Relationship of NEBAL to fertility

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

The detrimental effects of NEBAL or undernutrition in early lactation appear to be manifested as reduced fertility during the breeding period. In normal dairy herd situations direct assessment of energy balance in individual cows is not possible, but Ž . changes in body condition score BCS provide an indirect measure. With more Ž extensive loss of BCS, the reduction in conception rate becomes greater see Butler, . Ž . 2000 for review . Cows losing one unit or more BCS five-point scale during early lactation are at greatest risk for low fertility with conception rates of 17 to 38 reported in the various studies. A recent large survey study found that cows with a BCS Ž . of 3.0 at insemination were most likely to become pregnant Loeffler et al., 1999 . Although reduced fertility as a consequence of NEBAL in early lactation may be Ž explained by prolonged anovulatory anestrous in nearly 30 of cows Staples et al., . 1990; Rhodes et al., 1998 , our understanding of the linkage between NEBAL and suboptimum conception rates in ovulatory cyclic cows remains rather speculative. One important link between NEBAL and lower fertility appears to be through the aforemen- tioned effects on the timing of first postpartum ovulation. A positive association between the early commencement of ovulatory cycles and improved conception rate to insemina- Ž . tion is well documented Butler, 2000 . Minimizing the interval to first ovulation provides ample time for completion of multiple ovarian cycles prior to insemination, Ž . which in turn improves conception rate Butler and Smith, 1989 . Another important link between NEBAL and fertility may be the carryover effects on blood progesterone concentrations. Progesterone in the peripheral circulation increases Ž during the first two or three postpartum ovulatory cycles Villa-Godoy et al., 1988; . Spicer et al., 1990; Staples et al., 1990 and the rate of the increase in progesterone Ž levels is reduced or moderated by NEBAL early postpartum Villa-Godoy et al., 1988; . Spicer et al., 1990 . Cows with the most negative energy status during the first 9 days postpartum still had decreased serum progesterone levels during their third estrus cycles Ž . Villa-Godoy et al., 1988 corresponding to the start of the breeding period. The ability to produce and maintain optimum progesterone concentrations is important for fertility Ž . due to the effects of progesterone from one cycle to the next Folman et al., 1990 and Ž since plasma progesterone was higher in pregnant than in non-pregnant cows Butler et . al., 1996 . The ‘‘memory’’ by which NEBAL early in the postpartum period translates into Ž . reduced progesterone production two months later has not been established. Britt 1992 hypothesized that ovarian follicles are detrimentally affected by exposure to NEBAL during their early growth and development and that ovulation of affected follicles would lead to lower progesterone secretion. This hypothesis may well explain the pattern of serum progesterone concentrations in lactating cows, however, the effects of dietary intake on progesterone clearance must also be considered. In sheep, high dietary energy Ž intake increases metabolic clearance of progesterone from blood by the liver Parr et al., . 1993 . Plasma progesterone concentrations were about 25 lower in heifers fed a high-energy diet as compared to those fed a low-energy diet, presumably as a result of Ž . increased clearance Nolan et al., 1998 . In lactating cows high dietary protein may also Ž . increase metabolic clearance rate of progesterone Westwood et al., 1998 . During early lactation total dietary intake in dairy cows increases two-fold by the beginning of the Ž . breeding period Bauman and Currie, 1980 . During the breeding period any increase in Ž . progesterone clearance due to high dietary intake both energy and protein may be combined with the carryover effects of NEBAL to result in lower plasma progesterone concentrations and to reduce fertility.

4. Dietary protein intake and reproductive performance