Computation of heat unit accumulations in maize for practical application

Summary Every year maize breeders are facing the problem of trying to synchronize flowering of strains, hybrids and varieties to be crossed. Sowing in stages of one of the partners is the method most applied. In general this method gives good results, but there are years, such as 1954, when synchronization according to this method does not produce the results desired. The author discusses the possibilities to use heat unit accumulations for the synchronization of flowering of strains, hybrids or varieties to be crossed mutually.Sowing time trials over a number of years, conducted by the C.I.L.O. and the S.V.P. procured the data for the computations of the heat unit accumulations. Computations are made for threshold values of 0, 5, 7 and 10°C. It appeared that not all treatments have a similar threshold value, but that there are no great differences. In general they are approaching temperatures of 7 to 10°C.On assuming too low a threshold value the heat unit accumulations will appear to be too high, because temperatures are included which do not influence the development. This effect will be more prominent when more of these temperatures occur, for instance in a cold year or a year when sowing has taken place at an earlier date. In consequence the total of the heat unit accumulations can be seen to decrease, according as it is computed in a warm year or for a later sowing date. Conversely the heat unit accumulations will appear to be too low when a high temperature is assumed.If the precise threshold value is accounted for, the heat unit accumulation is equal, apart from the year or date of sowing.In 1955 a complete synchronization of the flowering of W 85 x W 15 and W 9 x WD was obtained by sowing them with such an interval as was indicated by the difference in the heat unit accumulations sowing-flowering in 1954. The computation of these heat unit accumulations was carried out on the basis of both air- and soil temperatures. It appeared that the air-temperatures alone are sufficient, provided sowing is done on the same type of soil as the previous year.A more accurate heat unit accumulation can be computed from the addition of the heat unit accumulation sowing-emergence (soil temperatures) and emergence-flowering (air temperatures).