Objectives: To gain further data on the hormonal control of pregnancy in the donkey and to obtain reference values for hormonal pregnancy testing.
Material and methods: Blood samples were collected at monthly intervals from 23 donkey mares with normal singleton pregnancies. Further samples were obtained from six mares displaying pregnancies with clinical disorders. Progesterone (P4), total estrone (TE), free (E) and conjugated estrone (ES) were determined using radioimmunoassay.
Results: Mean duration of pregnancy was 372 ± 16 days. It was longer (p < 0.05) in large (375.9 ± 5.7 days) and standard (385.8 ± 20.7 days) donkeys than in miniature donkeys (357.4 ± 5.7 days) and negatively correlated to the age of the mare (p = 0.043). P4-concentrations varied between 12-35 ng/ml during weeks 2-5 of pregnancy and increased thereafter in eight jennies concomitant with the formation of the secondary corpora lutea (sCL), reaching values of 40-110 ng/ml during weeks 12-17. The decrease observed thereafter resulted in concentrations between 5-16 ng/ml until week 46, followed by a slight increase in most of the mares prior to parturition. Concentrations of TE remained < 1 ng/ml until week 6. They increased thereafter to 600-2700 ng/ml during midpregnancy and displayed a decrease to 1-20 ng/ml during the last 2 weeks of pregnancy. The course of E and ES was correlated (p < 0.0001) and E concentrations were up to 1000 times lower than those of ES. The course of hormone concentrations did not provide any clear indications in relation to the observed clinical disorders.
Conclusion: The course of P4-concentrations resembles largely the situation in the horse. In contrast to the horse, the course of ES does not show an increase concomitant with the formation of the sCL. Breed-specific effects became apparent regarding pregnancy duration.
Clinical relevance: Hormonal pregnancy diagnostic in the jenny could be put on a solid basis with TE values > 5 ng/ml being indicative for pregnancy. At present, monitoring of P4 and estrone during pregnancy does not allow the prediction of clinical disorders.
Keywords: Donkey mare; hormone concentrations; pregnancy.