Cyanates, thiocyanates and cyanamides

Potassium cyanate (KOCN) is a herbicidal compound for the selective control of broad-leaved weeds in monocotyledonous crops, such as onion (Hedlin, 1948). It is readily soluble in water (65 g/100 ml at 10°C). Its usual rate of application is 6-14 kg/ha in the form of a 1-3% aqueous solution. Potassium cyanate is rapidly degraded in the soil. Its acute oral u ,o for rats is 850 mg/kg. 

All of the water-soluble thiocyanates are very phytotoxic compounds, ammonium thiocyanate (NH SCN) being the most active. According to Harvey (1931b) and Landen (1934) ammonium thiocyanate is a protoplasm poison, paralysing certain enzymes of the plant, (e.g. catalase) and coagulating cell proteins. 

Ranjau and Kaur (1954) found that a 2% aqueous solution of ammonium thiocyanate reduces the sprouting and respiration of potato tubers. 

Ammonium thiocyanate by itself is not used as a herbicide, although at a rate of 80-100 kg/ha it is suitable for total weed killing. At the same rates, it can be used for defoliation and for temporary soil sterilisation. It is rapidly absorbed both by the roots and the leaves of plants and is translocated. 

In agriculture ammonium thiocyanate is used in combination with aminotriazole (see triazoles), because it synergises the herbicidal action of aminotriazole. 

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