A four-year field experiment was used to evaluate the effects of sowing date and nitrogen fertilization on protein content and amino acid composition of grain of amaranth (Amaranthus cruentus L.). Seeds of A. cruentus cv.'G6' were sown at two sowing dates (May and June) and fertilized at rates to obtain three target levels of available soil mineral nitrogen (Nmin) set to 140, 200 and 260 kg N ha-1. Grain protein content and the sum of essential amino acids (EAA) were affected by growing season and date of sowing. There was a higher protein content (165 g kg-1) and lower EAA in protein (39.3 g kg-1) in grain of plants sown in June. Nitrogen application up to an Nmin target value of 140 kg N ha-1 raised protein concentration in grain, and maintained the content of EAA in protein. EAA in grain fertilized to the target value 140 kg N ha-1 was higher (397 g kg-1) than the standard requirement for preschool children (339 g kg-1). Among essential amino acids, only valine concentration responded to nitrogen supply. Leucine was the limiting amino acid in grain protein.
Key words: Amino acid composition, grain amaranth, nitrogen fertilization, Nmin, protein concentration, sowing date.
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