The two most commonly used summer cover crops in our region are annual buckwheat and sudan grass. Buckwheat is the fastest-growing summer cover crop, and when planted at a high enough density and irrigated up, annual buckwheat will outgrow and “smother” most of the fast-growing summer annual weeds such as pig weed and lambs quarter. Once incorporated, buckwheat residue breaks down easily, allowing for seedbed preparation soon after incorporation. Growers always comment on the noticeable improvement in soil quality following buckwheat. Note that buckwheat is not very drought tolerant and therefore must be sprinkler irrigated at least every 7 to 10 days on the Central Coast. If planting buckwheat with a drill, a good seeding rate is around 60 lbs per acre. If broadcasting, increase this amount to 80 lbs per acre. The advantages of Sudan are that it can be mowed and incorporated 40 days after planting when the plants are in full bloom and it is a good weed competitor. Sudan grass grows best during very warm weather, so during cool foggy periods its growth may be less than optimal for good biomass production and weed suppression. At the UCSC Farm we have had good luck intercropping vetch and sudan grass as a summer cover crop. The advantage of the intercrop is that if the weather is unseasonably warm during the initial growth stage the sudan will dominate and, conversely, vertical grow system if the weather is unseasonably cool the vetch will dominate.
In either scenario the cover crop will provide good biomass and weed suppression. Typically, when intercropping two different species it is advisable to plant each at half the recommended seeding rate. If planting a sudan grass/vetch mix with a drill, a good seeding rate is around 20 lbs per acre of each seed type. If broadcasting, increase this amount to 30 lbs of each per acre. AGS104 rye or Merced rye will both germinate well and provide excellent weed suppression when grown as summer cover crops in our region, and can be mowed numerous times to keep biomass manageable and to knock off developing seed heads of escaped weeds. Cereals like rye and oats are typically drilled at around 80 lbs per acre and broadcast at between 100 and 120 lbs per acre. Residue breakdown and subsequent seedbed preparation will depend on the length of time the rye, sudan or vetch covers are allowed to grow prior to termination. Sudan grass can be drilled at 40 to 50 lbs per acre and broadcast at 60 lbs per acre. The higher the seeding rate the finer the stem will be and the easier the breakdown will be at time of incorporation.The best tool for planting cover crops is either a no-till or conventional grain drill. Depending on the scale of operation, either three point or wider pull-behind drills can be used. All grain drills have single or double disc seedline openers, which facilitate planting into high residue situations often encountered when cover crops are planted following high residue cash crops such as corn or broccoli. Some drills, such as the no-till drills, have press wheels that run behind the disc openers, which help to re-establish capillarity to aid in bringing deeper soil moisture up to the seed; this feature greatly facilitates planting to moisture. The press wheels, which can be adjusted with spring tensioners, also facilitate accurately setting the planting depth, which is a critical factor as well when planting to moisture.
Accurate seed depth is also easily set with a drill and information on how to set depth can be found in the operator’s manual. Drills typically put down seed lines spaced from 6 to 7.5 inches apart, providing a close enough spacing for good early cover crop canopy closure, which will greatly reduce weed competition during the critical early cover crop establishment phase. Drills typically have adjustable seed drop openers that allow for some level of accuracy in setting seeding rates. It is advisable to “calibrate” a drill to improve the accuracy of seeding, and thus avoid either over planting and running out of seed or under planting and having seed left over . Drills are fast and efficient at field planting cover crops once the proper seed delivery rate has been determined. The double disc openers seldom clog, but it is not uncommon for clogging to take place in the drop tubes especially when the drill is being used to plant large seeds at a high rate. The drill operator must continually monitor the output of the drops to ensure that no clogging is taking place. It is advisable to check drop output visually from the tractor seat at the end of each pass. Another common problem is for the disc openers to pick up field trash that can jam the openers. The drill operator must also be cautious about not allowing the drill to move backwards while soil engaged, as this will often clog the openers with soil. A clogged drop is easily detected in the field since seed from the clogged tube will start to overflow at the top of the drop tube near the hopper, and a mindful operator will notice this overflow while running the drill. When using the drill after it has been parked for extended periods it is critical to blow out each of the drop tubes with compressed air or high-pressure water to clear out spider webs and other debris that can cause clogging.
When going into a field with the drill it often takes several feet of ground wheel operation for the seed to make its way through the delivery mechanism, down the drop tubes, and into the soil in the slot opened up by the disc opener. For this reason it is always advisable to make a final perpendicular pass along both edges of the field, filling in the areas that were potentially skipped as the drill entered and exited the field.If a drill is not available the next best option is to broadcast the cover crop seed with a relatively inexpensive, three-point tractor mounted broadcaster. Small-scale growers planting out small areas can effectively broadcast cover crop seed using commonly available and inexpensive hand cranked broadcast seeders. Seeding rates are challenging to set with broadcast seeders. Depending on the area to be broadcast it is often advisable to set the seed opening smaller than anticipated and make numerous passes over a field to improve overall uniformity of seed application. Note that it is important to measure out your field sizes and estimate the acreage prior to broadcasting so that you know exactly how much seed needs to be broadcast on each block. Recommended seeding rates are typically increased by 30% when cover crops are broadcast and harrowed, compared to drilling, to compensate for the lack of seeding depth uniformity. Once the seed is broadcast at the desired rate the grower must go back over the field with some type of secondary tillage implement to improve soil/seed contact to ensure adequate germination and minimize seed exposed on the surface. Secondary tillage implements commonly used to cover broadcast seed include spring tooth and spike tooth harrows and three point rototillers. Implement choice will often depend on the amount of residue in the field, since spring tooth and spike tooth harrows tend to bring residue to the surface, vertical grow system which can negatively impact cover crop stand establishment. The drawback to rototillers is that they are best operated at much slower ground speeds than other types of harrows, so covering large areas can be time consuming. Discs can also be used to cover broadcast cover crop seed, but setting the correct depth is critical to avoid placing the seed too deep and/or over mixing the soil. Tandem discs are better than offset discs for covering broadcast cover crop seed since they tend to move less soil and are less aggressive. If available, it is advantageous to pull either a ring roller or drag bar behind the disc or spring tooth harrow when covering cover crop seed to leave a uniform soil surface. When broadcasting and harrowing cover crop seed, it is inevitable that some seed ends up getting buried too deep and some seed may be left on the surface where it is less likely to germinate due to either bird feeding pressure or inadequate moisture.
Growers have several effective tools for dealing with weeds in cover crops. Perhaps the best tool is selecting the optimum seeding density and having the ability to plant uniformly, in terms of both density and seeding depth, in mid to late October for fall planted cover crops, when soil temperatures are conducive to quick cover crop germination; this allows the cover crop to effectively out compete weed seeds. Timing in relation to soil temperature is critical for success since cooler soil temperatures later in the fall will favor the success of winter weeds over the cover crops. Soil temperatures are not an issue with summer planted cover crops. Other weed management tools include the use of cover crops known for their ability to outcompete weeds through allelopathy. Good examples of these include mustards and many of the cereals—most notably cereal rye—when planted as monocrops. Though effective at outcompeting weeds, growers must be mindful of the challenges of spring incorporation of cereal cover crops when planted as pure stands. Although often difficult to achieve, one of the most effective winter cover crop planting strategies for good weed management is to drill cover crop seed into ground that has had a flush of weeds from either a light overhead irrigation or early rainfall event in the fall. Light tillage with a spring tooth cultivator or under-cutter bar at time of weed seed emergence will knock out the newly emerged weeds. If done correctly the cover crop seed can then be planted into residual moisture and will germinate without additional rainfall or irrigation. This scenario will provide a strong and weed free legume/cereal mix cover crop stand. This technique is dependent on the use of a drill for planting . Soil moisture is critical as well since too much moisture will have a potentially negative impact on soil compaction. An effective way to deal with emerging weeds in a newly planted cover crop is to go over the field very quickly with either a rotary hoe or a tine weeder just as the cover crop is emerging. This technique is referred to as “blind” cultivation and can effectively clean up a weedy cover crop field. If the timing is right, the cultivation from the rotary hoe or tine weeder will not negatively impact the emerging cover crop seed but will effectively disrupt, kill, and/or desiccate the newly emerged weed seeds that are much smaller and closer to the surface than the drilled cover crop seed. This technique depends entirely on timing in terms of the stage of development of the cover crop and the stage of development of the weed seeds as well as soil moisture. Tine weeders work best when they can be run perpendicular to the drill lines—particularly on soils prone to crusting.On the Central Coast of California, incorporation of high residue cover crops can be extremely challenging. Because of our mild maritime-influenced winters and relatively high rainfall rates , a legume/cereal mix cover crop may produce 2 to 3 tons per acre of residue calculated as “dry weight.” The average weight per acre of a standing legume/cereal mix cover crop just prior to incorporation can be over 20 tons per acre . At time of incorporation this residue typically has a very high moisture level and, depending on the level of maturity, can be carbonaceous and lignified. Because of these conditions it is advisable to flail mow the residue prior to incorporation to break up the stems into manageable sizes to facilitate incorporation into the soil. Timing of incorporation is directly linked to soil moisture and the level of maturation of the cover crop. Every spring is different and cover crop incorporation timing often involves a lot of guessing about potential rainfall patterns and soil moisture and cover crop maturation dynamics. A standing cover crop can transpire a tremendous amount of water and soil moisture can vary at different depths, making incorporation decisions challenging.