Application timing effects on weed control in rOUNDUP

Application timing effects on weed control in ROUNDUP-READYÒ sugar beetS

Michael J. Wille and Don W. Morishita

INTRODUCTION

The optimum timing of initiating RoundupÒ applications on Roundup-Ready sugar beets and subsequent sequential applications was investigated in a study established at the University of Idaho Research and Extension Center near Kimberly, Idaho.

MATERIALS AND METHODS

The experimental design was a randomized complete block with four replications. Individual plots were four rows by 30 ft. Sugar beets (‘Pillar RR’) were planted in rows 22-inches apart April 15, 1999, and thinned to a density of 47,520 seed/A on May 28. Roundup was applied beginning with 2-leaf, 4-leaf, or 6-leaf sugar beet. Following initial herbicide application to 2- or 4-leaf sugar beet, plots were treated either twice more at 10 day intervals, or once more at 20 or 30 day intervals. Plots initially treated at the 6-leaf stage were treated again at 20 or 30 day intervals. Roundup rate was 1 qt/A for all applications. Progress was included as a standard herbicide treatment, and applied at the sugar beet cotyledon stage and 7 and 14 days later. All herbicide treatments were broadcast-applied with a CO₂-pressurized bicycle-wheel sprayer calibrated to deliver 10 gpa using 11001 flat-fan nozzles, except Progress which was applied in a 10-inch band using 8001 even-fan nozzles calibrated to deliver 20 gpa. Kochia, common lambsquarters, and volunteer wheat were the major weed species. Application information and weed species densities are given in Table 1. Crop injury and weed control were evaluated visually 7 and 21 days after the last herbicide application (DAT) on July 23 and August 9. Sugar beets were harvested from the middle two rows of each plot September 22.

RESULTS AND DISCUSSION

None of the herbicide treatments injured sugar beet. Progress controlled common lambsquarters 93 and 98%, kochia 44 and 28%, and volunteer wheat 74 and 35% 7 and 21 DAT, respectively. All Roundup treatments controlled common lambsquarters, kochia, and volunteer wheat 100% regardless of application timing. All herbicide-treatments yielded more than the check. Yields from Roundup treatments ranged from 23 to 29 ton/A and did not differ from each other. Yield from plots treated with Progress averaged 17 ton/A, which was significantly less than plots treated with Roundup at the two-leaf stage and again 20 days later, but did not differ from other Roundup-treated plots.

Table 1. Application information and weed species densities.


Application date	5/6	5/16	5/24	6/3	6/7	6/14	6/17	6/24	7/2	7/9	7/15
Air temperature (F)	55	63	80	50	60	82	70	74	70	72	80
Soil temperature (F)	60	63	80	60	58	90	68	63	64	70	100
Relative humidity (%)	64	46	68	80	64	32	80	50	41	46	28
Wind velocity (mph)	0	4 to 7	2 to 5	0	4 to 7	2 to 4	0	2 to 5	4 to 6	3	5
Weed species (plants/ft²)
Kochia	4	--	6	--	7	7	--	--	--	--	--
Common lambsqsuarters	0	--	9	--	9	6	--	--	--	--	--
Volunteer wheat	5	--	7	--	6	6	--	--	--	--	--

Table 2. Crop injury, weed control, and sugarbeet root yield in Roundup-resistant sugarbeet.

					Weed Control^a
			Crop Injury		Colq		Kochia		Vol. wheat
Treatment	Rate^b	Application timing	7/23	8/9	7/23	8/9	7/23	8/9	7/23	8/9	Yield

	oz/A		-----------------------%-----------------------								ton/A

Check			-	-	-	-	-	-	-	-	11

Progress+ Upbeet Progress+ Upbeet Progress+ Upbeet	18+ 0.5 18+ 0.5 18+ 0.5	Cotyledon 7 d later 7 d later	0	0	93	98	44	28	74	35	17

Roundup Roundup Roundup	16 16 16	2 leaf 10 d later 10 d later	0	0	100	100	100	100	99	100	26

Roundup Roundup	16 16	2 leaf 20 d later	0	0	100	100	100	100	100	100	29

Roundup Roundup	16 16	2 leaf 30 d later	0	0	100	100	100	100	100	100	27

Roundup Roundup Roundup	16 16 16	4 leaf 10 d later 10 d later	0	0	100	100	100	100	100	100	26

Roundup Roundup	16 16	4 leaf 20 d later	0	0	100	100	100	100	100	100	26

Roundup Roundup	16 16	4 leaf 30 d later	0	0	100	100	100	100	100	100	26

Roundup Roundup	16 16	6 leaf 20 d later	0	0	100	100	100	100	100	100	23

Roundup Roundup	16 16	6 leaf 30 d later	0	0	100	100	100	100	100	100	24

LSD(0.05)			ns	ns	1	2	25	18	11	21	6

^{aWeed species evaluated were common lambsquarters
(Colq), kochia, and volunteer wheat (Vol. wheat).

^{bRate of application based on a broadcast basis. Band rate will vary
depending on width of band.}}