An evaluation of their management in winter fallow vegetable production beds in the Salinas Valley:
Cover crops grown during the winter fallow period on vegetable production ground in the Salinas Valley provides benefits for crop production and the environment. For instance, we measured increased yield head lettuce in the spring lettuce crop following winter cover crops, however, by the second (summer) lettuce crop there was no measurable effect of cover crops on yield. It is unclear why cover crops increase the yield of lettuce, but they clearly provide a benefit to the soil and crop that improves growth.
Regarding environmental benefits, cover crops reduced the quantity of runoff and improve the quality water that runs off winter fallow fields during winter storms. In this regard, cover crops are a key cultural practice for complying with water quality concerns during the winter for vegetable producers.
In spite of the benefits of cover crops, there are serious obstacles to their use in the Salinas Valley. One of the big obstacles is that they increase the risk of getting rained out of the fields in the spring and missing planting schedules. As a result they are used on a small percentage of winter fallow fields.
We have been working on low residue cover crops to try to overcome the disadvantages of cover crops during the winter fallow period. The concept of low residue cover crops is that they are allowed to grow only until they are large enough to provide water quality benefits (i.e. reduce erosion and nutrient loss), but then killed before they produce too much residue that would impede subsequent planting operations in the spring.
In this way, growers can utilize the cover crops as a tool to protect water quality while not running the risk of having too much residue to restrict crop production.
We reported on our first studies last fall (Monterey County Crop Notes, Sept. 2007). In general, low residue cover crops do provide a good measure of the benefits provided by full cover winter cover crops. They reduce sediment and nitrogen in runoff over the bare fallow treatment. Low residue cover crops incur extra costs for vegetable growers because they need to be seeded and then killed, once they have produced sufficient growth; as a result, we envision this technique being appropriate for fields that have the greatest erosion risks during the winter.
How much residue is too much was the question that we investigated last winter. We conducted a trial with a cooperating grower on a site with a substantial slope in the Castroville area. We included four treatments:
1) full cover Merced rye sprayed early in the growth cycle;
2) full cover Merced rye sprayed later in the growth cycle;
3) Trios 102 triticale planted on the furrow bottom and sprayed later in the growth cycle; and
4) bare control.
The cover crops were planted on Nov. 9, 2007; the grower planned to plant broccoli at the site in late February, 2008 and the cover crop residue was managed to not impede soil preparation for that planting. Merced rye seed was broadcast with a belly grinder seeded at 230 pounds/A and Trios triticale seed was spread by hand in the furrow bottom at 71 pounds/A. The seed was incorporated immediately following spreading with a lilliston. A light rain fell on Nov. 10 which initiated germination of the cover crop seed. All cover crops were evaluated for biomass, nutrient content and percent ground cover. The early controlled Merced rye treatment was sprayed with a 2 percent solution of glyphosate on Dec. 15 (35 days after seeding) and the later controlled Merced rye treatment was sprayed with the same rate of glyphosate on Jan. 3, 2008 (54 days after seeding).
The Trios triticale was sprayed with glyphosate on Jan. 17 (68 days after seeding). Each plot was 8 beds wide by the length of the field (app. 700 feet long). Water monitoring flumes were installed at the bottom of the hill to measure runoff and water quality. The plots were not replicated and none of the data presented was subjected to statistical analysis.
Results: Full cover Merced rye grew quickly and provided rapid soil cover. Merced rye that was killed early in its growth cycle (35 days after seeding) disintegrated rapidly; its biomass went from 0.41 T/A on Jan. 3 to 0.13 T/A on Jan. 17 (app. One month after being killed with glyphosate).
It also rapidly leaked the nitrogen that it contained in its biomass. In addition, it provided 70.4 percent ground cover on Jan. 3, but had declined to 9.4 percent by Feb. 7. Evidently, the plant is too succulent at this growth stage and rapidly decomposes.
The Merced rye treatment that was killed when it was 54 days old, was more resistant to rapid decomposition; it had 0.36 T/A of biomass on Jan. 17 when it was sprayed with glyphosate and still had 0.27 T/A on March 11; in addition, it provided 57.4 percent ground cover on Jan. 17 and still had 34.1 percent on March 11.
The Trios triticale had a peak biomass of 0.20 T/A on Feb. 7 that declined to 0.16 by March 11. Its percent ground cover peaked at 65.9 percent on Jan. 17 and declined to 26.3 percent on March 11.
Unfortunately, we did not have sufficient rainfall measure runoff in the plots and could not correlate them with the biomass measurements. However, one of the goals of this project was to get grower feedback on the acceptability of the levels biomass provided by the cover crops. The cooperating grower stated that the levels of biomass that were left from the cover crops did not cause disruption to ground preparation and broccoli seeding operations.
Summary: This trial provided an opportunity to evaluate low residue cover crops in an on-farm trial. These results will need to be confirmed in future trials, but some preliminary observations were made:
— Cover crops need to be controlled early in their growth cycle. In this trial, 54 day old Merced rye and 68 day old Trios triticale were sufficiently resistant to decay to provide good measure of cover to the soil. — None of the cover crop scenarios disrupted the subsequent broccoli planting operations.
Further studies will be conducted to evaluate these observations and to better understand the benefits and drawbacks of low residue cover crops for proving benefits to water quality during the winter runoff period.