Organic production: Broccoli and lettuce as protection crops
11 Nov 2020
Erich Brennan and a team of collaborators from the University of California-Davis began a two-year field study that evolved into long-term research on high-value organic crops. The project is called “Salinas Organic Cropping Systems Trial,”.
We can obtain important results derived from short-term studies; however, these do not always capture the variability of each year. Instead, “long-term research often provides more reliable results that are useful to growers,” says Erich Brennan, a horticultural expert with the USDA Agricultural Research Service (ARS).
Challenges in organic production
Production costs for high-value organic crops like broccoli and lettuce can be very high, so growers are looking to make better profits through an annual rotation of two to three crops.
Many growers keep crop fields barren during winter due to the additional work and expense of tilling cover crop residues into the soil. Also, planting a cover crop requires enough time for its residues to decompose before planting high-value crops in the same field. For this reason, production schedules can become very strict if the grower experiences prolonged rains in spring that prevent deep tillage of cover crops during winter.
Brennan designed a long-term research program with various strategies for cover crops in an annual production system with broccoli and lettuce. Six of the strategies involved working a protection crop each winter; the other two required cover crops every four winters.
Brennan selected three winter cover crops that occur frequently in the area – rye, mustard, and a rye-legume mix – and planted them at a typical planting density, and at a density three times higher. It should be noted that planting density influences the ability of the cover crop to smother weeds.
All systems received the same amounts of fertilizer and irrigation, while pest management was the same in all cases. The harvest and sale of the broccoli and lettuce crops were carried out by a marketing agent.
Study results: short and long term
Brennan’s results indicate that the three cover crops produced more dry matter than the recommended amount of crop residue (2 t / ha) to maintain the proper level of organic matter in the soil.
Rye-legume and rye protection crops produced 25% more dry matter biomass than mustard crops. However, to obtain effective weed suppression with the legume-rye crops, planting at a density of three times normal was required, while the rye and mustard crops suppressed the weeds adequately with typical planting densities.
“In previous short-term studies we did not observe any difference in dry matter production between the three types of protection crops. We also thought that higher stocking densities were always required, ”says Brennan.
“However, in the eight-year study it was clear that mustard does not produce as much biomass as the other two crops and that the rye-legume mix requires the highest stocking density to effectively suppress weeds.” The long-term study also provided Brennan with more data about the variations in yield that the rye-legume mix exhibited each year, including why legumes – which account for most of the cost of seeds – they are not that abundant.
“We believe that the colder weather early in the crop cycle is going to help legumes compete with rye,” says Brennan. “So, if growers are expecting a warm, dry fall; They may not want to spend so much on a legume protection crop; instead, they may decide to use only rye. “
Organic Production: Notes on Nitrogen
One of the benefits of many cover crops is that they absorb nitrogen from the soil, helping to reduce nitrate losses to groundwater; streams and canals.
When growers prepare fields for sowing, they disc till to mix the residues of the protection crop into the soil. In this way, the nitrogen accumulated by the protection crop is recycled once more in the soil to be absorbed by the high-value crops that will be planted later. Brennan’s data revealed that the long-term average nitrogen uptake of rye and mustard do not differ significantly (108 and 115 kg N / ha respectively).
However, the rye-legume mix accumulated nitrogen at a rate of 150 kg / ha, possibly due to the nitrogen fixed by legumes.
Waste management in organic production
Producers often measure the quality of cover crop residues by the carbon / nitrogen ratio. “High quality” wastes have lower carbon / nitrogen ratios; so the residues break down more quickly and can provide nitrogen for high-value crops that will be planted later.
However, during the wet periods of spring, the nitrogen released quickly by these residues can leach below the root zone more easily. On the other hand, “low quality” residues with high carbon / nitrogen ratios can temporarily deplete nitrogen from the soil; as organisms use it to break down waste.
Brennan found that the carbon / nitrogen ratios of the cover crops increased throughout the winter and were always higher in rye than in the other two cover crops.
The slow decomposition of low-quality residues may require more nitrogen from the soil than the decomposition of the other two cover crops; thus reducing the amount of nitrogen available for high value crops. The scientist also uses his data on the carbon / nitrogen coefficients of the crops, as well as the dry matter production of the protection crop; to calculate the contributions of this crop to soil carbon levels; which were 30% higher in the case of rye and the rye-legume mixture (1.4 t / ha) than in the case of mustard; which only added 1.1 tons of carbon per hectare.
The value of your investment in organic production
Brennan concluded that rye and mustard crops grown at typical stocking densities were the best-cost cover crops; that allow to maximize the production of dry matter and carry out weed control. Sowing 30 kilos of rye seed costs more than sowing 5 kilos of mustard seed. Seed costs to sow a rye-legume mix at three times the typical density (14 kg / ha are required for good weed suppression in winter); they were about 10 times higher. On the benefit side, legumes can help fix nitrogen in the soil.
Ultimately, “producers must decide whether it is worth bearing the additional cost of planting a rye / legume protection crop; that it would provide more nitrogen for the next high-value crop; or if it makes more sense to use an organic fertilizer pelleted from chicken manure and protein meals, ”explains Brennan.
“These findings can help growers involved in high-value horticultural production systems know where to invest their money,” he concludes. “And as it happens in conventional farming systems; protection crops can help organic producers improve their environmental footprint; sustainable practices that help reduce nitrogen losses in the soil; a problem that is becoming increasingly important for agricultural producers ”in various producing regions of the world.