Effects of high somatic cell counts in milk on reproductive hormones and oestrus behaviour in dairy cows with special reference to those with concurrent lameness.

The present investigation aims to establish the reason(s) why dairy cows with high somatic cell counts (SCCs; >100,000 cells/ml) are less fertile than cows with low SCCs alone. The objective of Study One was to determine whether differences in steroid hormone profiles could explain the low incidence of ovulation in cows with combined High SCC and lameness. Between 30 and 80 days post-partum, animals were scored for SCC and lameness and three groups were formed: Healthy (n=22), High SCC alone (n=12) or High SCC + Lame (n=9). The ovarian follicular phases of all cows were synchronised by administering gonadotrophin releasing hormone (GnRH) followed seven days later by prostaglandin F2alpha (PG). Milk samples were collected daily throughout the entire study period; twice daily during the follicular phase, blood samples were taken and the ovaries were monitored using ultrasonography. Progesterone concentrations were similar in all three groups during each of five specific time periods, i.e. throughout the five days before PG injection, the peri-ovulatory period, on Day 5 and on Day 7, and during the mid luteal phase 12-17 days after ovulation (P>0.13). Mean plasma oestradiol concentrations monitored every 12h during the 36h period before ovulation were similar in all groups (Healthy, 2.80±0.30pg/ml; High SCC alone, 3.82±0.48pg/ml; High SCC+Lame 2.94±0.51pg/ml; P=0.175). The objective of Study Two was to establish whether cows with High SCC (scored and synchronised as above) display different behaviours, especially the intensity and timing of oestrus. Intervals from PG to the onset of oestrus or to the first stand-to-be-mounted (STBM) were longer for the High SCC cows than the Low SCC animals (n=8 and 20; P=0.011 and 0.002, respectively). Also, cows with High SCC tended to have a less intense oestrus and a lower maximum oestrus score per 30-min period than Low SCC cows (P=0.063 and 0.066, respectively). In conclusion, High SCC±lameness did not affect progesterone or twice daily oestradiol profiles but the onset of oestrus was delayed and oestrus tended to be less intense in cows with High SCC. These factors could explain low fertility associated with High SCC.