Planting alfalfa-grass mixtures? Check the plant-back restrictions from previous herbicide applications.

Planting alfalfa-grass mixtures? Check the plant-back restrictions from previous herbicide applications

Mark Renz Extension Weed Scientist, University of Wisconsin-Madison

Alfalfa plantings are predicted to be higher in 2013 than the past several years in Wisconsin.  While most fields will likely be direct seeded with alfalfa, many will include a companion crop (e.g. oats), and some will include one to several perennial forage grasses.  Whichever options you consider, makes sure to check your field records for the previous years to ensure you will not be in violation of any plant-back restrictions BEFORE PLANTING.  While crop injury may not occur as a result of not following the plant-back interval on the label, residues of the herbicide could be passed through the harvested plant and into the food system (e.g. milk, beef). Below I discuss the plant-back restrictions for alfalfa, companion crops planted with alfalfa, and perennial forage grasses that many of you are facing in Wisconsin. For a summary of the restrictions for alfalfa, oats, ryegrass, and perennial grasses from herbicides that are commonly used in corn or soybeans in Wisconsin See table 1 at the end of this article.

Alfalfa: Historically we have seen injury to alfalfa from corn or soybean residual herbicides, but due to the reduction in use of active ingredients with a long residual in these crops many people may not be familiar with this.  Alfalfa plant-back restrictions are present on nearly all labels, providing a clear timeframe when you can and can’t replant.

Companion crops: While companion crops are widely used in Wisconsin when establishing alfalfa, I expect even more use in 2013 due to the reduced feed from the drought.  Oats, the most common companion crop planted, is listed on almost all labels so planting restrictions are clear.  Many are using alternative companion crops like Italian or annual ryegrass. I have yet to see annual or Italian ryegrass listed on ANY label. If not listed the maximum stated plant-back interval on the label must be followed. I have heard of several failed Italian ryegrass plantings with alfalfa (no harm to the alfalfa), and this could be from residual herbicides in the soil.

Perennial grasses: While use of perennial grasses (e.g. meadow fescue, orchardgrass, tall fescue) with alfalfa continues to increase in use, few labels mention these perennial grasses and plant-back intervals.  Read the plant-back sections carefully, as some may give timings for grass crops or forage grasses, instead of the specific species (e.g. Milestone). But again if the crop species is not listed the maximum stated plant-back interval should be followed.

While this can be confusing, it is important to follow the label instructions. So before planting take a moment to check the field history to ensure a mistake isn’t made. It is a lot easier to change a seed order than replant a field.

Table 1. Plant-back restrictions of common herbicides used in corn and soybeans for alfalfa, oats, Italian/annual ryegrass, and perennial forage grasses^{^}.

Product	Active ingredient	Alfalfa	oats	Annual or Italian ryegrass	Perennial forage grasses^
COMMON CORN PRODUCTS
MANY	atrazine	2 Y	2 Y	2Y	2Y
MANY	acetochlor	9 M	FY	18 M	18 M
MANY	glyphosate	0 D	0 D	0 D	0 D
Callisto	mesotrione	10 M	0 D	NOL = 18 M	NOL= 18 M
Dual II Magnum	metolachlor	4 M	4.5 M	NOL = 12 M	NOL= 12 M
Hornet	clopyralid + flumetsulam	10.5 M1	4 M	10.5 M	10.5 M
Laudis	tembotrione	10 M	4 M	BA	BA
Lumax EZ	metolachlor + atrazine + mesotrione	18 M	FY	18 M	18 M
Surestart/TripleFLEX	Acetochlor + flumetsulam + clopyralid	FY1	FY	26 M + BA	26 M + BA
COMMON SOYBEAN PRODUCTS
Authority First	sulfentrazone + cloransulam	12 M	12 M	30 M	30 M
MANY	glyphosate	0 D	0 D	0 D	0 D
FirstRate	cloransulam	9 M	9 M	18 M + BA	18 M + BA
Flexstar	fomesafen	18 M	4 M 2	18 M	18 M
Gangster	cloransulam + flumioxazin	30 M + BA	9 M	30 M + BA	30 M + BA
Prefix	metolachlor + fomesafen	18 M	4.5 M	18 M	18 M
Pursuit	imazethapyr	4 M	18 M	40 M	40 M
Valor SX3	flumioxazin	5M if tilled, 10 M no till + BA	5M if tilled, 10 M no till + BA	6M if tilled, 12 M no till + BA	6M if tilled, 12 M no till + BA

Abbreviations : Y=year, M=month, D = day, FY= full year, NOL = not on label, BA = Bioassay required.

^{^} Looked for meadow fescue, orchardgrass, tall fescue, or perennial ryegrass on labels.

¹Plant-back interval may increase in areas with less than 15 inches of precipitation and < 2% organic matter.

² Small grains listed as 4 months, oats not listed on label.

³ Application of reduced rates may result in lower plant-back intervals.