The Effect of Increasing Urea Intake in the Lactating Dairy Cow on Follicular Dynamics and Ovulation

This study was designed to investigate the effect of increasing the intake of QDN (in the form of urea) on the follicular dynamics of lactating dairy cows. Forty-two Holstein cows were selected from the ADAS Bridgets herd using inclusion criteria of third or subsequent lactation, calved in the previous 16 weeks and deemed suitable for rebreeding following veterinary examination. They were randomly allocated to one of two diets, control or high QDN. The control diet was based on maize and grass silage and fed as a TMR once daily, except for a sugar beet feed (2 kg/cow/d) which was fed to all cows separately on top of the ration in two equal feeds, after morning milking and after evening milking. It was formulated to meet metabolisable energy and metabolisable protein (ME & MP) requirements for early lactation cows according to the UK ME and MP systems. The high QDN diet differed from the Control diet only in the inclusion of 250 g urea per cow/day, this was mixed with the supplemental sugar beet feed to overcome any palatability problems associated with feeding high levels of urea. All cows were fed the control diet for an initial acclimatisation period of 3 weeks prior to the study commencing. At the end of this period the cows were synchronised and the cows on the high QDN treatment had their diet changed to the high QDN specification. The diet of both groups then remained unchanged for the remainder of the study period.

All cows were synchronised using an eight day progesterone/PGF_2α protocol. Rectal scanning was undertaken by means of a B-mode ultrasound probe (5 MHz) to measure follicular development. Cows were examined ten days before first insemination (the day of PRID^® insertion), seven days later (the day of prostaglandin injection) and daily thereafter for a maximum of five days to monitor the timing of ovulation. Each scanning occasion was recorded on video for subsequent measurements of the number and size of follicles, corpora lutea and embryos. Follicles were classed as small (<0.5 cm diameter) or large (0.5 cm or greater diameter).

No significant effect of diet on the number and size of follicles or number and size of corpora lutea was found at any time (P > 0.1), nor was there a significant effect of treatment on the timing of ovulation (P > 0.5). This suggests that increases in QDN intake have no significant effect on follicular dynamics or ovulation if the QDN is increased at least ten days before insemination