The objectives of this study were to relate concentrations of plasma (PUN) and milk (MUN) urea nitrogen to pregnancy rate in dairy cows and compare various methods of analysis and preparation of milk for measuring MUN. In two experiments, blood or milk samples were collected on the day of AI from Holstein cows (n = 160 and n = 155, respectively). Three methods of MUN analysis were compared. Two laboratory chemical procedures yielded similar results, whereas a quick dipstick method overestimated chemical analyses. Before and after milking strip samples had MUN concentrations equivalent to those in composite milk. Concentrations of PUN or MUN greater than 19 mg/dL were associated with decreased (P < .02) pregnancy rates (18 and 21 percentage point reduction in the two experiments). In two subset groups of cows (n = 51 and n = 23, respectively), plasma progesterone or MUN concentrations were monitored during the 5-d period after AI. Plasma progesterone concentrations increased similarly during the period for cows divided into low vs high PUN but were greater in pregnant than in nonpregnant cows on d 4 and 5 (P < .04). The MUN concentrations showed low within-cow variation (CV = 8%) but were lower in pregnant cows and had a decreasing trend over time compared with nonpregnant cows (P < .05). Based on this study, plasma and milk will yield similar results for monitoring urea nitrogen in dairy cows; PUN and MUN concentrations > 19 mg/dL were associated with approximately a 20 percentage point decrease in pregnancy rate after AI in lactating dairy cattle.

The objectives of this study were to characterize early postpartum (PP) follicular development in dairy cows and to evaluate the influence of energy balance (EB) and the level of dietary fat on dominant follicle development and function. Forty-five multiparous Holstein cows were fed either low (LF, 3.3%), moderate (MF, 5.2%), or high (HF, 7.1%) total dietary fat beginning at parturition, and daily EB was determined for all cows. Follicular development was monitored by ultrasonography, and blood plasma was analyzed for metabolites, metabolic hormones, progesterone, estradiol, and FSH. After an increase (p < 0.01) in mean plasma FSH during Days 1-5, all cows experienced a wave of follicular development during the second week PP regardless of diet or EB. Feeding the MF (p < 0.05) or HF (p < 0.06) diet resulted in a greater number of class IV (> 15 mm) follicles on Day 14 PP as compared to the LF diet. Cows fed the MF diet had higher (p < 0.05) peak plasma estradiol during the first follicular wave and a shorter (p < 0.05) interval to first ovulation than both LF and HF cows. Follicular development prior to first ovulation was characterized by either ovulation of the first dominant follicle (OV, n = 19), one or more waves of nonovulatory dominant follicles (NOV, n = 18), or the formation of a follicular cyst (CYST, n = 8). Ovulation failure during the first follicular wave PP was accompanied by lower (p < 0.001) peak plasma estradiol (0.96 +/- 0.36 vs. 5.0 +/- 0.35 pg/ml), a smaller (p < 0.01) maximum follicle diameter (16.0 +/- 0.9 vs. 20.0 +/- 1.0 mm), lower (p < 0.09) levels of plasma insulin-like growth factor-I (IGF-I; 73.5 +/- 10.1 vs. 102.9 +/- 9.9 ng/ml), and a longer (p < 0.02) interval to the EB nadir (14.6 +/- 1.9 vs. 6.4 +/- 1.9 days). The number of days to first ovulation was positively correlated with days to the EB nadir (r = 0.55), with dominant follicles that emerged after the EB nadir exhibiting enhanced (p < 0.01) production of estradiol and greater (p < 0.01) ovulatory success. The development of large (> 10 mm) dominant follicles was not a limiting factor in PP reproductive recovery, and moderate dietary fat shortened the interval to first ovulation. Follicular competence early PP was associated with higher plasma IGF-I and a shorter interval to the EB nadir.

The objectives of this study were to relate energy balance and metabolic hormones during the early postpartum period in dairy cows with dominant follicle development before first ovulation and to evaluate the effects of prilled lipid on follicular development during the first follicular wave after parturition and the postpartum anovulatory interval. At parturition, 42 cows received a control diet (4.8% fat) or a diet supplemented with prilled fatty acids (7.0% fat). Energy balance was determined daily. Ovarian follicular development was monitored by ultrasonography, and blood plasma or serum was analyzed for estradiol, progesterone, and metabolic hormones. Dry matter intake was lower in cows supplemented with dietary lipids during the first 4 wk of lactation, but energy intake, energy balance, and the postpartum anovulatory interval were similar between diets. A wave of follicular development occurred in all cows during the 2nd wk postpartum, and 50% of all cows ovulated their first dominant follicle. Numbers of follicles that were 3 to 5 mm, 6 to 9 mm, and 10 to 15 mm on d 8 postpartum were similar between diets and unrelated to energy balance or metabolic hormones. Diameter of the dominant follicle during d 8 to 14 postpartum and maximum diameter of the first-wave ovulatory follicle did not differ between diets. Cows with nonovulatory first-wave dominant follicles had lower mean plasma concentrations of estradiol during d 8 to 14 postpartum, a longer interval to the day of the energy balance nadir, lower serum concentrations of IGF-I, and higher 4% FCM yield than did cows with ovulatory first-wave dominant follicles. Serum IGF-I during d 1 to 13 was positively correlated with plasma estradiol during d 8 to 14 postpartum. Possibly because of reductions in dry matter intake, the consumption of prilled lipid by dairy cows during early lactation may be ineffective in altering energy balance, follicular development, and the postpartum anovulatory interval. Ovulation failure of dominant follicles early in the postpartum period is associated with greater production of 4% fat-corrected milk, a delayed energy balance nadir, and reduced concentrations of peripheral IGF-I.

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