Chapter 1.7
1.7 - Reducing seedling crowding after sowing (thinning)
Ethan Brock, University of Guelph, Canada
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. http://www.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 amongst smallholder farms, is to plant multiple seeds in the same place, due to the expectation of low rates of seed germination (Steenis, 1970). However, when crops are too close to each other, they will compete for the nutrients, water and sunlight they need (Steenis, 1970). Such 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 to start this dominant plant’s full rate of growth potential when it is 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 due to the benefit of reducing the plant population (thinning plants). For example, sowing one seed with an 85% germination success rate per space will result in only 15% empty space. Sowing two seeds with the same rate may decrease the empty space to only 2% but can add thinning labour by 72% (Dumroese, 2009). In this case, it would have been better to over sow a smaller percent of the crop to reduce the time required for subsequent thinning (Dumroese, 2009).
Grain versus Fodder
If a farmer has a need to grow fodder grain for livestock, there may be an option for them to produce fodder and grain more efficiently at the same time. A seed lot with an 80% germination rate is an example. The likelihood of at least one surviving seed in 96% of the seedling slots is achieved by sowing two seeds per slot, but this method necessitates thinning and still would leave 4% of the slots empty. These unfilled cavities can be justified by an over-sow factor of 105% at the very least. Over-sowing factors above this minimum can be used to raise the "green stem count." The green stem count is useful to raise fodder, 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 seeds of maize 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 producing grain, two plants per hole were allowed to grow. With no discernible impact on the grain production, increasing the planting density to three and four seeds per hole enhanced the output of green fodder. Without influencing the grain output, thinning large plants as opposed to small plants, resulted in more green forage thinning. On small farms, forage output could be increased by planting maize at higher densities than usual than thinning the crop (Methu, 2001).
Lessons From Weeds Among Maize
During its early growing stages, maize is the crop that is one of the most susceptible to weed competition (Rajcan, 2001). Weeds fight with maize plants for resources including light, nutrients, space, moisture, and water, 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, 2001). Lessons can therefore be learned about thinning, from weed studies. It has been shown that weed control is a crucial management technique for yield that should be implemented to guarantee the highest grain output months later. It has been shown that weed control 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 further than late in the season, and so by extrapolation, thinning should occur as early as possible after sowing (Rajcan, 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, 2004). The suggested planting method for Kenyan hybrids and synthetics is one seed per space, which many farmers typically implement to maximise grain yield (Njoka, 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 regiment during the vegetative phase of maize crops to reduce empty space in the crop and produce fodder through the season (Njoka, 2004).
In this study, the most suitable number of maize seeds per hole to produce fodder was 8. However, it produced the least amount of grain of any thinning strategy. It was hypothesised that the competition for available resources caused the grain yield to decline as the seeding rate increased. A significant amount of grain and fodder yields were produced with 2 seeds per hole and thinning (Njoka, 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) fertiliser rates (70, 105, and 140 kg N per ha) and three ages at forage removal (thinning), namely 15, 30, and 45 days after sowing for sunflower, and 30, 45, and 60 days for soybean (Nawar, 2020). The evaluated soybean and sunflower fodder characteristics were not significantly affected by changing the nitrogen rate. With increasing the nitrogen rate, soybean fiber content greatly dropped while the crude protein and dry matter contents of sunflower and soybean significantly rose. 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 fertiliser 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, optimised forage yield and quality (Nawar, 2020). Primarily attempting to produce seeds, delaying thinning until a later maturity stage, would have a negative impact on the final seed output. This method would optimise the intercropping system's advantages, particularly on smallholder farms (Nawar, 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 the operations of planting and thinning, which are carried out when there is labour scarcity, are made more difficult by closer spacing. Additionally, it makes labor-intensive processes like harvesting and sorting more difficult.
Typically, the first effort includes hand thinning. Initiatives that assist farmers in overcoming labour shortages in their fields during initial weeding would help to 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. To plant the crop, there is a basic fee per space. The price varies based on how many extra seedlings need to be thinned in each space. A percentage of seedling production costs can be considered to carry less weight when growing larger and more desirable seeds, which counter balances 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 they are sown. With a higher success rate of the seed, there is less needed waste in the form of putting 2 or 3 seeds in one space. This will encourage there to be less required thinning and less vacant space (Steenis, 1970).
As a lesson, a farmer can imagine a scenario of 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 the 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 think about sowing up to 2 seeds per slot if the growth space is limited, thus reducing the required growth space by 8%. Beyond 2 seeds in each space, it would become expensive, and the farmer is also not saving any further land area. Single seed planting becomes more economically feasible with smaller crop sizes, good for smallholder farms (Steenis, 1970).
It is expensive, risky, and takes time to transplant thinned seedlings into empty slot, 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 simply must be slightly increased. Compared to just planting multiple seeds and throwing away the thinned seedlings, far less seed is needed. By doing this, it is also possible to lower the green stem count and reach 100% fill. However, the seedlings may not survive transplanting (Dumroese, 2009). When transplanting is the plan, a transplanting tool can be very helpful.
Picture Based Lesson to Train Farmers
Practical Tips And Further Reading
Competition between plants: https://www.jstor.org/stable/2462213#metadata_info_tab_contents
Harvest and planting costs: https://www.fao.org/3/ca6411en/ca6411en.pdf
Growing cotton in Africa: https://doi.org/10.1017/s0014479700024856
Intercropping sunflower and soybean: https://orgprints.org/id/eprint/19489/4/19489.pdf
Effects of weeds within crops: https://zaszambia.wordpress.com/2020/11/22/effects-of-weeds/
References
1. Steenis, E. van. (1970, January 1). 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, from https://www.fs.usda.gov/research/treesearch/45630
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. https://doi.org/10.1016/s0378-4290(01)00159-9
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. From, https://hdl.handle.net/10568/29813
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. https://doi.org/10.4067/s0718-58392020000200142
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 https://www.fs.usda.gov/rm/pubs_series/wo/wo_ah730/wo_ah730_153_175.pdf
6. Meland, M. (2009). Effects of different crop loads and thinning times on yield, fruit quality, and return bloom inmalus×domesticaborkh. ‘elstar.’ The Journal of Horticultural Science and Biotechnology, 84(6), 117–121. https://doi.org/10.1080/14620316.2009.11512607
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. https://doi.org/10.1017/s0014479700024856
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. https://doi.org/10.1071/ea03015