Photocontrol – tilling in the dark?

E. Johnson - Scott Research Farm

Problem
Many buried weed species develop a ligh dependent stimulus for germination. This led to the concept of photocontrol, (excluding light during tillage), as a potential way to reduce weed seed germination. Much research has been conducted on photocontrol in Europe and South America, but very little has been done in western Canada. We examined the potential of photocontrol in western Canada with an extensive literature review to indicate whether further research is warranted.

Background
The soil contains a massive amount of weed seeds with estimates as high as 137,000 seeds per square meter. The density and composition of the seed bank is very heterogeneous within a landscape and is closely linked to the land’s cropping history but will also be influenced by tillage practices, soil water, and soil type. In temperate climates most seeds are dispersed near the end of the growing season. Spring annual weed seeds must avoid germination before entering the winter. Late summer germination is avoided by seeds’ primary dormancy, light induced dormancy, and normally, low amounts of available water. As autumn progresses low temperatures additionally inhibit germination. Seeds can become light-dependent for germination upon burial in the soil. The first plants to emerge after disturbance of natural vegetation or perennial cover arise from dormant seed in the soil rather than from freshly dispersed seed. Large populations of viable buried weed seeds were found in soil that had been in pasture for six years. Soil cultivation produced a large flush of germination during the subsequent four weeks. Exhumed seeds had low germination levels in the dark but a 90-second light flash was enough to cause germination of a large proportion of the seeds. Light stimulus from soil disturbance is an important evolutionary survival mechanism. Soil disturbance will reduce competition from established plants, thereby improving the seedling s chance of survival. Light requirement can be a signal that the seed is close enough to the soil surface so it has a high probability of successful emergence. The light stimulus is mediated by a sophisticated photoreceptor in plants known as phytochrome that is extremely sensitive to fluctuations in light intensity and light quality. Phytochrome functions at all stages of the plant's life cycle, acquiring information on the light environment, and giving the plant the capacity to adapt to light fluctuations. Studies have shown that excluding light during tillage can reduce weed seed germination. Photocontrol has been accomplished by tilling at night or covering the tillage implement with a lightproof cover. Weed emergence from light-induced tillage is the result of the light flash experienced during tillage and not from light that may reach the seed after emergence. Experiments where tillage was conducted in the dark and the area covered with opaque plastic sheets had similar emergence to plots tilled in the dark and not covered. Light-excluded tillage has generally caused a greater reduction in the number of dicots emerging, with less impact on grasses. Emergence of small seeded broadleaf weed species such as lambs quarters, pigweeds, and wild mustard were reduced with night tillage, but there was no effect on large seeded broadleaf species like velvetleaf. However, another study found no relationship between seed size and light-induced germination. Jensen found that weed emergence from daytime tillage was due to an increased emergence of weed seeds from deep levels where light could not normally penetrate. Night tillage may also slow weed seeds’ germination rates, thus allowing a seeded crop a chance to emerge before weed emergence. Time of season can influence the success of photocontrol since many species have seasonal dormancy periods. A study in southern Sweden found that night tillage was most effective in May, less effective in April and August, and ineffective in October. Reduction in weed emergence from nighttime tillage was quite small in this study, ranging from 5 to 30%. Success with photocontrol may be dependent on the type of tillage implement used. Nighttime tillage was effective in reducing seedling emergence when a moldboard plow was used, but was ineffective when a chisel plow was used. Moldboard plowing may have moved a higher percentage of deeply buried, light sensitive weed seeds to the surface than the chisel plow. Results from photocontrol have been inconsistent with germination reductions ranging from no effect to as high as a four-fold decrease in some cases. In other cases, a decrease in relative emergence of many annual species has been reported but absolute weed numbers in light-excluded plots were still unacceptably high. The inconsistent response to photocontrol is likely due to the complexity of the seed germination process. In addition to light, weed seed dormancy can also be broken by fluctuating soil temperatures, and soil nitrate level. Also, some weed seeds can lose their light dependency over time and there can be considerable genetic variation within a weed species in their response to light.

Conclusions
Weed seed-bank dynamics are affected by agronomic practices and environmental and soil conditions. Tilling in the dark has shown potential in some experiments; however, results have been inconsistent. Organic producers may want to evaluate this practice on their own farms; however, they should not have high expectations of success.

Funding
Provided by the Canada-Saskatchewan Agri-Food Innovation Fund


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