Harvest frequency or scheduling can have major effects on pests, particularly weeds and diseases, but harvests can also be used to mitigate insect infestations. Generally harvest schedules are dictated by the desire to maximize yield and quality and by practical considerations such as irrigation scheduling, labor, and machinery availability.
In the intermountain regions of California, 3 to 4 harvests are common, 5 to 7 harvests are practiced in the Sacramento valley, 6 to 8 harvests in the San Joaquin Valley, and 8 to 11 harvests in the low deserts of Imperial Valley and Palo Verde Valley. Timings range from a low of 24 days to over 40 days between harvests.
Harvest Considerations for Successful Pest Management
Earlier harvests generally result in higher quality, but lower annual yields and lower stand persistence. This is known as the yield/quality tradeoff. The choice of harvest time represents a compromise between the customer's demand for quality and the grower's desire to maintain high yield and a vigorous stand.
While early harvest results in highly digestible, high protein forage, harvesting after the plant has had the opportunity to replenish root reserves increases yield and significantly improves health. This improves the competitive ability of the alfalfa stand, and the plant's ability to tolerate pests, especially weeds. These benefits of a longer cutting schedule are often carried over from season to season. Decisions involving harvest timing, cutting height, windrow management, wheel compaction by harvesting equipment, and border harvesting can affect pest problems, as well as yield, quality, and profitability.
Staggered Harvest Schedules
Growers have a major incentive to harvest early for high quality forage and higher prices, and sometimes to mitigate insect pests. However, this should be followed by longer cutting schedules to enable root reserves to recover so that long-term productivity is not affected. Therefore, a staggered schedule of long-short intervals is usually recommended, providing some high quality hay while enabling crop recovery for improved stand persistence.
Effects of Wheel Traffic on Growth, Compaction, Water
There are typically four trips across a field with each harvest (swather, rake, baler and balewagon or accumulator). This can crush new regrowth and cause significant compaction lowering water infiltration and promoting diseases due to a lack of oxygen to the roots. Loss of plant stand due to traffic can be significant enough to allow weed intrusion. Efforts to control traffic (e.g., eliminating the bale collection step by depositing large bales at ends of fields) can help.
Harvesting Early When Pest Numbers Are High
Although this should not be done regularly, an early harvest can quickly reduce the effects of high numbers of insects, making an insecticide spray unnecessary. Early harvest may be a good strategy to avoid further damage when the alfalfa crop has a late infestation of alfalfa weevil in spring, or caterpillar damage in summer and can help avoid an insecticide spray. After harvest, however, growers need to monitor fields carefully to detect early damage to the young shoots, which can be devastating to regrowth and the following crop.
Watch pest feeding under windrows. Alfalfa weevils, alfalfa caterpillars, armyworms, and cutworms may feed on growth under windrows, so these areas should be monitored carefully.
Fall Harvest Management
Improperly timed fall harvests can result in stand decline and winterkill. Ideally, the last harvest of the season should be early enough to allow plants enough time to build up reserves before the first frost or late enough (weather permitting) that minimal regrowth occurs after the last harvest. The timing of the last harvest is not critical in more southern regions, where frosts are later or nonexistent, but can play a role when using soil residual herbicides for winter weed control. For some of these herbicides, application should occur when plants are dormant (or at least growing slowly), though soil should be exposed to maximize efficacy of the herbicide. If the last cutting is too early, significant growth can occur before it is cold enough to apply the soil residual herbicide and the canopy will interfere with the herbicide reaching the soil.
While a winter canopy helps to suppress winter weeds, during a wet foggy winter, a canopy can enhance conditions that allow Sclerotinia stem and crown rot to develop and spread. Clipping (even late fall clipping) is an important management tool when conditions are right for Sclerotinia or where meadow voles are a problem.
Border Strip Harvesting
Border strip harvesting involves leaving uncut strips of alfalfa at various intervals across the field as a refuge for natural enemies. This also retains lygus bug and other pest insects in the alfalfa (where they do no harm), keeping them out of neighboring crops such as cotton or beans where they cause significant damage. Research has shown that this practice significantly increases the numbers of parasitoids and predators of aphids, caterpillars, and other alfalfa insect pests.
Leave 10 to 14 feet wide uncut strips adjacent to every other irrigation border. At the following cutting, uncut strips are left adjacent to the alternate irrigation borders. Care should be taken when in mixing 'old' (e.g. 56 day) and 'new' (28 day) hay since blends of 75/25 new/old alfalfa and 50/50 new/old alfalfa may not qualify as prime feed for lactating dairy cows; such blends are acceptable for use as horse hay, and for beef and dry cows. In addition, uncut strips of alfalfa afford some protection to adjacent crops such as cotton and to a lesser extent dry beans, by providing habitat for lygus bug.
For more complete discussion of harvesting and harvest management, see Irrigated Alfalfa Management for Mediterranean and Desert Zones, UC ANR Publication 3512 and Cutting Schedule Strategies to Maximize Returns , 36th Western Alfalfa & Forage Symposium.