Both plasma
Ž glucose and insulin are decreased in NEBAL cows Beam and Butler, 1999; Butler,
. Ž
2000 and insulin is known to stimulate bovine follicular cells in vitro Spicer et al., .
Ž .
1993 and in vivo Simpson et al., 1994 . In addition, plasma levels of insulin-like Ž
. growth factor-I IGF-I are directly related to energy status and IGF-I is critical to
Ž .
ovarian follicular development Beam and Butler, 1999 . In postpartum dairy cows the levels of IGF-I were 40–50 higher during the first 2 weeks in cows in which the
dominant follicle would ovulate as compared to levels in cows with non-ovulatory Ž
. follicles Beam and Butler, 1997; 1998 . Furthermore, plasma estradiol concentrations
Ž .
were highly correlated with plasma IGF-I levels Beam and Butler, 1998 . This is consistent with the demonstration that changes in systemic concentrations of IGF-I and
IGF-binding proteins affect their concentrations in follicular fluid and follicular develop-
Ž .
Fig. 2. A schematic model describing dominant follicle development circles and function in relation to Ž
. changing metabolic and reproductive hormones, and energy balance EB , during the first follicular wave
Ž .
postpartum in dairy cows. The first-wave follicle either ovulates or undergoes atresia. LH pulse frequency
is modulated by the day of the EB nadir and, to a lesser extent, the level of EB. The large upward arrows Ž
. indicate increased insulin-like growth factor-I IGF-I and insulin leading to improved responsiveness to LH
Ž .
Ž and greater estradiol E2 production by the dominant follicle Beam and Butler, 1999, by permission of J.
Ž .
. Reprod. Fertil., Suppl. 54 1999 411–424 .
Ž .
ment in heifers Cohick et al.,1996; Perks et al., 1999 . During the early NEBAL period, the ability of follicles to produce sufficient estradiol for ovulation seems to depend on
the availability of insulin and IGF-I in serum and the changing energy balance profile. A model for control of follicular development in postpartum cows is presented in Fig. 2
Ž
. Beam and Butler, 1999 .
Since the extent of NEBAL depends upon dietary energy intake relative to require- ments, nutritional strategies to minimize NEBAL are of great interest. Increasing dietary
energy intake by feeding more concentrates has limitations due to problems with digestion, milk composition and health. Alternatively, increasing dietary energy density
by increasing the lipid content could ameliorate the limitation of intake and better meet the energetic demands of lactation. The effects of supplemental fat in the diet of
lactating cows on postpartum ovarian activity and reproductive performance have been
Ž .
mixed Staples et al., 1998 . Fat supplementation increases follicle diameter, but does not consistently advance first ovulation. However, most studies with supplemental fat
reported some improvement in conception or pregnancy rates.
3. Relationship of NEBAL to fertility
