Reducing Seedling Crowding After Sowing (Thinning)
1.7 - Reducing seedling crowding after sowing (thinning)
Ethan Brock, University of Guelph, Canada
Related video(s): Sowing and thinning sorghum (Source: Access Agriculture)
Suggested citation for this chapter.
Brock, E. (2022) Reducing seedling crowding after sowing (thinning). In Farmpedia, The Encyclopedia for Small Scale Farmers. Editor, M.N. Raizada, University of Guelph, Canada. farmpedia.org
Introduction to Seedling Crowding
The success or failure of a smallholder farmer's crops is influenced by several variables, including germination percentage, seed planter accuracy, cost per seed, plant space per block, and costs of handling, thinning, and transplanting labour, among others (Steenis, 1970). The most common direct sowing practice, especially among smallholder farms, is to plant multiple seeds in the same place, due to the expectation of low seed germination rates (Steenis, 1970). However, when crops are too close to each other, they compete for the nutrients, water, and sunlight they need (Steenis, 1970). This competition results in lower initial growth rates, but ultimately one plant in the crowd becomes dominant, taking up most of the nutrients in the space. Thinning should be done as soon as possible so this dominant plant can reach its full growth potential while it is still young.
Thinning Requires Labour
It is evident that eventually, adding more seeds does not increase the final yield but rather overcrowds the plants and adds labour, given the need to reduce the plant population (thinning). For example, sowing one seed with an 85% germination success rate per space results in only 15% empty space. Sowing two seeds at the same rate may decrease the empty space to only 2%, but can increase thinning labour by 72% (Dumroese et al., 2009). In this case, it would have been better to over-sow a smaller percentage of the crop to reduce the time required for subsequent thinning (Dumroese et al., 2009).
Grain versus Fodder
If a farmer needs to grow fodder grain for livestock, there may be an option to produce fodder and grain more efficiently at the same time. Consider a seed lot with an 80% germination rate. Sowing two seeds per slot achieves at least one surviving seed in 96% of seedling slots, but this method necessitates thinning and still leaves 4% of slots empty. These unfilled cavities can be justified by an over-sow factor of 105% at minimum. Over-sowing factors above this minimum can be used to raise the "green stem count," which is useful for fodder production, but otherwise oversowing should stay to the minimum, followed by thinning if required. No more than a small percentage of slots should be vacant after seeding (Steenis, 1970).
A test was done with maize, revealing a method to follow when thinning that allows farmers to grow more green fodder. Variations of two, three, and four maize seeds were sown in each hole with regular spacing, and after eight and fourteen weeks the smallest plant or the second-largest plant was removed from each space. For grain production, two plants per hole were allowed to grow. Increasing the planting density to three and four seeds per hole enhanced green fodder output with no discernible impact on grain production. Thinning large plants rather than small plants resulted in more green forage without influencing grain output. On small farms, forage output could be increased by planting maize at higher densities than usual, rather than by thinning the crop less (Methu et al., 2001).
Lessons From Weeds Among Maize
During its early growing stages, maize is one of the crops most susceptible to weed competition (Rajcan and Swanton, 2001). Weeds compete with maize plants for resources including light, nutrients, space, and moisture, similarly to overplanting in crops. It is the same concept as when two seeds are sown together and one plant takes nutrients from the other, resulting in a lower or lesser-quality yield (Rajcan and Swanton, 2001). Lessons can therefore be learned about thinning from weed studies. Weed control has been shown to be a crucial management technique for yield that should be implemented to guarantee the highest grain output months later, and it is essential during the first 4 to 6 weeks after crops are planted to prevent weeds from reducing crop yields. The point is that early-season weed competition lowers yields more than late-season competition, and so, by extrapolation, thinning should occur as early as possible after sowing (Rajcan and Swanton, 2001).
Lessons From Maize in Kenya
Most Kenyan small-scale farmers sow a sizable number of acres of maize each year, making it the country's main food crop, with grain being the primary reason maize is farmed (Njoka et al., 2004). The suggested planting method for Kenyan hybrids and synthetics is one seed per space, which many farmers typically implement to maximize grain yield (Njoka et al., 2004). This method does not, however, allow for the thinning-based generation of feed. Farmers can use high planting densities along with a suitable thinning regimen during the vegetative phase of maize crops to reduce empty space in the crop and produce fodder through the season (Njoka et al., 2004).
In this study, the most suitable number of maize seeds per hole to produce fodder was 8; however, this produced the least amount of grain of any thinning strategy. It was hypothesized that competition for available resources caused grain yield to decline as the seeding rate increased. A significant amount of both grain and fodder yield was produced with 2 seeds per hole and thinning (Njoka et al., 2004).
Lessons From Intercropped Sunflower and Soybean
A field study was conducted to examine the forage potential, in terms of productivity and quality, of thinned sunflower and soybean intercrops. The study used three nitrogen (N) fertilizer rates (70, 105, and 140 kg N per ha) and three ages at forage removal (thinning): 15, 30, and 45 days after sowing for sunflower, and 30, 45, and 60 days for soybean (Nawar et al., 2020). The evaluated soybean and sunflower fodder characteristics were not significantly affected by changing the nitrogen rate. As the nitrogen rate increased, soybean fiber content dropped substantially, while the crude protein and dry matter contents of both sunflower and soybean rose significantly. The study's findings showed that after thinning, the removed sunflower and soybean plants could be used as feed, while the remaining plants in the field could be used to produce seeds. Nitrogen fertilizer rate had no effect on forage yield and quality in the trial; however, forage removal at 30 and 45 days after sowing for sunflower and soybean, respectively, optimized forage yield and quality (Nawar et al., 2020). When primarily attempting to produce seeds, delaying thinning until a later maturity stage would have a negative impact on final seed output. This method would optimize the intercropping system's advantages, particularly on smallholder farms (Nawar et al., 2020).
Lessons From Cotton in Africa
Contrary to many other annual crops, cotton yields are mostly unaffected by differences in plant spacing (Robinson, 1994). The main issue is that planting and thinning operations, which are carried out when labour is scarce, are made more difficult by closer spacing. Closer spacing also makes labour-intensive processes like harvesting and sorting more difficult.
Typically, the first effort includes hand thinning. Initiatives that help farmers overcome labour shortages during initial weeding would help expedite thinning, which might increase yields by 10 to 15 percent. A common spacing used by smallholders in Africa for cotton is 37,000 plants per ha (Robinson, 1994).
Labour and Cost of Thinning
The cost per seed, if determined, offers a compelling reason to spare seed. There is a basic fee per space to plant the crop, and the price varies based on how many extra seedlings need to be thinned from each space. A percentage of seedling production costs can be considered to carry less weight when growing larger and more desirable seeds, which counterbalances the cost of thinning. This has to do with how much it costs to have each empty space, which is more expensive for higher-quality seed. A more expensive seed can raise costs but also lower costs and labour when it comes to thinning after sowing. With a higher germination success rate, less seed is wasted by planting 2 or 3 seeds in one space, which encourages less required thinning and less vacant space (Steenis, 1970).
As a lesson, a farmer can imagine a scenario with a one-cent-per-seed price based on Steenis' study. Depending on a farmer's circumstances, this may appear high or low. If the germination rate was 94% and single seeding resulted in a 1-cent increase in the minimal cost per seedling, and any increase in sowing raised the cost of each seedling by similarly substantial amounts from seed and thinning expenses, then planting up to 2 seeds per space progressively increases costs; but after that, the law of diminishing returns prevails. In this instance, 1 seed per slot (hole) results in the lowest cost per seedling generated; however, one can consider sowing up to 2 seeds per slot if growth space is limited, thus reducing the required growth space by 8%. Beyond 2 seeds per space, costs rise without any further savings in land area. Single-seed planting becomes more economically feasible with smaller crop sizes, which suits smallholder farms (Steenis, 1970).
It is expensive, risky, and time-consuming to transplant thinned seedlings into empty slots, but if done correctly, it may be rewarding. This strategy can be applied to lower both the sowing factor and the oversowing factor. To generate enough thinned seedlings to enable transplanting to 100% crop fill, the sowing factor only needs to be slightly increased, requiring far less seed than simply planting multiple seeds and discarding the thinned seedlings. This approach also makes it possible to lower the green stem count while still reaching 100% fill; however, the transplanted seedlings may not survive (Dumroese et al., 2009). When transplanting is the plan, a transplanting tool can be very helpful.
Picture Based Lesson to Train Farmers
Practical Tips and Further Reading
References
1. Steenis, E. van. (1970). Calculating optimum sowing factor: A tool to evaluate sowing strategies and minimize seedling production cost. US Forest Service Research and Development. Retrieved November 10, 2022. Link
2. Rajcan, I., & Swanton, C.J. (2001). Understanding maize-weed competition: Resource competition, light quality, and the whole plant. Field Crops Research, 71(2), 139-150. DOI
3. Methu, J.N., Owen, E., Tanner, J.C., & Abate, A.L. (2001). The effect of increasing planting density and thinning on forage and grain yield of maize in Kenyan smallholdings. Tropical Science, 41(2), 68-73. Link
4. Nawar, A.I., Salama, H.S.A., & Khalil, H.E. (2020). Additive intercropping of sunflower and soybean to improve yield and land use efficiency: Effect of thinning interval and nitrogen fertilization. Chilean Journal of Agricultural Research, 80(2), 142-152. DOI
5. Dumroese, R.K., Luna, T., & Landis, T.D. (2009). Nursery manual for native plants: A guide for tribal nurseries. U.S. Dept. of Agriculture, Forest Service. Link
6. Meland, M. (2009). Effects of different crop loads and thinning times on yield, fruit quality, and return bloom in Malus × domestica Borkh. 'Elstar'. The Journal of Horticultural Science and Biotechnology, 84(6), 117-121. DOI
7. Robinson, J.B. (1994). Improving cash crops in Africa: Factors influencing the productivity of cotton, coffee and tea grown by smallholders (World Bank Technical Paper No. 216) by S.J. Carr. The World Bank, Washington DC. DOI
8. Njoka, E.M., Muraya, M.M., & Okumu, M. (2004). Plant density and thinning regime effect on maize (Zea mays) grain and fodder yield. Australian Journal of Experimental Agriculture, 44(12), 1215. DOI