Establishment of oilseed rape: the influence of physical characteristics of seedbeds and weather on germination, emergence and seedling survival

Abstract

From autumn through winter into early spring, many factors can operate to reduce plant populations, and a key part of ascribing 'cause and effect' is to determine precisely when plants are lost and relate this to the seedbed characteristics, weather and biotic pressures operating during that phase. This report examines the detailed relationships between the physical characteristics of the seedbed, the interactions with the weather and the performance of the seeds and seedlings in the absence of pests and weeds.

A previous study by McWilliam, Stokes, Scott, Norton, Sylvester-Bradley and Davies (1995)(HGCA Project report OS13) examined the effect of cultivation equipment on seedbed production in the presence of cereals residues and on subsequent seedling survival. This analysis clarified the effects of cultivation equipment on seedbed structure, identifying that a seedbed could be grouped into one of three broad categories: uncultivated (e.g. direct drilled), mixed (e.g. disced) and inverted (e.g. ploughed) and that the effects on the seed and seedling were largely the result of the structure of the seedbed and were not strictly linked to the type of implement used. Furthermore, this work showed conclusively that it was necessary to identify precisely when plant losses occurred so they could be related to the specific factor (or combination of factors) operating at the time of loss. The work reported here, builds on these initial investigations by providing more detailed information about the relationship between the individual physical factors operating within a range of seedbeds.

A series of experiments were conducted in both controlled environments and small scale field sowings at the University of Nottingham. The main findings from this work are reported below.

Germination was progressively restricted as soil moistures fell below critical levels. The 'availability' of water in a soil can be measured in terms of the pressure required to 'pull' water from the soil surface (water potential). Water potentials greater than -1.5 MPa severely restricted germination. The precise soil moisture at which germination was restricted was specific to an individual soil type because different proportions of clay, sand and silt modify the availability of water in the soil. For example, a sandy loam with 10% clay particles had to be dried to 5% moisture before seeds could not draw water, whereas in a clay soil with 50% clay particles, this point was reached at a moisture content of 23%. These results demonstrate the importance of understanding the water release characteristics of soils when determining the amount of rainfall required to raise exceptionally dry soils to water contents where water release will be sufficient to get most seeds through the germination and emergence stage. Furthermore, as seedbeds dried and approached the limiting deficit of -1.5 MPa, there was a progressive decline in germination not a sudden cut-off. Thus at the scale of the seed, some niches of the seedbed were 'wetter' than others or seed/soil contact was greater.

Laboratory investigations showed that during the early stages of germination when seeds were taking in water (imbibing), severe desiccation before radicle (root) protrusion did not kill the seed and once water was again available, germination proceeded. However, if drying occurred after the radicle had protruded through the seed coat, the seeds were usually killed. Short term desiccation often did not kill the seed directly, but resulted in abnormal roots with fewer root hairs, thus impairing subsequent growth of the tap root rendering the plant vulnerable to further desiccation events.

In situations where soils are likely to dry rapidly, consolidation with a roll is common practice. Whilst soil drying is reduced and seed soil contact is increased, it is important not to over consolidate. Seedling emergence was shown to be very sensitive to compaction especially on clay soils: emergence from 2cm in an alluvial clay was significantly (P<0.001) poorer at bulk densities heavier than 1.1 gcm-3. At a limiting bulk density of 1.2 gcm-3, selecting large seed >2mm in diameter increased the number of seedlings which emerged.

In laboratory investigations, seed sown deeper than 5cm in sand usually failed to emerge. In a clay soil, this 'critical' depth is less because of the greater resistance to emerging seedlings exerted by adhesion between clay particles. It was concluded that sowing depths between 2-3cm provided the best compromise between the risk of desiccation and losses resulting from failure to emerge from depth, especially when a large proportion of the seeds is less than 2mm in diameter.

In the field, 19 sequential sowings produced a wide range of seedbeds and variations in weather during each of the stages of establishment: sowing to germination, germination to emergence and emergence to final establishment when plants would contribute to yield. Detailed measurements of germination, emergence and subsequent plant loss allowed the losses in each phase to be quantified.

Germination in the field, under normal autumn temperatures where water supply was unrestricted, was above 95% i.e. comparable to germination in the laboratory. Poor germination did result from drilling into cold wet soils and where soil temperatures were below 3° C germination fell to 70%. Although such conditions are unlikely following drilling at more normal time this finding does indicate that there are important weaknesses in some seedlots which need to be investigated further.

There was a strong relationship between accumulated soil temperature (at 2cm depth) and the thermal time between sowing and 50% emergence. In most situations, 160° C d were required for 50% emergence. In autumn field experiments, between 30 and 40% of seeds sown failed to emerge. Much of this was attributed to deep sowing with many seeds sown below 5cm; incorporation of cereal residues in the surface 10cm layer of the seedbed reduced average drilling depths by reducing drill penetration. Straw in the top 10cm of the seedbed acted as a mulch and increased the soil moisture content relative to seedbeds where the straw had been buried at depth or removed prior to cultivation. This additional moisture was beneficial during germination and emergence in dry conditions but exacerbated the effects of wet conditions over winter. The effect of aggregate size on germination and emergence were smaller than initially thought. Experiments and observations from the field where water supply was not limiting, suggest that as long as seedbeds do not contain large, platy aggregates which physically impede emergence or have loosely packed aggregates which result in excessive drilling depths, then germination and emergence are unlikely to be restricted. However, cloddy, loosely packed seedbeds dry more rapidly than fine consolidated seedbeds and hence consideration must be given to water supply to the germinating seed and emerging seedling where soil moisture is marginal.

In the absence of effects of pigeons and slugs, post emergence losses from September and October sowings on a non-calcareous alluvial clay were 30%, but on a lighter sandy loam only 11% of plants were lost. In both cases, the main losses were associated with freezing conditions; in the case of the heavier, less well drained clay, these were exacerbated by waterlogging.

This report shows that the main phases during which losses occur are emergence and during frost conditions post emergence. More importantly, the key parameters responsible for these losses have been quantified, namely water supply, resistance to emergence and sowing depth during emergence and in post emergence losses time of sowing and drainage were critical factors. Surprisingly, straw and large aggregates where not deleterious to establishment as initially expected.

Combining this analysis with the previous studies of cultivation equipment suggests that the physical condition of the seedbed can be optimised for a given set of conditions by using appropriate cultivation equipment to exploit the natural tilth if present. In addition, improved seed quality (e.g. seeds with greater vigour or more reserves - large seed) is likely to improve germination and emergence where conditions are not ideal e.g. cold seedbeds, compact soils, surface crusting and deep sowing.