Effects of nitrogen supply on grain yield and protein content in hybrid grain sorghum

Abstract

Considerable concern has been expressed by both domestic users and importers of Australian sorghum over the low protein content of some of the grain being offered for sale. Some grain samples have been found to contain as little as 5% crude protein, compared with the 9.2% minimum required for export sales. There has been a tendency both in Australia and overseas to associate the problem of low protein levels with increased use of hybrid sorghum varieties.This study examines, under closely controlled conditions of nitrogen supply, the effects of nitrogen supply at particular stages of growth, on both grain yield and protein concentration in the high yielding hybrid variety, Texas 610.A method was developed to control the nitrogen status of sorghum plants grown in solution culture by the use of small frequent additions of nitrogen, the size and frequency depending on the desired growth curves of the plants.In these solution culture experiments it was found that nitrogen stress could cause substantial reductions in the number of florets initiated, and the number of florets which survived to anthesis. Nitrogen stress also caused substantial reductions in the number of viable pollen grains per flower.Contrary to what has been reported for many other crops, the sorghum plant retained the capacity to absorb large amounts of nitrogen throughout the post-flowering period and grain nitrogen concentration depended almost entirely on the level of nitrogen supply during this period.Results are presented which indicate that in plants grown under non-limiting nitrogen supply conditions, grain nitrogen content is restricted by sink size. When grain size vas increased by reducing the number of grains, both grain nitrogen content and concentration increased considerably. It would appear that changes in grain structure and in the capacity of the endosperm to store nitrogen are at least partly responsible.A large field trial was conducted to determine if the results observed in solution culture under glasshouse conditions held true in the field situation. Once again it was found that a shortage of available nitrogen during the pre-flowering period had major effects on yield, and that grain protein concentration depended completely on the level of available nitrogen during the grain filling period. Side dressings of nitrogen at the boot stage or at anthesis were found to cause substantial increases in grain protein concentrations even when 300 lb of nitrogen per acre had been applied at planting.Results of the present study strongly suggest that low grain protein contents in high yielding grain sorghum varieties result from inadequate nitrogen nutrition during the grain filling period. For high yields of high protein grain, the fertilizer programme must be adjusted to ensure that the crop has an adequate supply of nitrogen up to and including the grain filling period.