3Land, Air & Water Resources Dept., UC Davis

This trial is being conducted in a Merlot vineyard on 5BB rootstock; the vineyard was planted in 1993. The vineyard is located in central Sacramento County. It was planted on a 7 x 11 ft. spacing. The soil type is San Joaquin silt loam. The vineyard is drip irrigated, with sprinklers available for frost protection and cover crop germination. Herbicides are applied on the berms in a 4-ft. wide strip.

The experiment consists of five treatments and four replications in a randomized complete block design. The treatments used are as follows:

1. California native perennial grass mix (Calif. barley, Calif. brome, ‘Berkeley’ blue wildrye) – mowed once in early spring and again after reseeding; 40 lbs. N/planted acre were applied
2. Winter annual forage mix (bell beans/field peas/common vetch/barley) – planted every fall after disking, and mowed and disked in late spring; no additional fertilizer
3. Winter annual reseeding clover mix (subclover, crimson clover, rose clover, bur medic) – mowed once in early spring and again after reseeding; no additional fertilizer
4. Annual cereal mix (barley/oats) – planted every fall after disking, mowed once in late winter, then mowed and disked in late spring; 40 lbs. N/planted acre were applied
5. Resident vegetation – disked periodically late winter through fall, no additional fertilizer

The native grass treatment was planted in October 1996 and the other mixes were planted in October 1997. The annual forage and cereal mixes were again planted in October 1998.

Each treatment occupies the row middle on either side of one entire 175-vine row; i.e., one plot consists of two adjacent middles. Each plot is separated by one disked middle. Unless otherwise indicated, all plant and soil measurements are made from or adjacent to 10 contiguous vines. Drip irrigation is applied uniformly across all treatments, and a typical fertigation program for the vines is used during the growing season. A 4-ft. herbicide strip is maintained in each row of this vineyard.

During this El Nino spring, the grower was unable to mow until June, and the cover crops grew exceptionally well. There were substantial differences in biomass of the cover crops (Table 1). Annual clovers grew little during the winter and did not catch up to the other species one month later. The annual forage mix produced the greatest biomass by April. Weed suppression was also slightly greater in this treatment. The biomass of the resident vegetation (control) was about a quarter to a third that of the cover crop treatments.

The total N content of the forage mix increased dramatically in March and early April, and was about 50 percent greater than that of the clover mix in April (Table 1). On a per-acre basis, the forage mix added just over 100 lbs. of N; this amount may be excessive if continued over time.

The cover crops used in this study had minimal effects on the vines in the first year (Table 2). Vine water stress, although minimal, was greatest in the native grass and clover mixes before the first irrigation only. Petiole nitrate-nitrogen content was somewhat higher in the forage mix treatment and lower in the native grass treatment. However, yields were similar and juice analyses were not greatly different.

In the first year, the use of the native grass is substantially more expensive than the other mixes because of the cost of seed (Table 3). However, the cost of maintaining these grasses in future years is relatively low. The cost of annual clovers is lowest in the long term if the stand does not require replanting in future years. Overall, planting seeds and disking, when needed, are the costliest operations involved in using cover crops.

Gophers. We searched the study site for pocket gopher mounds and feed holes in January, February, and March. We divided the study site into sample cells defined by row and vine number (total of 1392 cells), and recorded the number of cells in which sign was present. In January, both old and fresh sign was recorded. In February and March, only fresh sign was recorded. The results provide an indication of food and habitat preferences of pocket gophers within the site (Table 4).

Table 4. Proportion of total pocket gopher activity in each cover crop treatment

In all months, gophers showed a distinct preference for the clover cover crop. No fresh activity was observed in the barley/oat cover crop or the control rows. Low abundance of preferred food and disking may have deterred pocket gophers from the control rows. Unfortunately, with pocket gophers exhibiting such a preference for clover, we cannot comment on the relative preference of other cover crops to pocket gophers in the absence of clover. Many studies have shown that pocket gophers are extremely adaptable in their feeding habits and eat alternate food sources when preferred foods are scarce. It is likely that pocket gophers would switch to a different cover if clover weren’t present. However, reproductive output under those circumstances may be reduced.

Data similar to that above will be collected in 1999 and 2000. In addition, the Lodi-Woodbridge Winegrape Commission has funded Kate Scow (LAWR Dept., UCD) to examine the soil microbial content in these treatments.

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