At Arlington and Lancaster, IT/IR corn hybrids performed equal to nontolerant isolines. At Janesville, IT/IR corn hybrids performed better than nontolerant isolines.
IT/IR corn hybrids, whether sprayed with imazethapyr (Pursuit) or unsprayed, performed similarly to unsprayed nontolerant isolines.

The imidazolinone herbicides were discovered in the 1970's by scientists at American Cyanamid Company. In 1982, American Cyanamid began collaborating with Molecular Genetics, Inc. to develop imidazolinone tolerant corn. By 1984, Molecular Genetics, Inc. had successfully regenerated corn with imidazolinone tolerant (IT) genes which then allowed backcrossing to inbred lines. In 1992, the first corn hybrids were produced for commercial sale. These hybrids had 2X to 4X crop safety to imidazolinone herbicides. In 1993, Pursuit (imazethapyr) herbicide was labeled for use in corn.

Pursuit can be used preplant incorporated, preemergence, and early postemergence at a rate of 4 oz/A. Post applications of Pursuit must include a nonionic surfactant at 0.25% v/v or 2 pint/acre crop oil concentrate and fluid fertilizer (i.e. 28-0-0 at 1-2 quart/acre). Tank mixtures of Pursuit can be made with alachlor, metolachlor, EPTC, butylate, atrazine, dicamba, dimethenamid, pendimethalin, bentazon, bromoxymil and metribuzin.

Imidazolinone herbicides are taken up by root and leaf tissue. Transport of the herbicide is to meristems via both phloem and xylem. The herbicide stops the activity of the enzyme, acetolactate synthase (ALS), which is involved in branch chain amino acid synthesis (i.e. valine, leucine and isoleucine). Selectivity of the imidazolinone herbicides is through differential absorption, translocation, metabolism and ALS enzyme sensitivity. Of these methods of selectivity, metabolism is most important. The herbicide is detoxified by hydroxylation and conjugation with glucose. Imidazolinone herbicides are selective in legume crops and the herbicide controls both dicot and monocot weeds.

Imidazolinone symptoms on nontolerant corn include stunting, chlorosis and/or interveinal reddening. Lateral roots can also be stunted, giving the roots a bottle brush-like appearance. Imidazolinone activity increases when temperatures are cool. Rainfall enhances herbicide activity.

Imidazolinone tolerance in corn is due to a single semidominant allele. To produce IT corn inbreds, corn with the IT allele is backcrossed to a recurrent inbred parent. Approximately seven generations are required to get 99% recovery of a recurrent inbred parent's genetic material. The inbred is then selfed to derive a homozygous IT inbred. Hybrid corn is sold as IT or IR. IT hybrids have the gene from one inbred parent, while IR hybrids have the tolerant gene from both inbred parents.

There is a potential for linkage of the IT allele to undesirable alleles which may decrease hybrid performance. The objective of these studies was to compare performance of IT/IR corn hybrids with nontolerant isolines. A second objective was to compare IT/IR corn hybrid performance in cropping systems using commercial rates of Pursuit.

In 1994, experiments were conducted at Arlington, Hancock and Janesville, Wisconsin. The experimental design was a randomized complete block in a split-plot arrangement with four replications. Main plots were 10 corn hybrids and split-plots were hybrid isoline pairs with either IT/IR tolerance or nontolerance.

At Arlington in 1994, a separate experiment was conducted to compare IT/IR corn hybrid performance using commercial rates of Pursuit. The experimental design was a randomized complete block in a split- split-plot arrangement with four replications. Main plots were: 1) no herbicide application, and 2) Pursuit applied preemergence at 4 oz/A. Split-plots were 10 corn hybrids; split-split-plots were IT/IR tolerant versus nontolerant isolines. Measurements included grain yield, moisture, lodging, plant population, vegetative development, plant height and silking date.

Weather during the 1994 production year was ideal for corn. Record yields were observed statewide. Warm, early season temperatures with little rain during the month of May promoted early planting and fast early corn development. No problems were encountered with stand establishment of the plots. Differences in harvest plant population or broken stalks and grain moisture were observed, but were related to hybrid performance rather than the presence or absence of the IT/IR alleles.

At Lancaster and Arlington IT/IR hybrids performed similarly to nontolerant isolines (Table 1). At Janesville IT/IR hybrids produced 12 bushels per acre (6%) more than nontolerant isolines. In 37 of 40 cases, IT/IR hybrids performed similarly to its nontolerant isoline. In 2 of 40 cases, the IT/IR hybrid performed better than its nontolerant isoline. In one case, the IT/IR hybrid yielded less than its nontolerant isoline. Averaged across four locations, 9 of 10 IT/IR hybrids performed at least equivalent to, if not better than, its nontolerant isoline. Ciba 4393IMR yielded less than its nontolerant isoline, Ciba 4393.

IT/IR corn hybrids, whether sprayed with Pursuit or unsprayed, performed similarly to unsprayed nontolerant isolines (Table 2). Grain yield decreased from 178 to 165 bushels/acre when nontolerant corn was sprayed with imazethapyr. Greater yield reduction might be expected in years with more rainfall during May.

The availability of IT/IR corn hybrids offer new options in our cropping systems. First, Pursuit and the imazethapyr containing products of Pursuit Plus and Resolve can be used on these hybrids. Using these products may improve control of certain weeds or reduce the cost of a herbicide program compared to standard corn herbicide programs. Second, the IR hybrids have enough tolerance that corn injury caused by certain herbicide interactions with organophosphate insecticides are not a problem. With IT and nontolerant corn hybrids there are restrictions on the use of Counter to avoid injury from Accent or Beacon. Third, if carryover from a previous imidazolinone application is a concern, IT/IR corn hybrids can be planted and will have tolerance to such residues.

IT/IR corn hybrids offer flexibility when using Pursuit or Pursuit premixes. This creates one potential disadvantage with Pursuit use on IT/IR corn hybrids. Wisconsin producers can now use Pursuit year after year, even in a corn and soybean rotation. Pursuit is in a group of herbicides (ALS inhibitors) where there is a high risk of herbicide resistant weeds developing. Five to six years of continuous use of Scepter, a related imidazolinone herbicide, in soybeans has selected for Scepter-resistant cocklebur. This cocklebur is also resistant to Pursuit. If growers plan to use Pursuit in IT/IR corn, the weed management plan in all crops should be reviewed to determine if appropriate strategies are being used to prevent or delay the development of resistant weeds.

Table 1. Relative yield of IT/IR corn hybrids compared to its nontolerant isoline when grown at four locations in Wisconsin during 1994.
Hybrid	Arlington 1	Arlington 2	Janesville	Lancaster	Average
Pioneer 3751IR	105	92	114	90	100
ICI 8692IT	* 87 *	121	109	108	* 106 *
ICI 8532IT	104	94	114	98	103
Asgrow RX623T	99	103	100	98	100
Asgrow RX747T	100	100	129	94	106
Pfister 3000R	95	99	100	103	99
Growmark FS 466PT	105	99	102	99	101
Cenex/LOL 522PT	102	100	94	100	99
Cenex/LOL 622PT	107	101	* 106 *	112	107
Ciba 4393IMR	92	85	98	94	* 92 *
Mean	100	100	100	100
, * IT/IR hybrids which were significantly different from the NT isolines at 0.05 and 0.01 probability levels.

Table 2. Interaction between Pursuit (imazethapyr) applied at 4 oz/acre and corn hybrid isolines.
Herbicide Application	IT/IR Tolerant	Non-Tolerant
Control	177	178
Pursuit	181	165
LSD(0.05)	8

Does An Imidazolinone Tolerant Corn Hybrid Yield Less Than It's Nontolerant Isoline?

Does An Imidazolinone Tolerant Corn Hybrid Yield
Less Than It's Nontolerant Isoline?