Oestrous detection is crucial for successful dairy cow reproduction. Bulls identify cows in oestrus by oestrous‐specific odours especially in urine and vaginal fluid. These have been used to train dogs to detect cows in heat. To improve and simplify the dog training, a spray containing synthetic oestrous molecules was developed. The objective of this study was to test the spray on similarities to the natural substance thus to assess its suitability as a training substance for heat detection dogs. Ten privately owned dogs of various breeds were trained. Dogs should be trained either to differentiate natural vaginal fluid from cows in oestrus and dioestrus (n = 5), or spray with or without synthetic oestrous molecules (n = 5). Dogs trained on natural fluid and on spray could detect the oestrous odour they had been trained on with an overall accuracy of 69.0% and 82.4%, respectively (p = 0.019). To validate the synthetic molecules, dogs trained with synthetic molecules had to detect oestrous odour in natural fluid without further training (accuracy 37.6%). Dogs trained on natural fluid detected the synthetic molecules with an accuracy of 50.0% (50% vs 37.4%, p < 0.05). Dogs can recognize natural vaginal fluid from cows in oestrus after they have been trained with synthetic oestrous molecules, but accuracy needs to be improved.