Factors Affecting Alfalfa Stand Longevity in Montana
The second year of this three-year investigation into factors affecting alfalfa stand longevity in Montana was recently completed. Alfalfa remains the major forage crop grown in Montana with over 1.8 million acres in production supporting the $1.1 billion livestock industry. Alfalfa is grown on both rainfed “dryland” (57%) and irrigated (43%) acreage. Very little is known about the role of the clover root curculio (Sitona hispidulus) and other injurious Sitona species in the root and crown rot complex or stand longevity of alfalfa in Montana. Plant sampling and bi-monthly sweep net sampling are being used to concurrently evaluate major alfalfa insect pests including clover root curculio (CRC), alfalfa mosaic virus, and the complex of root and crown rot pathogens. The combined results from 2005 and 2006 are highlighting some of the key factors affecting alfalfa stand longevity. With three full years of data and observation, culminating in 2007, these factors and their trends will be better understood.
Fourteen alfalfa fields established in 2004 are being monitored over a three-year period. Forage production records are provided by the cooperating producers. Bi-monthly insect sweep samples are provided by county agents to the laboratory for identification and enumeration. Prior to first and final seasonal harvests, MSU specialists sample each field site and monitor the development of apparent clover root curculio feeding damage to roots, foliar disease symptoms, root and crown rot symptoms and stand measurements. The combined results from 2005 and 2006 are highlighting some of the key factors affecting alfalfa stand longevity. With three full years of data and observation, culminating in 2007, these factors and their trends will be better understood.
RESULTS and DISSCUSSION
Hay harvests. Dryland hay was cut once in late June to early July (as normal) and averaged 1.6 tons per acre. Three of the four dryland fields were grazed in fall or winter. Eight irrigated sites were cut an average of three times, with one field cut twice and one field cut four times. Irrigated fields averaged 2.6 tons per acre for the first cutting early to mid-June, 2.0 tons per acre for the second cutting mid-July to early August, and 1.2 tons per acre for the third cutting late August to early October.
Applications. Six of the producers applied fertilizer early in 2006. One producer applied the herbicide Buctril, targeting shepherdspurse and kochia. Two producers sprayed insecticides to control alfalfa weevil populations in June.
Pests. Bi-monthly sweep samples taken from early May to late August at all field sites, contained three main pests in 2006: alfalfa weevil larvae, pea aphids, and leaf hoppers. Alfalfa weevil larvae counts were highest before the first cutting, averaging 8-20/sweep in all fields, except Rosebud1, Gallatin, and Teton, which averaged less than 1/sweep.
Aphid counts varied throughout the state with half of the field sites averaging 5-12/sweep in the middle of the growing season, and the other half, 1-2/sweep. The Teton County site had 40 aphids/sweep during mid-June. Leaf hopper numbers ranged from 2-8/sweep in most fields.
Beneficial Insects. Most field sites were low in parasitic wasp and lady beetle numbers and high in nabid and minute pirate bug populations.
Pest Management. Two field sites, Rosebud1 and Custer2, were treated with insecticides (Mustang Max and Lorsban, respectively) to control alfalfa weevil in early and late June. Sweep samples from Rosebud1 did not indicate the need for alfalfa weevil control, showing than one alfalfa weevil adult or larvae per sweep. The Custer1 field site was cut early in order to reduce damage done by high alfalfa weevil populations.
Damage. Mild insect damage on alfalfa foliage in spring 2006 was seen in all fields on at least 8 of the 10 plants taken from each field. No foliar damage was noted in fall samples.
Diseases. Although plant samples were not tested for Alfalfa Mosaic Virus (AMV) in 2006 by DAS-ELISA, leaves were examined in the spring for yellow and green striping “mosaic” on the leaves, a typical indication of AMV. All but two of the field sites showed these physical characteristics of AMV on at least 1 of the 10 sampled plants, and in the case of Rosebud2, on all plants sampled. However, as learned in 2005, the ELISA test for AMV was not strongly correlated with physical display of mosaic. In 2005, some foliage appeared perfectly healthy, yet tested positive for AMV. In 2007, all plants will be tested for AMV using DAS-ELISA as well as examined for the physical attributes of the virus. This thorough analysis could potentially explain the discrepancy between the physical appearance of AMV and a positive test for carrying the virus. In addition, further study could reveal a trend of percent accuracy in predicting AMV infection by physical examination without having to perform a lab test.
Spring black stem (SBS) is a disease manifesting as black or brown spots or blackened areas on leaves and stems in early spring. The disease is caused by the fungus Phoma medicaginis, and is also associated with crown rot that can lead to a decline in stand density and vigor (UC-IPM, 1981; Graham et al., 1979). Symptoms of SBS appeared in more fields during 2006 than in 2005, ranging from higher to lower incidence depending upon field sampled (Table 2). General crown and root rot incidence also increased in 2006, a typical indication of an alfalfa stand getting older. Increased root and crown rot appears to be associated with clover root curculio larval feeding on the crown (Table 2). Fall samples indicated that three fields had mild symptoms of a disease that could not be identified, but physically appeared similar to that of SBS. Fall samples were not examined for crown or root rot.
Clover Root Curculio
Results from this study demonstrate an increase in clover root curculio crown damage in 2006, and a general correlation with the incidence of crown and root rot (Table 2). The CRC feeding sites allow crown and root rot pathogens such as Fusarium spp. to penetrate root cortical tissue and increase disease severity. Root infections reduce the cold and drought tolerance, and impede the over-all vigor of the plant (Hill et al, 1969). The combination of CRC and the crown and root rot disease complex could be a major factor limiting alfalfa stand longevity in Montana.
Sweep samples continued to yield low numbers of adult clover root curculios throughout 2006. From 1 to 6 CRC were collected per 25 sweeps, with the highest numbers found around the first cutting date. It remains unclear if adults lay most of their eggs in the fall or early in the spring. In either case (whether the adults overwinter or not), eggs hatch on the soil surface in the spring. The larvae burrow into the soil to feed on lateral roots and migrate to the taproot where they do the most damage as older instars. Adults emerge from pupation, perhaps in mid-June when highest population numbers are noted. This could explain why there was less increase in damage between fall 2005 and spring 2006 in comparison to spring 2006 and fall 2006 (Fig. 2). Most of the larval feeding (in all fields but Gallatin) must be taking place after our spring plant sampling in early May.
Root ratings from clover root curculio feeding continue to be correlated with soil type (Fig. 2). Our results agree with those of Pacchilo and Hower (2004) who demonstrated that silt-clay loam provided soil cracks larger than 1 mm for CRC larvae, increasing access of first instars to alfalfa root nodules. Sandy loams do not contain large enough pores and the texture is abrasive to the larva cuticle. In fall 2005, sandy soils had significantly lower rates of damage than silty and clay soils (P = 0.003). In spring 2006, alfalfa roots in sandy soils were significantly less damaged than those in clay soils (P = 0.045). Silty and sandy soils were not significantly different from one another, although roots in sandy soils had a lower mean damage rating than that of silty soil, 0.38 and 1.12, respectively. Fall of 2006 showed no significant differences (P = 0.074). However, the trend continues with lower root ratings in sandy soil (mean = 0.5) as compared to silty (1.7) and clay (1.5) soils.
Alfalfa yield in this study is monitored by producer field records for each harvest. First-harvest yields of the eight irrigated sites ranged from 1.9 to 3.3 tons per acre, and dryland fields 1.3 to 1.6 tons per acre in 2006 (Table 3). Depending on sites and production practices, irrigated alfalfa is harvested two to four times per season in Montana. Many producers opt to take two harvests by early August, and utilize the aftermath for fall grazing. For these reasons, the final harvest dates are variable across this study. For the final harvests in 2006, the irrigated yields ranged from 0.5 to 2.0 tons per acre (dryland sites had insufficient alfalfa regrowth for harvesting).
One component of this study is to evaluate alfalfa stand performance over time by stand and stem density measurements used in other regions. Specifically, spring stem counts are used to determine the economic feasibility of maintaining an alfalfa stand for the current year (stem count per square foot): maximum yield (>55 stems), 75 to 92% of possible yield (40 to 50 stems), too weak to maintain (<40 stems) [Banks, 2000]. Other workers have developed regression equations to predict alfalfa yield directly from stem counts per square foot (Undersander et al. 2004). Currently, no recommendations are available to use stem counts to assess alfalfa productivity in Montana. Similarly, no data exist comparing alfalfa stands for stem density over time concurrently with known pests that disrupt longevity. Field stand data were collected at all sites just prior to spring and fall harvests in 2006 (Tables 3 and 4). Despite the common age of the stands, there was a large degree of variability in stem numbers per plant or per square foot. Irrigated fields had higher stem counts per square foot than dryland fields in spring 2006 (Table 3). This likely reflects lower seeding rates and stand densities of dryland fields (data not shown). For the first harvest, the three fields in excess of 55 stems per square foot had an average yield of 2.6 tons per acre. Two fields with 40 to 50 stems per square foot had an average first-harvest yield of 2.4 tons per acre. However, the three irrigated fields with <40 stems per square foot had an average first-harvest yield of 2.8 tons per acre. Fall stem counts per square foot tended to decline at most sites. Stem count does not appear to be strongly correlated with alfalfa yield in these new stands. Dryland production systems widely utilize very winterhardy cultivars with deep, broad crowns such as ‘Ladak 65’. It is unlikely that a single criterion such as stem count will accurately reflect yield of all varieties across wide environments and stand ages. However, we will continue to monitor these traits and do more thorough analyses with total annual yield and the pests that are monitored.
Impacts and Contributions/Outcomes
The targeted outcomes of this project will be a better understanding of biological interactions contributing to alfalfa stand decline and tools for alfalfa producers and consultants. While the project is not yet completed, some significant impacts have occurred as a result of the project: 1) in 2006, two of 12 producers applied insecticides for the control of alfalfa weevil (AW) larvae, and one producer harvested early to avoid AW feeding losses – concurrent sweep net sampling confirmed that one of the sprayed fields was below the economic threshold for AW control and this was a “teachable moment” for the county agent and producer; 2)increasing incidence and severity of crown and root rot decay appear to be correlated with clover root curculio (CRC) feeding damage to roots; 3) as reported in 2005 and in the literature, CRC feeding damage on alfalfa is more prevalent in silt loam and clay loam soils than in sandier soils; and 4) alfalfa yield assessments from spring alfalfa stem densities do not seem to be accurate.