Seed vigour and the effects of soil physical conditions on sugar beet emergence
Sugar beet (Beta vulgaris. L.) is sown to a stand in commercial agriculture. Consequently establishment of < 70 % can lead to an important loss in final yield and it is essential to obtain seed quality and pellet types that can give consistently high percentage emergence even under adverse soil conditions. This depends on suitable field testing and in use of vigour tests that can adequately simulate soil physical conditions in the field. In this study field and laboratory experiments were carried out to determine the effects of seed vigour, pellet type and soil physical conditions on the emergence of different sugar beet (Beta vulgaris. L.) seed lots. Field experiments were performed at three contrasting sites with seed lots of three different levels of vigour usually in four different types of pellet. The importance of imposing a stress was identified after the first field trial, in which few significant differences in percentage emergence were found. Therefore field trials with irrigated plots were carried out to determine the effects of heavy rainfall immediately after sowing. The result was a decrease in emergence percentage in irrigated plots and more significant differences (P <0.05) were found between seed lots. All field trials discriminated between high and low vigour seed (in common with laboratory tests) and between pelleted and unpelleted high vigour seed, but field tests did not rank order or discriminate as consistently between variants as did laboratory tests. Results from laboratory tests that were designed to simulate field conditions were compared with field emergence. Laboratory tests in soil or an artificial mixture (Erd and Sand-peat) under non-stressed conditions produced between 97.5-89.9 % emergence and did not differentiate well between different seed lots. In contrast 'wet' Erd and Sand-peat tests that had similar matric suctions to the irrigated field trials, gave between 90.1 to 50.4 % emergence, discriminated between different seed lots, correlated well with irrigated and other more stressed field trials. A model (BeetEM) predicting seedling emergence is also described and applied to sugar beet. The input variables are soil temperature at seed depth and the depth of sowing. The time for the hypocotyl to reach the soil surface is calculated using a base temperature and thermal time for hypocotyl appearance, and base temperature and thermal time and controlling the rate of pre-emergent shoot growth. Predicted emergence times were shorter than the observed. When the conditions were not stressed or seed lots were high in vigour the prediction was closer to the observed (within ½ - 1d out of a total of 12-14 d). However, in irrigated field trials low vigour seed lots took > 2 d longer to emerge than predicted.