Factors affecting cereal establishment and its prediction
About this project
Previous HGCA-funded research has shown that significant reductions in spring plant population are possible compared to the traditional figure of 275 plants per m2 (Spink et al. 2000). As growers move towards lower target plant populations the margin for error in terms of estimating the percentage of sown seeds that will produce plants is reduced. Improving prediction of seedling emergence and over-winter survival, through a better understanding of the major factors affecting germination and seedling growth, is therefore key to optimising seed rates and plant populations in cereals.
A large number of factors affect establishment. This review attempts to quantify the effects of a range of factors with a view to improving prediction of percentage establishment likely in any given situation. Two approaches have been used: a review of existing scientific literature; collation and analysis of experimental results where establishment was measured.
The collated database consists of some 1250 individual records from a broad range of experiments from 27 different sites over the last 25 years. The overall average autumn establishment was 67%; this however masked significant variation with actual establishment varying from 2 - 100%. The average establishment is surprising and should be an immediate cause of concern, as the generally accepted benchmark figure for wheat is 85%. Additionally, experimental work is usually sited in the better areas of fields and receives greater care and effort than do many commercial crops; as such establishment achieved would be higher than in commercial practice.The literature review indicated that the physiological processes involved in germination are well understood. This understanding, whilst of little immediate practical use, forms the basis for understanding how factors affecting establishment in the field affect the physiological process. A number of seed factors also have consistent, although generally not large, effects on establishment. For example, there are many reports that large seed results in better establishment, but reducing seed size from 61 to 26 g thousand grain weight only reduced establishment by 6%. There are also a number of factors influenced by parent crop management and seed storage, which could improve seed quality and may be worthy of further investigation and ultimately incorporation into seed production protocols. These factors could not be further investigated as few records of seed sown are maintained as a routine in field experiments.
The literature review indicated the importance of a range of soil characters that affect the ability of the seed to germinate and subsequently emerge. Those affecting germination could be summarised as those which affect seed-to-soil contact or soil water status and therefore the rate and duration of seed imbibition. Factors affecting emergence were largely those that influenced impedance, for example clod size distribution or sowing depth. These factors could not be tested directly in the data review; again few quantitative records of sowing depth, seedbed quality or soil moisture are taken routinely in field experiments.
Records of cultivations are fairly routinely made, so these were investigated. There were no large or consistent differences between primary methods of cultivation on establishment. There were however significant differences between secondary cultivation methods and with rolling. Rolling is generally carried out to improve seed-to-soil contact and, assuming it was only carried out in appropriate situations, would be expected to improve establishment, which it did by about 10%. Three main forms of secondary cultivation were used: power harrowing, discing or tined cultivations. Tined cultivation was the most successful with 80% establishment compared to 62-66% for the other two methods. No immediately obvious explanation could be found, and it may be worthy of further investigation. Meteorological records were collected for each of the sites and rainfall pre- and post sowing and soil temperature investigated to look for consistent effects. Perhaps surprisingly there were no effects of rainfall, but there were consistent effects of soil temperature. Contrary to many laboratory based experiments, which have been reported indicating optimum temperatures of 20-25oC, optimum soil temperatures (at a depth of 10cm) in the data review were 8-12oC for earlier drillings and 12-16oC for later drillings. The different optimum temperatures according to sowing date were thought to be due to interactions with soil moisture. These soil factors vary in the degree to which the grower can manipulate them; they all, however, are open to measurement using precision farming techniques. If such methods could be developed this would enable improved precision in seed rate used within fields resulting in more uniform crop stands, and less within-field variation to subsequently manage.
Other factors considered in the data review, which did not figure significantly in the literature, were soil type, previous cropping and variety. A large number of soil types were included in the data set but all soils were grouped into sands, loams and clays. There was little difference between the clays and loams with establishment averaging 60-65%; sands were significantly better however with establishment averaging nearly 90%. Although a grower has little control over their soil type, an effect as large as this is worthy of inclusion in any future establishment prediction system. Previous cropping had a significant effect on establishment; wheat crops following oats had the best establishment (79%) and the worst was following beans at 54%. The degree to which these effects were real, or due to confounding of, for example, crop type and soil type or sowing date was hard to establish. However, there were cases where previous crop may have affected pest or disease incidence and therefore establishment of the following crop.
Perhaps the most surprising result was the significant effect of variety. Seven varieties for which there were a sufficiently large number of records were assessed. Establishment varied from 73% for Claire down to 61% for Spark. This effect may in part be due to seed size but there may be other inherent genetic effects that could be exploited by breeders.
Over-winter survival is important as spring plant population influences yield potential. The literature review indicated that little work has been carried out on the subject. Ongoing seed rate work has, however, indicated that in some circumstances it can be a considerable effect. Additionally, in the data review, whilst there was insufficient data where both autumn and spring population was recorded to do a formal analysis, spring populations in some regions were about 15% lower than autumn populations.
A number of significant biotic factors were identified. Again these are rarely quantitatively recorded in field experiments and could not be further investigated in the data review. The major pests identified included slugs, frit fly, leather jackets, wheat bulb fly and wireworm. Whilst a reasonable amount is known about each in terms of life-cycle, and factors which affect pest population, little has been done to quantify damage they cause and how this interacts with the crop to cause yield loss. Research to investigate these interactions could contribute significantly to prediction of establishment but also targeting of pest control treatments. One main group of pests largely absent from the literature was birds; this probably reflects the difficulty of carrying out research into the damage they do, rather than their importance.
Significant improvements in establishment prediction could be made if suitable experiments to understand more fully their importance and suitable mitigation strategies can be designed.
The two main disease problems identified in the literature were fusarium seedling blight and septoria seedling blight. Whilst both can significantly reduce establishment, both are easily controlled by either use of clean seed or seed treatment fungicides.
Seed or parent crop management was identified as one area that has received less attention. The impact of nitrogen strategies on grain nitrogen, grain size and homogeneity may all impact on likely field establishment, especially where crops are sown in less than ideal conditions.
Optimising management of seed crops could improve growers' confidence in expected establishment. Several models have attempted to predict establishment. Bouaziz and Bruckler (1989b) under dry conditions in S. America focused on factors of most influence in that environment, namely water potential and temperature. Techniques to characterise seedbeds by video image analysis (Stafford & Ambler, 1990) and sensing of soil water content from mobile machinery (Whalley & Stafford, 1992) provide tools and real time information on seedbed quality. However interpretation of this information alone is insufficient to provide the basis of a model of establishment. This review and data analysis indicates that, although soil moisture and temperature have significant effects, many other factors have to taken into consideration if a model of establishment is to be safe and accurate in a given field situation. A predictive model for use under UK conditions could make use of the available algorithms. A model that fully accounts for establishment under UK conditions would also need to consider water availability, waterlogging and oxygen availability. Such a model would be improved by accounting for both soil and seed characteristics, over-winter survival, and the potential risk of pest damage.
This review of the scientific literature and the collation and analysis of results from a large number of experiments has identified many factors which are important in seedling establishment. However, it has also identified a serious deficiency in the monitoring and appropriate measurement of seedbed characteristics, which are essential in predicting seedling establishment. This lack of monitoring of soil and seedbed conditions even extends to those experiments claiming to compare different soil management and seedbed cultivation systems.
Research focused on improving the growers ability to predict establishment more accurately by developing simple measurements and observational techniques, will increase growers confidence in reducing seed and cultivation costs. This information could provide both direct information to the grower as well as be used to strengthen a predictive model of establishment.
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