Insect Update – Cutworms

Here in Northeast Nebraska, cool temperatures and welcome rain have slowed insect activity. Some cutworm damage has been reported, all in corn that has been planted into spring killed alfalfa. As reported in an earlier issue, alfalfa nearly always is a home to species of cutworms that overwinter as larvae (i.e. dingy and claybacked) and damage from these species is quite common in these situations. With the advent of transgenics (Herculex) and systemic seed treatments (Gaucho, Poncho, and Cruiser), farmers have alternatives to use at planting time rather than the traditional liquids (pyrethroids like Asana, Pounce, Warrior, Mustang Max, Baythroid, and also the organophasphate Lorsban).

However, transgenics and seed treatments require that the plants be fed on, since most control is obtained through systemic activity. This leads to some nervous moments for growers as they see the feeding damage on corn leaves. We have little data to give us insights as to how these new technologies work on overwintering cutworms, since most cutworm data is obtained by artificial infestation with black cutworms. While our gut feeling tells us that they should work, we would appreciate any feedback from “real-life” situations where these technologies have been used and how they fared against cutworms. (KJ)

Insect Update – Alfalfa Weevil

Alfalfa weevils are larger now and some damage is being observed in alfalfa fields. This damage is not economic and even if it were, the best solution at this time would be to harvest the alfalfa as soon as possible. However, regrowth after the first cutting needs to be carefully watched to see if the weevils (adults or larvae) hold back the new growth. (KJ)

Well Capacity Issues

Listed below are minimum net system capacities needed to meet crop water demands 90% of the time based upon soil type. The numbers in column 3 come directly from NebGuide G89-932, Minimum Center Pivot Design Capacities in Nebraska. Numbers in Column 4 and 5 provide an indication of how much the flow rate should be if the system is under a load control program and is shut off either 12 or 24 hours per week. The last line of numbers is the capacity needed to meet crop water demand of 0.35 inches per day 100% of the time.

Divide these numbers by 0.85 water application efficiency to get the gross system capacity you would need to irrigate effectively. To get the system flow rate simply multiply by the number of acres irrigated ( Ex: Sandy loam, 12 hours downtime: 4.85/0.85= 5.71 gpm/acre x 125 acres = 713 gpm ). Note that soils with higher water holding capacities need lower flow rates. This is because the soil helps meet crop water requirements during periods when the irrigation system flow rate is too low.

Soil Texture	Available WaterCapacity (inch/ft.)	Region I Net Capacity* 9 of 10 years
		Minimum	12 Hours Downtime	24 Hours Downtime
Loam, silt loam, very fine sandy loam w/silt loam subsoil	2.5	3.85	4.17	4.51
Sandy clay loam, silt loam, very fine sandy loam w/salty clay subsoil	2.0	4.13	4.47	4.84
Silty clay loam, Clay loam, Fine sandy loam	2.0	4.24	4.59	4.97
Silty clay	1.6	4.36	4.72	5.11
Clay, Sandy loam	1.4	4.48	4.85	5.25
Loamy sand	1.1	4.83	5.23	5.66
Fine sands	1.0	4.95	5.36	5.81
PEAK ET**		5.65	6.11	6.63

*Data taken from von Bernuth, R.D., D.L. Martin, J.R. Gilley and D.G. Watts. 1984. Irrigation System Capacities for Corn Production in Nebraska. Transactions of ASAE 27(2): 419-424, 428.

Even though it is likely too late to make major adjustments for this growing season, the system flow rate can be evaluated to determine if changes are necessary for 2005. The reason for going through this exercise is to show you how much water you need to irrigate effectively. Unfortunately some areas cannot get wells that produce water at these flow rates. Still others have seen their systems lose capacity as the season progresses. So what are they to do?

There are several options, however, if the capacity of the well is below the level calculated based upon the numbers in this table, you should first realize that full irrigated yields may not be obtained every year. Management of water application and crop selection will be key ingredients to achieving the greatest return for the amount of water pumped each year. So if the water supply is consistent during the year but below the minimum level you could plant crops that use water during different portions of the growing season, planting 25% of the acres to a different crop increases the effective capacity of a well by 33%. Crops like winter wheat, oats, or alfalfa would provide a lot of yield if irrigated only during May and June and would free up the system to concentrate on the remaining acres in July and August. Plant half a quarter to crops with different water use patterns and the effective flow rate for each half of the system doubles.

Though soybeans are drought tolerant, their water use pattern nearly mimics that of corn. By mid-July the daily crop water use rates are quite similar under fully irrigated conditions. Thus, while planting soybeans provides some relief, the two crops will require full irrigation during the heat of the summer so fields with low capacity wells will generally experience some yield loss.

For those with flow rates that drop during the irrigation season, the most noticeable thing is a loss of operating pressure. What is less obvious is that the water application uniformity will also normally be reduced. Consequently, it might be best to adjust the system for the lower flow rate that can be pumped consistently through the summer so that the water application uniformity will be high all season long Blower total water application but uniform. When the new operating conditions have been set, options like those discussed above should be considered if the capacity is well below the minimum values in the table.

Of course another option to consider if the flow rate of the system is dropping off due to declining groundwater levels is to return a portion of the field to dryland production. This is only economical if the flow rate is low enough to make the options listed above a stretch. In this case returning 25% of the acres to dryland will have the same impact as planting different crops (i.e., the effective flow rate increases by 33%).

Finally, use of the No-Sooner-Than date of irrigation scheduling is an option. In this case, irrigation begins as soon as the soil is able to hold the water being applied. Keeping track of daily crop water use rates or frequent soil sampling is necessary to make this management style work. On good soils this might mean if the plan is to apply 1.0 inch of water, when the crop has used 1.0 inch of water, the irrigation system is started. This approach tries to maintain as much of the soil water holding capacity as possible to help prevent crop stress when the system is incapable of meeting crop demand. The down side is that the soil is always near field capacity so larger rainfall events may result in leaching losses.

The bottom line is that one needs to evaluate the system flow rate to determine if enough water can be pumped to meet crop water needs. If the answer is yes, than load control and reserving some soil water storage for large rainfall events become very viable options. If the answer is no, then water management will need to increase and any downtime means reduced water application and potential yield reductions. (BK)

Hoary Vervain Control In Pasture

Hoary vervain (Verbena stricta), also known as wooly verbena or tall vervain, is a commonly found native weed in northeastern Nebraska on over-grazed rangeland, prairies and disturbed sites in all soil types. There are several other types of vervain in Nebraska (prostrate, white, and blue), which have similar growth forms and habits as hoary vervain. Hoary vervain is a perennial forb from the vervain family (Verbenaceae) that reproduces by seeds. The taproot (perennial structure) produces individual erect plants. The stem is nearly round, simple or branched above and can be up to 5 ft tall, covered with soft white hairs. Leaves are opposite, leaf blades are ovate with many teeth. The lower surface is pubescent with highly visible veins. Like many other plant species, the overall growth and development depends on the amount and timing of rainfall. Hoary vervain, in Nebraska, can flower from May to September, with blue or purple flowers positioned on the top of the main stem and branches and producing a two-seeded fruit.

Hoary vervain provides forage for deer while seeds are important food source for small mammals and upland birds. Native Americans also made a tea from the leaves to treat stomachache. Hoary vervain has no value to livestock because of its low palatability.

This weed can be controlled by various means. Mowing the plants when they are 3-5 inches tall can reduce vervain population considerably for the season. Mowing can be done one or two times per season depending on the amount of rainfall during the season. One mowing done in mid June can be effective (>75% control) if the season is dry, due to lack of moisture needed for weed regrowth. If the season is wet, an additional mowing is needed in July-August. Herbicides can be also very effective in providing a season long control. Herbicide application should be conducted when vervain plants are 3-5 inches tall, which is usually in early part of June. The list of effective herbicides, their rates and cost per acre includes: Salvo (12 oz/acre, $4), Grazon P+D (32 oz/acre, $8), Weedmaster (32 oz/acre, $6), Ally (0.25 oz/acre, $8), and Vista (22 oz/acre, $8). (SK)

Western Ragweed Control in Pasture

Western ragweed (Ambrosia psilostachya) is a commonly found native weed in northeastern Nebraska rangeland, prairies and disturbed sites in all soil types. It is a perennial forb from the sunflower family (Asteraceae) that reproduces both by seeds and rhizome. Rhizome is a horinzontal creeping root system growing within the top 5-10 inches of the soil surface. The plants usually grow in sparse groups (patches or clusters). The stem is very erect, up to 3 ft tall, with many branches and long hairs giving the stem a coarse feeling. Leaves are alternate on the upper part of the stem, opposite on the bottom, with many divisions and teeth. Like many other plant species, the overall growth and development depends on the amount and timing of rainfall. Western ragweed, in Nebraska, can flower from July to October, with greenish-yellow flowers positioned on the top of the main stem and branches, and produces an inch long bur-like fruits with a single seed within each bur.

Western ragweed provides forage for deer and the fruits are an important food source for upland game-birds, wild turkeys and songbirds. Native Americans also made a tea from the whole plants to treat colds and cramps. Western ragweed has almost no value to livestock because of its low palatability. With other forage limited, it may be eaten. Pollen produced in late summer causing late summer hay fever in many people, due to presence of volatile oils, which can also cause skin irritation.

Due to its low value for livestock forage, it is a concern to livestock producers and ranchers. This weed can be controlled by various means. Mowing the plants when they are 4-6 inch tall can reduce ragweed population considerably for the season. Mowing can be done one or two times per season depending on the amount of rainfall during the season. One mowing done in mid June is effective if the season is dry, due to lack of moisture needed for weed regrowth. If the season is wet, an additional mowing is needed in July-August. Herbicides can be also very effective in providing season long control. Herbicide application should be conducted when ragweed plants are 3-5 inches tall. The list of effective herbicides and their rates per acre includes: Salvo (12 oz/acre), 2,4-D-Ester (1 qt/acre), Grazon P+D (32 oz/acre), Weedmaster (32 oz/acre), Ally (0.25 oz/acre), and Vista (22 oz/acre). (SK)