Weeds resistant to glyphosate and the potential of increased resistance received considerable attention this winter in the press, at conferences and at county production meetings.

One of the most serious threats seems to be the possibility of a glyphosate-resistant pigweed. This has prompted the question “If pigweed does become resistant to glyphosate on a large scale, what will be the effect on our cost of cotton production in the Mid-South?” To answer this question, start with the 2007 cost of production estimates for cotton in Arkansas.

We estimate that producing an acre of Roundup Ready Flex/Bollgard II cotton in northeast Arkansas this year (assuming center pivot irrigation) will generate $643.68 of total specified cost. This does not include land rent, overhead or returns to management, but it does include the cost of a glyphosate-resistance management program.

In order to delay the occurrence of glyphosate-resistant pigweed in your Roundup Ready Flex cotton fields, our weed scientists recommend you include two of the following three herbicides in your weed control program starting right away: (1) metolachlor, (2) prometryn and (3) fomesafen or flumioxazin. All of these herbicides are effective at controlling pigweed and they have some residual activity.

This year, a producer planting Roundup Ready Flex cotton has a choice. He can choose an in-season weed control program consisting of only glyphosate, or he can choose an in-season program that includes residual herbicides that are effective on pigweed so as to delay glyphosate resistance.

We estimate the cost of the glyphosate alone program to be approximately $22 per acre. The program designed to manage resistance is estimated to cost approximately $35 per acre. Thus, for a $13 per acre investment, a producer expects to delay the onset of resistant pigweed in his Roundup Ready Flex cotton fields.

If and when glyphosate-resistant pigweeds become established in your cotton fields, our specialists recommend including all three of the chemistries identified above plus fluometuron pre-emergence and flumioxazin at burndown. All of this adds up to $57 per acre for in-season weed control and the added burndown and preplant needed when glyphosate resistant pigweeds are present.

The table shows a scenario of the two alternatives described above. In scenario one the farmer does not invest in resistance management. His in-season weed control cost is $22 per acre. By 2010 he is faced with glyphosate-resistant pigweed and is forced to spend $57 per acre for in-season weed control and the added burndown and pre.

In scenario two the farmer practices resistance management, spending $35 per acre for in-season weed control. This delays the onset of glyphosate-resistant pigweed until 2012, when he too is forced to spend $57 per acre due to the inevitable occurrence of glyophosate-resistant pigweed even following a resistance management plan.

At the end of six years the farmer who utilized resistance management spent $5 per acre less on weed control than the farmer who did not.

There are a lot of unknowns in these scenarios. We don't know how many years until glyphosate-resistant pigweeds will be common if resistance management is not practiced. We don't know how many years resistance management will delay the inevitable, and we don't know when new technology may be available that is a better alternative than the $57 per acre one identified for this article.

No doubt spraying large quantities of glyphosate alone causes weeds to develop resistance. No doubt spending extra on weed control now will delay the occurrence of resistance. For every year you invest $13 per acre in resistance management, a year in the future when you can save $22 per acre (due to not having a resistance problem) is needed in order to recoup your investment.

For more information on managing pigweed to prevent glyphosate resistance, contact local cooperative Extension staff.