No description available.

No description available.

Five experiments were undertaken to identify metabolites and hormones that could mediate the effect of dietary energy status on reproduction, particularly pulsatile secretion of luteinizing hormone (LH) postpartum dairy cows. / In the first two experiments, the concentration of progesterone (P4) in tailhead adipose tissue and plasma in 12 cows at different stages of pregnancy and lactation was determined as was P4 produced in vitro by explants of tailhead adipose tissue. Concentration of P4 in adipose tissue was correlated with that of plasma P4 near estrus and during the luteal phase of the estrous cycle, and P4 was released in vitro by fat mobilization. / In the third and fourth experiments, blood was collected continuously for 16 h from four ovariectomized cows offered maintenance or restricted energy diets after priming with P4 or estradiol (E2) using a crossover experimental design. The results indicated that P4 released during body fat mobilization is minor and is not related to LH secretion. Dietary energy restriction influenced plasma LH concentration and pulse amplitude but the effect was modulated by P4 and E2 priming. Dietary energy restriction decreased glucose concentration but did not influence plasma non-esterified fatty acids (NEFA), cortisol, P4 and insulin levels. Cortisol was negatively related to LH pulse frequency. Glucose and insulin were positively and negatively correlated with LH pulse amplitude, respectively. Cortisol, NEFA and glucose jointly had a negative correlation with LH concentration. / In the fifth experiment, blood samples were collected daily for 60 d and every 10 min for 8 h on 18, 36 and 54 d postpartum from 24 cows (12 ovariectomized) fed low (1.4 Mcal/kg DM) (L) or high (1.7 Mcal/kg DM) (H) energy in a 2 x 2 factorial treatment design. LH pulse frequency was reduced at 18 d postpartum in ovariectomized cows, but not in intact cows, fed L. First postpartum ovulation occurred later in intact cows fed L compared to those fed H. Energy balance and plasma glucose concentration were lower, but plasma NEFA, $ beta$-hydroxybutyrate (BHB) and E2 concentrations higher, in cows fed L compared to those fed H. E2 concentration in intact cows fed L was elevated for a prolonged period prior to first ovulation. Diet had no influence on plasma P4 and insulin concentrations. Plasma E2 and BHB concentrations were positively correlated with LH pulse frequency in intact cows across diets and ovariectomized cows fed L, respectively. NEFA were negatively correlated with LH pulse amplitude in ovariectomized cows fed L. Glucose, NEFA and P4 were negatively, but BHB, E2 and insulin positively correlated, individually or in association, with LH concentration. / Overall, the results suggest that the effect of dietary energy status on LH patterns and timing of onset of postpartum ovulation is modulated by priming with or presence of ovarian steroids. The relationships of metabolites and hormones with LH patterns appear to change with dietary energy level, ovarian status and mutual associations among the metabolites and hormones. These parameters, especially glucose and BHB, may be potential mediators of the effect of dietary energy status on LH patterns. (Abstract shortened by UMI.)

No description available.

No description available.

Experiments were conducted to investigate the effects of formulating rations using different ingredients and of manipulating the cation-anion balance of the rations on production, acid-base status, metabolism of macrominerals and renal function of dairy cows. The effects of dietary cation-anion balance were studied during early, mid and late lactation as well as during the dry period. Feeding an alfalfa-haylage based diet vs a corn silage based diet, both having the same cation-anion balance did not affect the voluntary consumption, milk yield or milk composition of cows in early lactation. The acid-base status of the animals was not affected by dietary treatment. The lower urinary bicarbonate concentration observed with the alfalfa haylage-based diet was not associated with a lower total urinary bicarbonate excretion since urine volume was significantly higher than when cows were fed the corn silage-based diet. Manipulating dietary cation-anion balance, however, resulted in changes in acid-base status at all stages of lactation studied. Urinary bicarbonate concentration and total daily excretion were increased by a higher dietary cation-anion balance at all stages of lactation. Similar effects of dietary cation-anion balance on urinary bicarbonate did not, however, translate into similar changes in production or intake by cows at differing stages of lactation. Cows in early and mid lactation seemed to have benefited more from a highly positive dietary cation-anion balance than cows in late lactation or dry period.