Brassica nitrogen dynamics: Fertility planning and seasonal N carryover

As this season is winding down and planning for next year begins, it’s a great time to consider nitrogen applications for your brassica crops as well as what nitrogen credits are made available to the crop following your brassicas.

There’s tremendous variability in the rates and application methods that growers use. Many fail to maximize the efficiency of their fertilizer by applying it at times when the crop doesn’t need it but weeds will hungrily accept it.

Most brassicas have similar seasonal N uptake demands, as depicted in the graph below (fig 1.), which plots nitrogen uptake over a typical cauliflower growth cycle. You can see that N demand dramatically increases around 25 days after transplanting, and 75% of the plant’s nitrogen needs are during the last half of its lifecycle.

Fig 1. Cauliflower seasonal N uptake [1]

Recent research from Cornell shows that when growers apply the same rate of fertilizer but choose to split-apply their nitrogen instead of only applying pre-plant, average yields increased by close to 5% representing an increase from 32.8 ton/A to 34.2 ton/A ^[2]. Across multiple farms and three years of study, the researchers found a 50:50 ratio of fertilizer application (at-planting : side-dress) produced the greatest yields when the side-dress application was banded at 30 days after planting.

By split-applying your nitrogen, not only are you matching your fertility to the demands of the crop, you are also minimizing the fertility available to your weed bank and reducing losses through leaching. And, by banding your fertilizer, the nutrients in the areas between rows are minimized whilst your brassica crop has maximum availability.

Brassicas, especially cabbage, are excellent N scavengers. This coupled with the fact that much of the plant is left in the field after harvest means that you can expect residual N the following year of up to 10 lb/A. A typical cabbage crop requires 200 lb/A of N over a growing season. After harvest, there is roughly 100 lb/A in the leaves, stump, and roots left on the field. This residue breaks down and is mineralized, ready for plant uptake the following season. Plant available N levels the season following cabbage averaged 8-10 lb/A ^[3]. This might not seem like much, it’s clearly no legume—but it’s definitely worth considering as fertilizer costs steadily climb. 

 

Tim Morcom

 

  

References:

[1] UC Davis, Cauliflower uptake and partitioning, http://geisseler.ucdavis.edu/Guidelines/N_Cauliflower.html#References

[2] Hoepting. C, Understanding Nitrogen Use In Cabbage: New York Study, CCX Cornell Vegetable program, https://rvpadmin.cce.cornell.edu/uploads/doc_857.pdf

[3] Hoepting. C, Understanding Nitrogen Use In Cabbage: New York Study, CCX Cornell Vegetable program, https://rvpadmin.cce.cornell.edu/uploads/doc_857.pdf

 

Cover crop videos

Perennia's Sonny Murray and Rosalie Gillis-Madden
discussing cover crops.

Did you plant a cover crop this summer?  Curious about how it will perform through the fall and into the winter?  Check out Perennia’s Cover Crop video series!  
To highlight a couple of the hits:

• Brassica Cover Crops

• Brassica Cover Crops through the Fall and Winter  

• Using Cool Season Grasses as Cover Crops 

• Legumes as Cover Crops 

• Legume Cover Crops Through the Fall and Winter  

• Oats and Winter Rye as Cover Crops  

Time to keep an eye out for cabbage maggot

With a cool start to the spring this year, insect pests have been a little later getting out and about than we would typically see. Now that there are warmer days upon us, it's important to pay attention to when those insect pests will be active so that control measures can be taken in a timely fashion.

Cabbage maggot (Delia radicum) is a particular challenge in brassica crop production. It overwinters in the pupal stage, emerging in the spring, usually coinciding with the bloom of yellow rocket and serviceberry. From a degree day perspective, emergence begins around 161 Degree Days (DD) at a base of 4°C, with peak flight occurring around 250 DD for the first generation of cabbage maggot. As of Friday May 22, 2020, 175 DD (base 4°C) had been accumulated in Kentville. Other Delia species, Delia platura (seedcorn maggot) and Delia florilega (bean seed maggot) are also on the move in the Valley.

Cabbage maggot larvae in soil. Photo UMass Extension.

Once the first generation of adults has emerged in the spring, they take flight and lay their eggs. It is important to know when peak flight is taking place so that you have the opportunity to alter planting times or deploy control measures, such as insect netting, accordingly.  For scouting purposes, eggs can usually be found small clumps on the soil around the base of your seedlings. For more information on cabbage maggot, including concerns about pesticide resistance, check out Perennia’s fact sheets “Cabbage Maggot” and “Chloropyrifos Resistance in Cabbage Maggot”.

Combating Clubroot

Clubroot is a soil borne disease that can survive in the soil for very long periods of time. The spores can go dormant when a host plant is not present, leading to the need for long crop rotations between brassica crops to reduce spore load in the soil. Equipment is the main source of disease spread as the spore-containing soil gets picked up and carried from field to field on tires and tillage equipment.

Soil on equipment and tires transfers clubroot from field to field.
Photo: Canola Council of Canada.

At Horticulture Congress in January 2020, Dr. Mary Ruth McDonald from the University of Guelph spoke about research she is conducting on clubroot control. One of the points that really stood out was the use of grass cover crops to help with clubroot control. A grass cover crop will help to keep the soil in place, therefore reducing the spread of clubroot by equipment. Planting a grass cover crop on the headlands or at the entry to fields with a history of clubroot could be beneficial in reducing this spread.

The clubroot fungus causes abnormal growth of root tissue, resulting
 in clubbing. Photo: Rosalie Gillis-Madden, Perennia.    

There is also some evidence that certain species or varieties of grass cover crops could be contributing to the management of clubroot populations by acting as bait crops. Perennial ryegrass has been seen to stimulate the germination of resting clubroot spores, allowing them to be taken up into the root hairs of the grass, but preventing them from completing their lifecycles and being released back into the soil. Dr. McDonald reported the resting spore concentration of bare soil compared to soil that had been planted with grass cover crops. Three varieties of perennial ryegrass (vars. Norlea, All Star, Fiesta), one of smooth bromegrass (common lot), and one of meadow bromegrass (var. Fleet) were tested, resulting in significantly lower resting spore concentrations in the Fiesta perennial ryegrass and smooth bromegrass.

Further research conducted by Sarah Drury, masters student from the University of Guelph under the advisement of Dr. McDonald, tested the effect of several common cover crops on the clubroot resting spore concentration in soil. Under greenhouse conditions, soils were inoculated with resting spores and then seeded with barley, spring wheat, field pea and perennial ryegrass. One treatment was also left fallow. Post-planting spore concentrations were measured and showed a significant reduction in initial levels for the barley treatment. All others were comparable to the initial spore concentrations.

Perennial ryegrass has been shown to stimulate germination of resting clubroot spores, in some studies.
Photo: Sonny Murray, Perennia.

Other management strategies include liming to a pH of 7.2 or higher, which inhibits the germination of the resting spores, and crop rotations of at least 4-5 years while controlling alternate weed hosts.

New pesticide registrations

Agri-Réseau is a resource available through The Centre de référence en agriculture et agroalimentaire du Québec (CRAAQ).  Every month, they publish a list of new phytoprotection registrations.  At Perennia, we are fortunate to have Mélanie Leclerc as part of our staff, and she has translated the March list of new pesticide registrations for Fruit Trees, Field Crops, Vegetables, Greenhouse, Berries, Ornamental, and "Other" (hops, hemp, quinoa, and basil), which can be found here.

From the Vegetable side, the main excitement is a few more weed control options in asparagus, Brassicas, and shallots, and some new disease management options in asparagus, kohlrabi, Brassica leafy greens, and bulb vegetables (Group 3-07A and 3-07B, so bulb onions, garlic, shallots, green onions, leeks, etc.)