Monograph Table of Contents

Effects of Biostresses on Alfalfa Quality and Quantity

Todd DeGooyer

INTRODUCTION

Alfalfa, Medicago sativa L., is the most valuable cultivated forage crop in North America (Fick and Mueller 1989). In the United States, over 11 million ha of land are committed to alfalfa production (Melton et al. 1988). Alfalfa is predominantly grown in the Northeastern and Northcentral regions of the U.S. (Bolten et al. 1972). In the Northcentral region, alfalfa usually is harvested 3 to 4 times a year during the late-bud to early-flower stages of growth. Harvest at these stages allows for maximum quality and quantity of forage. Alfalfa is fed as hay, silage, greenchop, pellets or cubes to a variety of livestock, but it also is grown for pasture and seed production (Fick and Mueller 1989).

The perennial nature of alfalfa makes it a very important crop in terms of energy and soil conservation. It functions in crop rotation, fixing nitrogen for subsequent crops, improving soil structure and fertility, and reducing pest problems for other crops (Fick and Mueller 1989). But, alfalfa's perennial nature and sensitivity to most herbicides makes weed management difficult. Furthermore, alfalfa's life cycle also permits pests to cause greater damage than they might cause if associated with annual crops (Grau et al. 1985). An overview of the effects of three biostresses, including weeds, insects and diseases, on alfalfa quality and quantity follows.

EFFECTS OF WEEDS ON ALFALFA QUALITY AND QUANTITY

When weeds are present in an alfalfa field, they affect yield and quality because they compete with the alfalfa plants for light, nutrients, and moisture (Oloumi-Sadeghi et al. 1989). The competition between weeds and alfalfa can alter the crude protein content, palatability, and digestibility of alfalfa (Mueller and Fick 1987, Cords 1973, Temme et al. 1979). Mueller and Fick (1987) found that crude protein values were highest when both broad leaf and grassy weeds were controlled, but found overall yields (total plant biomass) to be highest when there was no control of weeds. Furthermore, Cosgrove and Barrett (1987) found that first-harvest forage yields (alfalfa plus weeds) were either unchanged or less in herbicide-treated plots. These authors also found first-harvest alfalfa yield and crude protein content increased in the herbicide treatments, but in vitro digestible dry matter and acid detergent fiber content were not affected. Temme et al. (1979) identified three weed species, including Pennsylvania smartweed (Polygonum pensyvanicum L. ), shepherds-purse (Capsella bursa-pastoris (L.) Medic), and yellow foxtail (Setaria lutescens (Weigel) Hubb.) as the three weeds that had the greatest influence on lowering forage quality. In this study, these weed treatments produced the lowest percentages of crude protein and IVDDM and the highest percentage of acid-detergent fiber when compared to the pure alfalfa treatment.

EFFECTS OF INSECTS ON ALFALFA QUALITY AND QUANTITY

Insect pests have long been recognized as an important threat to the sustained production of high quality alfalfa (Osborn 1896). There are numerous insect pests that are injurious to alfalfa. These pests can be classified, based on the type of injury they cause, into two broad categories: leaf-mass consumers and assimilate removers. Leaf-mass consumers cause damage by either defoliation of aerial portions of the canopy or by defoliation of the stubble after regrowth (Hutchins et al. 1990). Assimilate removers characteristically possess piercing-sucking mouthparts that injure plants by extracting plant juices or by disrupting translocation (Medler 1941).

Leaf-Mass Consumers

Alfalfa weevil.

The alfalfa weevil , Hypera postica (Gyllenhal) (Coleoptera: Curculionidae) is mainly a pest of first-growth alfalfa in the Northcentral United States. The larval stage of this insect injures alfalfa plants by using its chewing mouthparts to defoliate the alfalfa. In some instances, when large weevil populations are in a field or when weevil phenology is delayed, injury to regrowth of the second-growth alfalfa also is possible (Foster 1986). Economic alfalfa weevil injury occurring during the first growth of alfalfa has been shown to reduce the quality and quantity of the first cutting of alfalfa and to reduce yield of subsequent cuttings (Berberet et al. 1981, Berberet and McNew 1986), although some studies have shown no significant carryover effect (Godfrey and Yeargan 1987). Furthermore, Fick (1976) demonstrated that even if root reserves were depressed in the growth cycle subsequent to plant injury, storage carbohydrates usually are replenished before the next cutting. However, because maturity of the injured alfalfa regrowth was delayed at harvest, crude protein and IVDDM was enhanced in the remaining herbage (Fick 1976).

Lepidopteran pests.

Several lepidopteran species are occasional pests of alfalfa. All of these species are leaf-mass consumers. The variegated cutworm, Peridroma saucia (Hübner) (Lepidoptera: Noctuidae) causes considerable damage to alfalfa by feeding on regrowth after the cutting. Buntin and Pedigo (1985) showed that variegated cutworm reduced dry matter accumulation and delayed alfalfa development. These authors also found that the overall leaf/stem ratio, crude protein, and IVDDM were enhanced in regrowth from injured plants. This was attributed to the younger physiological age of the defoliated plants. A lepidopteran pest complex (webworm [Loxostege cereralis (Zeller)], green cloverworm [Plathypena scabra (F.)], corn earworm [Helicoverpa zea (Boddie)], soybean looper [Pseudoplusia includens (Walker)], and velvetbean caterpillar [Anticarsia gemmatalis Hübner]) reduced stem length, crude protein content, IVDDM and dry matter yield of alfalfa in Louisiana. Although, in California, Summers (1989) found that low populations of the a lepidopteran pest complex (alfalfa caterpillar [Colias eurytheme Boisduval], beet armyworm [Spodoptera exigua (Hübner)], and western striped armyworm [S. praefica (Grote)]) produced effects that resulted in a yield increase in alfalfa when compared to the insecticide-treated treatments.

Assimilate Removers

Potato leafhoppers.

The potato leafhopper, Empoasca fabae (Harris) (Homoptera: Cicadellidae) is a major pest of the second- and third-growth alfalfa in the Northcentral and Northeastern U.S. (Lamp 1991). Potato leafhoppers have piercing-sucking mouthparts that are inserted into the alfalfa to remove plant juices (Hutchins and Wintersteen 1988). Injured plants exhibit a distinct inverted V-shape yellowing of the leaf (chlorosis), commonly referred to as "hopperburn" (Pedigo 1989). Injury to the alfalfa plant results from phloem destruction and clogging from cell hypertrophy induced by saliva injected during repeated insertion of the pest stylet (Medler 1941). The injury to alfalfa plants results in reduced photosynthesis (Pedigo 1989), which translates to reductions in plant height and dry matter production (Hutchins and Pedigo 1989). In addition to reduced biomass, alfalfa quality is altered by leafhopper feeding. Faris et al. (1981) also have identified reductions in crude protein and digestible dry matter associated with excessive leafhopper feeding. However, Hutchins et al. (1989) found only small effects of potato leafhopper feeding on forage quality and concluded that management programs for potato leafhoppers should be based on the likelihood of yield reduction or nutrient yield instead of alfalfa quality.

Aphids.

Aphids are another assimilate remover that are an important pest of alfalfa. Several species of aphids damage alfalfa, including the pea aphid (Acyrthosiphon pisum (Harris), spotted alfalfa aphid (Therioaphis maculata (Buckton)) and blue alfalfa aphid (Acyrthosiphon kondi Shinji). The primary effect of aphid feeding is it stunts the growth of alfalfa and subsequently reduces the quantity. Summers (1989) found that repeated subeconomic populations over several cuttings reduced yield late in the season. Pea aphid feeding was shown to reduce stem length (Harper and Kaldy 1982) as well as reduce final biomass yield (Cuperus et al. 1982). Kindler et al. (1973) found that the main effect of spotted alfalfa aphid injury was dry matter yield losses of 28% and stand reductions of 6%. Blue alfalfa aphid feeding was shown to affect leaf retention which stunted plant growth (Manglitz and Ratcliffe 1988).

EFFECTS OF DISEASES ON ALFALFA QUALITY AND QUANTITY

Leaf and stem diseases reduce yield and quality of alfalfa. Several diseases cause leaf spots, defoliation, and stem blackening (Willis et al. 1969). In the cool weather of the spring and fall, Phoma medicaginis Malbr. et Roum and Leptosphaerulina briosiana (Poll.) Graham and Luttrell cause brown lesions on leaves and stems (spring black stem), respectively. Control of these pathogens with Dithane M-45 increased yields from 4 to 19% in four alfalfa varieties (Willis et al. 1969). At high temperature, the principal disease is Cercospora zebrina Pass. (summer black stem and leaf spot), which causes gray-brown leaf lesions and stem blackening. In the same study, Dithane M-45 increased yields from 26 to 38% for the third and fourth cuttings combined. Anthracnose is a disease causing oval- or diamond-shaped lesions. These lesions can enlarge and join together to girdle and kill one or several stems on the plant, ultimately resulting in stand loss (Erwin 1990). Anthracnose causes yield reductions of up to 25% (Undersander et al. 1994). Bacterial wilt is a disease caused by the bacterium Clavibacter michiganense subsp. insidiosum (McCull.) Davis et al (syns. Corynebacterium insidiosum (McCull.) H. L. Jones, Corynebacterium michiganense subsp. insidiosum (McCull.) Carlson & Vidaver, and Aplanobacter insidiosum McCull.). Severely infected plants are stunted, which results in yield losses (Undersander et al. 1994).

INTERACTIONS OF BIOSTRESSES ON ALFALFA QUALITY AND QUANTITY

The interactions of biostresses imposed on alfalfa by weeds and insects also have been studied. Waldrep et al. (1969) showed that H. postica larval feeding damage (mainly reduced yields) increased when ground cover by henbit, Lamium amplexicaule L., was 50% or greater. Moreover, Berberet et al. 1987 demonstrated that insect feeding damage and weed interference reduced forage yield and stand longevity. These authors also found, in alfalfa left unharvested through the fall and winter, that the combined effects of weed competition and H. postica larval feeding damage were greater than the sum of the losses caused by each kind of pest occurring separately. In contrast, the removal of winter annual weeds from alfalfa in California increased populations of Egyptian alfalfa weevil, Hypera brunneipennis (Bohman), 20-50% (Norris et al. 1984), which probably increased the feeding damage.

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