Extension Ag Update
November/December 2004
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Research

Soy Set to Withstand Exotic Aphid

Jan Suszkiw, ARS News, USDA, 301-504-1630, jsuszkiw@ars.usda.gov

A key genetic discovery by Agricultural Research Service (ARS) and university scientists opens the door to breeding soybeans that can resist Chinese soybean aphids. Since first being detected in Wisconsin in 2000, the soybean aphid (Aphis glycines) has spread across the Midwest and into the Deep South, causing millions of dollars of losses to the legume crop. Growers have fought back with insecticide spraying, a practice that adds $12 to $25 per acre to their production costs.

ARS plant pathologist Glen Hartman and University of Illinois (UI) collaborators at Urbana have worked to find cheaper, longer-term alternatives. In early 2004, their efforts paid off with the discovery of Rag1, a single gene conferring resistance to the exotic aphid in two southern cultivars that are no longer grown.

Normally, the sap-sucking pest causes harm in the form of stunted growth, disfigured leaves, poor pod formation, and the plant's eventual death. But in tests, neither wingless female aphids nor their nymph offspring survived for long when confined to the resistant beans' leaves. Typically, 94 to 100 percent of female aphids died within 10 days--compared to 17 percent on "Pana," a nonresistant variety--reports Hartman, at ARS' Soybean/Maize Germplasm, Pathology, and Genetics Research Unit in Urbana. Nymphs suffered a similar fate, he adds.

Hartman and UI collaborators Curtis Hill, Shawn Carlson, Brian Diers and Yan Li identified the aphid resistance after screening 800 commercial soybean cultivars and 3,000 germplasm accessions managed by ARS in Urbana. Since publishing their finding in Crop Science, the team has mapped Rag1's genetic whereabouts on the resistant beans' DNA (deoxyribonucleic acid). They've also identified marker regions and devised technology to detect them so that soybean breeders can rapidly identify resistant plants.

New, high-yielding cultivars bred to express Rag1 could be available by 2008. Meanwhile, the team's search for other resistance genes continues.

MANAGEMENT MUN - A Reliable Readout Of Milk Urea Nitrogen

Michel Wattiaux, University of Wisconsin, 608-263-3493, wattiaux@wisc.edu

Too much phosphorus fed to dairy cows creates pollution, costs producers, and provides no benefit to cows.  Likewise for too much protein in dairy diets, says a University of Wisconsin-Madison researcher.  Keeping tabs on milk urea nitrogen can help farmers optimize the amount of protein they feed - with benefits for both the bottom line and the environment, says Michel Wattiaux, a dairy nutritionist at the UW-Madison's College of Agricultural and Life Sciences.

"Assuming that protein levels in diets are OK is risky," Wattiaux says.  "Changes in feed, errors in mixing, and other factors mean a cow may be eating something different from what you think she's eating." When you feed a cow, she'll use some of the dietary protein to produce milk protein, but she will excrete the dietary protein she doesn't use in the form of urea in her urine. Excess protein in the diet means more urea in urine. That's money down the drain and ammonia into the atmosphere.

Milk urea nitrogen, or MUN, is the best way to determine whether you're feeding too much or too little protein, Wattiaux says, because MUN is an indicator of nitrogen not used by the cow to produce milk.  Urea nitrogen is excreted in milk in small amounts    (5 to 25 milligrams per deciliter, versus milkfat, for example, which is 3,000 to 4,000 milligrams per deciliter).  MUN levels are closely correlated to the amount of urinary nitrogen that the cow excretes. MUN levels predict how much urine nitrogen the cow produces - in other words, how much nitrogen she has not used to make milk.

For example, a 1,500 pound cow that tests 14 mg/dl for MUN excretes 271 grams of urinary nitrogen per day.  By dropping MUN to 10 mg/dl, she would excrete 194 grams per day, with no difference in milk protein levels.  The 77-gram difference is the amount of nitrogen in 2.2 pounds of 48-percent soybean meal or 0.38 pounds of urea, according to Wattiaux.  Multiply those supplement costs by a year of feeding and you're looking at some real money, he says.

"Until recently, many people didn't appreciate the close connection between MUN and urinary nitrogen," Wattiaux says.  In part, that was because all the factors that could influence MUN - and give misleading results - hadn't been accounted for in a systematic way.  Management MUN, a service now available on AgSource Cooperative Services' herd data summaries, accounts for those factors and gives producers a reliable monthly update on their cows' dietary protein efficiency.

To develop Management MUN, Wattiaux analyzed more than 400,000 AgSource cow records from herds in Wisconsin, Iowa, Illinois, Michigan and Minnesota.  He looked at factors not directly related to nutrition that systematically influenced MUN - for example, breed differences, lactation number, 2x versus 3x milking, AM versus PM sampling, seasonal effects, and milk production levels.

Wattiaux analyzed the impacts and interactions of these factors, then adjusted all the values to a common base.  Producers who use Management MUN know that seasonal differences, time of sampling, and the other factors that could give misleading MUN numbers have been accounted for.  "When you look at Management MUN versus lab MUN, Management MUN should be a more reliable indicator of the nutrition of the cow," Wattiaux says.

For typical Midwest cow diets, the lower the Management MUN (down to 10 mg/dl), the higher the protein yield and the higher the efficiency of dietary nitrogen utilization, according to Wattiaux.  Aim for 10 mg to 12 mg if you're paying close attention to your nutritional program, he suggests.  This tells you that the cows are eating a diet of 16.5 percent to 17 percent protein, which is ideal.

If you don't closely manage your nutrition program, don't worry until Management MUN levels exceed 14 mg, because you probably don't have the management tools to fine-tune the diet, Wattiaux says.  At levels above 14 mg, either you're wasting money on unnecessary protein supplements or there's some other problem in your feeding program because your cows aren't utilizing nitrogen efficiently.  For example, rumen acidosis from too much dietary starch can increase MUN because the cow isn't able to use protein efficiently. Management MUN, due to seasonal and geographical adjustments, is for upper Midwest producers only.  So far, Wattiaux has developed adjustment factors for Holsteins, Jerseys and Brown Swiss.