Effects of N-fertilisation on amino acid composition and nutritive value of spinach, kale, caulifower and potatoes.

Abstract

AbstractSpinach, kale, cauliflower and potatoes were grown in pots at differing levels of nitrogen fertilisation. Increased nitrogen in the soil led to increased total‐N and nitrate contents but decreased amino acid concentrations (g/16 g total‐N) in spinach and kale. Similar treatments on cauliflower and potatoes also increased the total‐N but the nitrate concentrations were negligible; amino acid concentrations in the crude protein decreased except for aspartic and glutamic acids in potatoes. The essential amino acid indices and chemical scores decreased with increasing N‐content in all crops. When compared with egg protein, methionine and cystine were generally the most limiting amino acids. At low N‐levels, methionine and cystine were generally above the FAO amino acid reference limits. In rat feeding experiments, increased total‐N concentrations were associated with higher true digestibilities of the crude protein in all four crops, and the biological values tended to increase. Addition of synthetic methionine to potatoes increased their biological values by 35 and 12% at low and high N‐levels. Increasing total‐N contents increased the net crude protein utilisation in the four crops. Increased N‐applications decreased the nutritive value of the crude protein when judged on the basis of amino acid determinations, but the opposite conclusion was reached from the results of the feeding experiments.