Effect of rate and time of nitrogen fertilizer application on Striga hermonthica infestation in field-grown maize

The effects of urea and sulphate of ammonia applied at 0, 90, 120, 150 and 180 kg N ha'\\ (from 1989 to 1992) and the time and rate of N application (1992) on the severity of Striga hermonthica infestation and maize yield were studied in order to compare the two sources of N fertilizer; commonly available to farmers in Ghana, in reducing S. hermonthica infestation, and to determine the rate and time of application that would minimise the deleterious effect of Striga on maize. The experiments were carried out under natural field infestations on farmers' fields (6-10 locations each year) in the Guinea Savanna zone of Ghana and replicated three times at each location on plots 5 m long and 3.6 m wide. In 1989 and 1992, higher S. hermonthica population density was recorded when urea was applied compared to sulphate of ammonia. However, there were no differences in Striga population density between the two sources of nitrogen from 1990 to 1991. Applying N at either 4 or 6 weeks after planting (WAP) maize did not affect the number of emerged Striga shoots. The rate of nitrogen applied did not influence the emergence of Striga from 1989 to 1991 as compared to 1992 when Striga population declined with increased rate of nitrogen fertilizer. Maize grain yield was unaffected by the source of nitrogen applied from 1989 to 1991. Similarly, applying N fertilizer at 4 or 6 WAP did not affect maize grain yield in 1992. However, the application of slllphate of ammonia resulted in higher maize yield compared to urea in 1992. Maize grain yield increased with increased nitrogen rate up to 90 kg ha'\\ beyond which no appreciable increase was observed. A minimum of90 kg N ha'\\ is therefore required for any meaningful reduction in the de1eterious effects of Striga on maize. In addition, to reduce the population of Striga in maize, it is essential to apply N fertilizers early