Base temperature and thermal time requirements for germination and emergence of temperate pasture species

The base temperature (Tb) and thermal time (Tt) requirements for germination and emergence of temperate herbage species were defined using a linear model of development rate against temperature. A T-b of less than or equal to 4 degrees C was found for all species. The Tt requirement for germination was lower for legume than grass species and generally lowest fur small seeded species. The poor relationship (R-2 = 0.42) between Tt requirements for germination and 50% field emergence indicated that germination results could not be extrapolated to predict the rate of field emergence. The Tt for 50% field emergence was highest for the small-seeded grass species cocksfoot (220 degrees Cd) and timothy (200 degrees Cd). The times (days and Tt) for emergence of white clover and perennial ryegrass were similar from five autumn sowing dates but 57 days after sowing on 21 March 1996 ryegrass seedlings were 12 times heavier. The implication of these results for establishment of pasture mixtures is discussed together with the need for accurate measurement of soil temperatures to assist the prediction of field emergence.