Determinants of nectar production in oilseed rape

With the aim of contributing to the development of models forecasting the melliferous characteristics and the potential honey yield of oilseed rape, we analyzed data on the temporal pattern of number of oilseed rape flowers, nectar secretion, sugar concentration, and weight of bee hives in relation to meteorological parameters, across three seasons of oilseed rape production in Denmark. The analyses showed that the time of flowering of oilseed rape is governed by degree days, while the number of flowers at the onset of flowering is determined by the preceding climatic conditions, with long frost-free periods in the beginning of a year (January-March) leading to the setting of more flowers. On the other hand, the decrease in the number of open oilseed rape flowers during the flowering period seems to be governed exclusively by inherent characteristics of oilseed rape. A late flowering in 2013 was associated with significantly reduced quality of the nectar in terms of both volume and sugar concentration, and our results suggest that the oilseed rape in this year did not contribute notably to bee colony development. In years in which the oilseed rape flowered around the normal time (beginning of May), the amount of secreted sugar decreased over the flowering period, being governed by inherent oilseed rape characteristics modified by climatic factors preceding the onset of flowering, as well as climatic conditions during flowering. The exploitation of oilseed rape nectar by bee colonies was found to be influenced by the amount of secreted sugar, as well as by the temperature conditions and the strength of the colony during flowering. Our results will assist beekeepers in predicting nectar production and sugar yield in oilseed rape and the ability of bees to exploit the sugar.