Genetic shift in Setaria sphacelata in response to grazing

Abstract

Seven populations of Setaria sphacelata cv. Nandi were studied as spaced-plants. Six populations were derived from nitrogen-fertilized pastures grazed continuously at high, medium or low stocking rates and fertilized with high or low levels of N fertilizer. The seventh was derived from a commercial seed sample of the cultivar. Plants grown from the commercial seed sample were more erect, more variable in growth habit and taller than plants derived from the pastures. Genetic differences in the populations from the grazed pastures were largely associated with flowering date, with progressively earlier flowering in populations from more heavily grazed pastures. This was evident in both the heavily and lightly fertilized pastures. The earlier flowering was associated with a larger number of flower heads at a set date but was not associated with any detectable yield difference. Heavily grazed pastures were characterized by populations which were more variable in growth habit and sometimes more variable in flowering date than lightly grazed pastures. This could have been due to a random effect of faecal areas in heavily-grazed pastures promoting growth but reducing palatability, hence resulting in the retention of high variability. The populations did not differ significantly in chemical composition or in vitro digestibility.