A prepartum diet supplemented with rolled canola seed reduced pituitary sensitivity to GnRH in dairy cows during second week postpartum
Cows fed a prepartum diet supplemented with rolled canola seed (high in oleic acid, OLA) had longer interval from calving to first ovulation than cows fed diets supplemented with either linola (high in linoleic acid, LA) or flax (high in linolenic acid) [Colazo et al. 2009; JDS, 92:2562]. We hypothesized that the delayed ovulation in canola-fed cows occurred through suppression of pituitary LH since adding OLA to culture medium suppressed GnRH-induced LH release from porcine pituitary cells in-vitro [Barb et al. 1995; JAS, 73:1416]. To test our hypothesis, pregnant Holstein cows, blocked by BCS, were assigned to 1 of 3 prepartum diets supplemented with canola (high OLA, n=10), sunflower (high LA, n=10), or control (no oilseed, n=11) from 35 d before expected calving date until parturition. The concentrate portion of OLA- and LA-diets contained 0.99 kg rolled oilseeds providing 0.27 kg/d OLA or 0.31 kg/d LA. Average DMI ± S.E. (kg/d) was higher in control (15.30±0.63) than in canola (13.54±0.54), sunflower (13.31±0.57) diets. Blood was sampled during the first (6±1.00 d, n=15) or second (9±1.20 d, n=16) week postpartum, every 15 min for 6 h to measure LH pulsatility. Thereafter, 100 µg GnRH was administrated im and blood was sampled for 4 h to measure induced LH release. Treatments did not affect LH pulsatility during the first and second week postpartum. Mean, minimum, maximum LH, pulse amplitude, and frequency were 0.30, 0.11, 0.82, 0.40 ng/ml, and 4.24 pulses per 6h, respectively, and they were not affected by treatments or weeks. GnRH-induced LH release was not influenced by dietary treatments during the first week postpartum, but cows fed a prepartum diet high in OLA had lower mean LH (1.70±0.20 ng/mL) than in control (2.40±0.20 ng/mL; P=0.02) during the second week postpartum, but it did not differ from LA (1.80±0.20 ng/mL; P>0.05); LA versus control, P=0.09. After GnRH administration, diets did not affect LH peak (3.39 ng/ml), interval to peak (47.40 min) or area under curve (7.01 ng/ml per 4h). In summary, although, prepartum diets did not affect pulsatile and GnRH-induced LH release during the first week postpartum, cows fed a prepartum diet supplemented with rolled canola had lower mean GnRH-induced LH than those fed no oilseed.
Keywords: Oilseed, Prepartum, Luteinizing hormone