284 THE EFFECT OF FEEDING PROPYLENE GLYCOL TO DAIRY COWS DURING THE EARLY POSTPARTUM PERIOD ON SERUM INSULIN CONCENTRATION AND THE RELATIONSHIP WITH OOCYTE DEVELOPMENTAL COMPETENCE

Abstract

High yielding dairy cows are typically in negative energy balance postpartum (pp). It has been shown that initiation of the first pp ovulation and, therefore, the resumption of normal oestrous cycles is delayed in high genetic merit dairy cows and is associated with lower circulating insulin concentration (Gutierrez et al., 1999 J. Reprod. Fertil. 24, 32 abst). Evidence shows that propylene glycol (PG) rapidly elevates systemic concentrations of insulin (Bremmer et al., 2000 J. Dairy Sci. 83, 2239–2251). The aim of this study was to determine the effects of PG feeding to dairy cows in the early pp period, on serum insulin and ovarian function, and on oocyte developmental competence after in vitro maturation, fertilization, and culture. Thirteen Holstein-Friesian cows were assigned to PG (n = 6) or control (n = 7) groups. Each treated cow received 500 mL of PG and each control was given 500 mL of water daily from Day 5 pp until day of AI. Blood samples for insulin were collected at 0, 30, 60 and 90 min post-drenching on Days 5, 15 and 25 pp. All cows were fed 3 kg concentrates at milking (twice daily) and had ad libitum access to a 50 : 50 maize silage : grass silage forage from the time of last blood collection. Oocytes were collected by ovum pick-up (OPU) in four sessions (following treatment with pFSH) beginning on Day 25–35 pp. The recovered oocytes were graded (Grade 1 to 4) in terms of their surrounding cumulus cells and the appearance of the cytoplasm. Grade 1–2 oocytes were matured in vitro, then fertilized using frozen-thawed bull semen, and subsequently cultured up to Day 8 in synthetic oviduct fluid. All data were analyzed using SAS version 6.12 and split-plot designs, following square root or arc sine transformation, if appropriate. PG significantly increased (P < 0.001) serum insulin concentration (0 min: 1.55 ± 0.19; 30 min: 4.48 ± 0.82; 60 min: 4.74 ± 0.72; 90 min: 4.10 ± 0.56) compared to the control group (0 min: 1.91 ± 0.28; 30 min 1.96 ± 0.27; 60 min: 2.37 ± 0.44; 90 min: 2.04 ± 0.26). The follicle size distribution was similar between treated and control cows for categories 2–4 mm (4.0 ± 0.47; 4.3 ± 0.70), 8–10 mm (3.2 ± 0.47; 2.5 ± 0.39), and >10 mm (0.42 ± 0.12; 0.67 ± 0.17). However, there were significantly more follicles in the 5–7 mm category (6.2 ± 0.82 v. 3.3 ± 0.43; P<0.05) for treated cows. The number of follicles punctured (13.8 ± 1.02; 10.7 ± 1.04), the number of oocytes recovered (4.5 ± 0.53; 3.5 ± 0.61), and the number of Grade 1–2 oocytes (2.8 ± 0.35; 1.8 ± 0.35) were not different between treated and control cows. Although cleavage rate (68.3 v. 58.9%) and blastocyst yield (25.3 v. 14.4%) were higher for treated cows, the differences were not significant. In conclusion, these results indicate that feeding cows with PG during the early pp period increased the circulating insulin concentration. However, the developmental competence of the recovered oocytes did not differ between the groups.