Winter and Spring Sheep Grazing to Manage Alfalfa Weevil Populations
By Hayes Goosey, Pat Hatfield, and Dennis Cash
Fall regrowth of alfalfa serves as a major source of winter pasture for Montana sheep producers. In years of drought, alfalfa fields are extensively winter/spring
grazed; however, the impact on crop health is unknown. Fall regrowth also is utilized as overwintering habitat by the adult alfalfa weevil which hibernates in leaf litter or around plant crowns. In the southern U.S., the majority of weevil eggs are oviposited (laid) in alfalfa regrowth during fall and winter months, making fields with abundant fall regrowth more attractive. However, in colder northern states, such as Montana, temperatures restrict weevil winter activity and little to no oviposition (egg laying) occurs during winter months. Because alfalfa weevil adults aestivate during summer, emerging in fall, they are in a resting state when temperatures are low during winter. 
Research was conducted during two study years near Dillon, MT. During winter and spring, alfalfa fields were continuously grazed for 95 days in the first year and 98 days in the second year. Grazing dates were 19 January to 3 May during the first year and 5 February to 15 May during the second year. We collected data from grazed and non-grazed areas on alfalfa regrowth, alfalfa nutrient quality, and alfalfa weevil densities. Sheep grazing reduced alfalfa biomass by 98 percent during year one and 73 percent during year two. We did not record any differences in yields in either study year between grazed and non-grazed areas (Table 1).
Table 1. Yield, maturity, and forage quality of alfalfa continuously grazed from 19 January through 3 May of year one and 18 January through 15 May of year two near Dillon, MT.
| Treatment | Pre-graze biomass(kg/ha) | Post-graze biomas (kg/ha) | DM Yield (kg/ha) | DM (%) | CP (kg/ha) | ADF (kg/ha) | NDF (kg/ha) |
|---|---|---|---|---|---|---|---|
| Year 1 | |||||||
| Grazed | 3410 | 60 | 5081 | 92.7 | 1197 | 1460 | 1986 |
| Non-grazed | 3415 | 3692 | 5003 | 94.0 | 1116 | 1350 | 1828 |
| S.E. | 251.8 | 193.64 | 222.26 | 0.21 | 47.8 | 71.1 | 98.6 |
| P-value | 0.99 | <0.01 | 0.80 | <0.01 | 0.25 | 0.30 | 0.29 |
| Year 2 | |||||||
| Grazed | 2636 | 711 | 5559 | 93.2 | 1015 | 1424 | 1994 |
| Non-grazed | 2636 | 711 | 5559 | 93.2 | 1015 | 1424 | 1994 |
| Non-grazed | 2509 | 2833 | 5906 | 93.8 | 1071 | 1748 | 2471 |
| S.E. | 84.54 | 163.45 | 370.87 | 0.05 | 105.9 | 103.4 | 141.3 |
| P-value | 0.32 | <0.01 | 0.50 | <0.01 | 0.71 | 0.05 | 0.04 |
Levels of crude protein (CP), and acid (ADF) and neutral (NDF) detergent fibers did not differ between alfalfa samples taken from grazed and non-grazed areas, during year one and CP did not differ during year two (Table 1). Levels of ADF and NDF correlate with digestibility and animal intake. Relatively high values of either ADF or NDF correlate with lower quality feeds. Our study recorded no differences between grazed and non-grazed alfalfa ADF and NDF, during year one, for two reasons: 1) alfalfa weevil larval numbers were not great enough, in non-grazed plots, to reduce forage quality and 2) extensive sheep grazing, in grazed plots, did not reduce forage quality. However, during year two greater levels of ADF and NDF were recorded from non-grazed alfalfa (Table 1). We believe this to be a direct response to numbers of feeding alfalfa weevil larvae in non-grazed plots. Montana’s alfalfa weevil economic threshold is 400 larvae per 20 sweeps, which was exceeded in non-grazed plots during year two (Fig. 4). Feeding weevil populations cause economic losses by consuming plants leaves, which are high in cell solubles (i.e., sugars), and leaving plant stems, which are high in structural carbohydrates (i.e., ADF and NDF). Conversely, alfalfa weevil larval numbers were kept below the economic threshold in sheep grazed areas (Fig. 4) and a relative increase in forage quality was the result (Table 1).
Plant percent dry matter (DM) was greater in the non-grazed than grazed plots during both study years (Table 1). We were unable to find any other study indicating percent DM to be an indicator of plant maturity. However, our data suggest that as plant maturity increases so does plant percent DM. The level of damage to growing alfalfa plants by alfalfa weevil larvae was less in grazed areas (Figs. 1 and 2). Additionally, our study recorded fewer alfalfa weevil larvae in samples taken from areas where sheep had grazed compared to non-grazed areas in both years (Figs. 3 and 4).
We recorded 40 to 70 % reduction of alfalfa weevil larvae in grazed compared to non-grazed areas. This reduction may have been a result of reduced biomass, relative humidity and/or temperature, making the grazed areas less attractive for ovipositing alfalfa weevil adults moving into the fields following hibernation. Additionally, biomass was greatly reduced in the grazed plots (Table 1). In this scenario, any alfalfa weevil eggs successfully laid in grazed areas would be quickly consumed by grazing sheep resulting in reduced weevil densities.
These data express potential of grazing alfalfa regrowth with sheep as both a source of winter pasture and means of managing damage caused by alfalfa weevil in Montana without impacting spring regrowth, crop yields or nutritive characteristics.
Figure 1. Plants damaged by alfalfa weevil larvae in year one
Figure 2. Plants famaged by alfalfa weevil larvae in year two
Figure 3. Alfalfa weevil larvae captured from grazed and non-grazed alfalfa in year one
Figure 4. Alfalfa weevil larvae captured from grazed and non-grazed alfalfa in year two
