The metabolic and hormonal responses of dairy cows to lactation are a vital area for investigation to further stimulate lactational and reproductive performance. Successful management of lactating dairy cows needs to integrate the disciplines of reproduction and nutrition with standard postpartum herd health programs to optimize both milk and reproductive performance. Working in close cooperation with Dr. Charles Staples (University of Florida, Animal Sciences Department), we have integrated systems of reproductive and nutritional management. The achievement of high-energy intake, to bring cows out of a decreasing negative energy status as early as possible postpartum, is critical for both reproductive and lactational responses. In the majority of lactating dairy cows, development of dominant follicles on the ovary occurs very early in the postpartum period. However, functional competence of these follicles varies in association with concentrations of IGF-1 in plasma and energy status in which the majority of these follicles emerged after the nadir in energy status. Both regulation of IGF-1 and preovulatory surges of LH appear to be critical to the efficiency of this process. Subsequent timing of ovarian cycles, measured by formation of CL, also is related to postpartum concentrations of IGF-1 and energy status. It is clear that the anestrous condition impacts reproductive efficiency to timed insemination systems such as Ovsynch and that nutritional programs such as fat feeding may reduce the incidence of anestrus and thus benefit herd reproductive management.

Fats (concentrated energy sources) can be incorporated into the diet of cows in early postpartum in order to try to minimize the differences between energy intake and energy output. Absorption of total fatty acids by the ruminant is linear up to 1200 g/day (Staples et al., 1993), which is about 6% of DMI. Typical nonfat-supplemented diets contain about 2 to 3% fat. Therefore it appears that there is significant room to increase the use of fat in diets without loss of efficiency. Feeding supplemental fat has proven effective in improving reproductive performance of lactating dairy cows. Conception rates were improved by feeding calcium salts of long chain fatty acids (Garcia-Bojalil et al., 1998a and 1998b, Schneider et al., 1988; Sklan et al., 1989; Sklan et al., 1991). We have identified that the polyunsaturated fatty acids (i.e., EPA and DHA) in fish oil can suppress the uterine secretion of PGF2a. These basic studies have led to ongoing field studies to determine if calcium salts of EPA and DHA will specifically improve embryo survival. Results in cooperation with Dr. Jose Santos (University of California, Davis-Tulare) look encouraging, and such approaches involve targeting of specific nutrients to improve reproductive function may change markedly management approaches to improve pregnancy rates.

The detrimental effects of feeding a high degradable intake protein (DIP) diet on reproduction can be alleviated by supplemental fat feeding (CaLCFA ). One possibility of how high protein feeding may adversely affect reproductive performance is the increased energy costs to the animal for detoxification of ammonia resulting in a "weakening" of the cow's energy state. This energy cost is likely to push early postpartum cows even further into negative or less positive energy states, thus delaying return to normal ovarian activity. To test the effects of intake of energy and DIP on reproductive performance of lactating dairy cows, 45 cows were assigned at calving to 20% CP diets containing either 15.7% or 11.1% DIP and 0 or 2.2% CaLCFA (MegalacR; Garcia-Bojalil et al., 1998a, Garcia-Bojalil 1998b). Treatments continued through 120 days in milk. Cows fed the highly degradable protein diets had greater BUN values (22.0 vs. 17.3 mg%). Based upon progesterone concentrations of blood samples taken three times per week, cows fed the 15.7% DIP diets experienced more days to first luteal phase postpartum than cows fed other diets (39 vs. 25 days). All cows on experiment were synchronized to estrus between days 50 and 57. Cows not cycling prior to synchronization were assigned 50 days to first luteal activity. If cows had not been synchronized, the number of days to first luteal activity likely would have been even greater for cows fed the 15.7% DIP diets. Four out of 10 cows fed 15.7% DIP diet without CaLCFA were anestrus at synchronization compared with only three out of 35 cows fed the other dietary treatments. These prolonged days to restoration of ovarian activity and the anestrus condition were matched with greater loss of body weight and body condition by these cows. Cows fed 15.7% DIP diets lost more body weight and for a longer period of time compared with cows fed 11.1% DIP diets. The absence of CaLCFA resulted in a 10 kg greater loss in BW of cows fed 15.7% DIP diets. In addition, body condition loss was greater and more prolonged by cows fed the CaLCFA-free, 15.7% DIP diet.

Results indicate that dynamics of postpartum ovarian activity can be suppressed indirectly by feeding of high DIP (15.7%), but this adverse effect can be alleviated partially by feeding of CaLCFA. Also of interest was the observation that pregnancy rate by 120 days postpartum was increased from 52.3% to 86.4% when CaLCFA was supplemented and evaluated as a main effect across diets. This study demonstrated the specific benefit of feeding by-pass fat to increase ovarian cycles and reduce the incidence of anestrus in the postpartum period which is a major impediment to herd reproductive efficiency as described above.