In the preliminary trial, the E1S concentration of the 5 pregnant sows was found to increase markedly from day 22–24 to day 26–28 (Fig. ).
Oestrone sulphate in serum of 5 individual sows bled on alternate days during early pregnancy.
In trial 2, 12 of the animals were mated or inseminated once, 55 were mated or inseminated on 2 consecutive days, while 23 were mated or inseminated on 3 consecutive days. Mating and insemination work started 4 h earlier for each consecutive day, so that the interval between matings or inseminations was approximately 20 to 22 h.
A total of 77 sows and 11 gilts farrowed with a mean total litter size of 13.8 piglets (s.e.m. = 0.3) and 10.3 piglets (s.e.m. = 0.9), respectively. The difference in subsequent litter size between gilts and sows was significant (p < 0.001). Litter sizes ranged between 3 and 21 piglets. Mean previous lactation length was 33.9 days (s.e.m. = 0.2 days).
The mean number of AI or matings was similar for both gilts and sows (1.82 and 2.17, respectively, p > 0.10). Between animal groups with different numbers of AI or matings, the litter sizes were similar (total number born = 13.1, 13.4 and 13.6, for 1, 2 and 3 matings, respectively, p > 0.10). No significant differences were found in E1S level on day 24 or on day 28 between animals with different numbers of matings or AI. For triple inseminated or mated animals there was a tendency toward lower E1S levels on day 24 with a proportionately higher increase until day 28 compared to animals inseminated or mated only once (p = 0.07 and p = 0.08 for day 24 level and percentage increase, respectively).
Mean serum E1S level on day 24 for the 88 pregnant animals was 4.1 ng/ml (s.e.m. = 0.2 ng/ml) while mean level on day 28 was 8.8 ng/ml (s.e.m. = 0.3 ng/ml). The E1S levels on day 24 and day 28 within animal were correlated (r = 0.35, p < 0.001).
Subsequent litter size was found to have a strong positive linear relationship with the day 24 E1S level (p < 0.001), while parity was only slightly correlated (p < 0.10). The R2 of this model was 0.26. Neither litter size nor parity was related to serum levels on day 28 (p > 0.10). The relationship between litter size and serum levels of oestrone sulphate on day 24 and 28 is shown in fig. .
Relationship (raw data) between oestrone sulphate concentration in serum 24 and 28 days after first mating or AI and total number of piglets subsequently born per litter.
When litter size was ranged in classes A-C and adjusted for parity, there was a significant relation with day 24 E1S concentration (p < 0.01), whereas parity was less strongly related (p < 0.10, the R2 of the model being 0.25). Least squares mean differences between day 24 E1S concentrations in the 3 litter size classes are shown in Table . Repeating the model with day 28 E1S concentrations resulted in non-significant parity differences (p > 0.10), while litter size classes were significant (p < 0.05). However, the only significant difference in this model was between class A and class B.
Table 1 Least squares mean differences in serum levels of oestrone sulphate between litter size classes A, B and C in sows and gilts 24 days after first AI or mating. (Litter size class A included 3–9 piglets; class B included 10–14 piglets and (more ...)
In the logistic procedure, E1S concentration on day 24 was negatively related and the number of matings or inseminations tended to be negatively related to the probability of a litter size <10 piglets (odds ratios for small litters = 0.16 and 0.21; p < 0.01 and p = 0.055, respectively). The oestrone sulphate level was divided into 5 groups with the mean value and the mean value ± 1 and 2 standard deviations, respectively, as midpoints for each of the groups. Estimated probability curves and proportions of small litters in the proposed E1S classes are shown in Fig. .
Figure 3 Observed proportions and estimated probabilities of litter size smaller than 10 piglets at term based on serum oestrone sulphate level 24 days after first mating. The probability curves reflect different numbers of consecutive days of mating or insemination. (more ...)