Could a single P4 determination predict the days for successful breedings in bitches ?

Serial determinations of progesterone (P4) in serum or plasma provide a reliable estimation of the optimal breeding time in bitches (1). It is generally accepted that the optimization of the chances of pregnancy is obtained with matings or inseminations during the fertilization period, preferably 2 to 5 days after the ovulation day (2). In clinical practice dog owners and breeders often demand a reduction of the number of P4 measurements for cost saving, without affecting the pregnancy rate. The aim of this retrospective data analysis was to identify whether a single P4 determination could predict the days for successful breedings in bitches. Materials and methods. Fifty-nine heats of 42 Great Dane bitches aged between two and seven years were included in this study. Bitches with excellent health conditions and no previous reproductive problems were mated twice in 24h. A total of 107 determinations of P4 by Enzyme Linked Fluorescent Assay (MiniVidas, BioMerieux, France) were analyzed. The interval between the day of P4 concentration of 4-10 ng/ml (ovulation time) and the first breeding, with studs of proven fertility, was 1 to 3 days. Successful breeding was considered when the litter size was >3 (mean ± SD: 6±3) and the influence of bitches age on this parameter was evaluated by logistic regression. Concentration of 0.9 ng/ml was considered as P4 baseline. Exponential trendline between P4 levels (y) and days (x), with day 0 as the first day of vulval serosanguineous discharge, was established and the timeframe between day 0 and P4 of 4-10 ng/ml was obtained by a logarithmic function. Successful breeding rates were compared with Fisher’s and Chi-square test with significance set at P<0.05. Results. The rates of successful breeding were not affected by the age of the bitches. Exponential trendline resulted in y=0.9e0.2285x. The logarithmic function showed that P4 level of 4-10 ng/ml occurred from 6.78 (day 7) to 11.26 (day 11) after day 0. As no differences were observed in successful breeding rates of bitches bred 3, 2 or 1 day after P4 levels within 4-10 ng/ml, a ±2 days offset suggested day 9 as a proper day for a single P4 determination. On day 9 the logarithmic function showed a P4 level around 6-8 ng/ml; thus, P4 data from 4 to 10 ng/ml were divided into 3 groups: 4-6 ng/ml, 6.01-8 ng/ml and 8.01-10 ng/ml. Successful breeding rates were then compared in bitches bred 3, 2 or 1 days after P4 levels of 4-6 ng/ml, 6.01-8 ng/ml and 8.01-10 ng/ml and no differences were observed. Conclusions. It is well known that serial measurements of P4 for the identification of the basal value, the initial rise and the ovulation time increase the reliability of the estimation of the optimal breeding time. However, as in some circumstances a single determination is only permitted, the results of this analysis suggest that a P4 determination performed on day 9 after the onset of vulval discharge (when there is the highest possibility that the P4 concentration is 6-8 ng/ml), could predict the days for successful breedings in bitches.