Summer and Fall Cover Crops for Annual Rotations
Mark Van Horn
Student Experimental Farm, University of California, Davis
CA 95616
The ability of winter cover crops to add significant amounts of nitrogen to annual cropping systems has been clearly demonstrated in recent years. However, fitting such cover crops into annual rotations in the Sacramento Valley has proven problematic in many situations. Often, the winter cover crop has not grown sufficiently to contain large quantities of nitrogen by the time the grower wants to plant the following cash crop. This is particularly common with early planted crops such as some tomato and safflower plantings.
Under some circumstances, an irrigated summer cover crop be utilized. Recent work with relatively drought tolerant warm season legumes (e.g., cowpeas, lablab, crotalaria and sesbania) has shown that such crops can add 90 to 150 lbs. of nitrogen per acre with just two irrigations (and may add more with additional irrigation). Typically, such cover crops would be planted in July and incorporated into the soil 60 to 80 days after planting. Thus, this strategy will not work unless the preceding cash crop is harvested by early July.
An alternative strategy would be to plant a cover crop in August or early September. We have conducted trials with both warm season (e.g., cowpea and lablab) and cool season (e.g., purple vetch and 'Lana' vetch) cover crops planted in August and September. As with July-planted cover crops, two irrigations are required for satisfactory performance from these cover crops. For both August and September planting dates, the vetches contained more nitrogen than either of the warm season cover crops and the lablab contained more nitrogen than the cowpea. For early August planting dates, vetches produced over 175 pounds of nitrogen per acre (200 kg per hectare) by November 1 and continued to grow and accumulate nitrogen into the winter. However, given sufficiently dry fall weather, such vetch cover crops could be incorporated around November 1 and contribute a large amount of nitrogen to the soil without necessitating a standing cover crop during any of the winter.
The summer cover crops grow very slowly, if at all, after November
1 and frost kills these warm season species. Preliminary studies
with some lines of lablab show that its frost-killed residue may
inhibit winter weeds. Thus, it may be possible to plant lablab
in August (or earlier), allow the frost to kill it in the fall,
and leave it in the field to suppress winter weeds. Flail mowing
or otherwise chopping the lablab residues in the early spring
may be the only pre-plant field work needed if the rotation has
been sufficiently planned and the lablab is, indeed, effective
at suppressing the winter weeds. On-farm trials with lablab have
not adequately tested this strategy to determine if it is a feasible
one on a farm scale.
Cover Crop Research and Education Summaries