TIMING OF VOLUNTEER POTATO CONTROL IN

ROUNDUP READY^® SUGAR BEETS

Joel Felix and Joey Ishida

Malheur Experiment Station

Oregon State University

Ontario, OR, 2008

Introduction

Volunteer potatoes are a common occurrence in sugar beets grown in rotation with potato in the Treasure Valley of eastern Oregon and southwestern Idaho. Potato tubers that drop out of the harvester are generally small but tend to survive the mild winter temperatures in this area and compete with subsequent crops grown in rotation. The treatments were designed with a belief that the starch-filled potato tuber could provide energy for new shoot regrowth following herbicide interruption of the original shoot.

Materials and Methods

A field experiment was conducted in 2008 at the Malheur Experiment Station near Ontario, Oregon to determine the optimum timing of volunteer potato removal from Roundup Ready^® sugar beets using glyphosate. The predominant soil was an Owyhee silt loam with 1.9 percent organic matter, a pH of 7.1, and a cation exchange capacity of 11 meq/g of soil. The study was laid out in a randomized complete block design with four replications. Individual plots were 7.33 ft wide (4 rows) by 25 ft long. To determine potato interference with sugar beets, whole potato tubers of variety 'Russet Burbank' averaging 3 oz each were planted in each treatment (except the control) at 8,168 tubers/acre. Roundup Ready sugar beet variety 'BTSCT02RR07' was planted on April 10 in 22-inch rows using tractor-mounted flexi-planter units with double-disc furrow openers and cone seeders fed from a spinner divider that uniformly distributed the seeds within the row. Sugar beet seeds were dropped at the rate of 8 seeds/ft of row and thinned to 8-inch spacing between plants within a row on May 28, 2008. Counter^® 15G (7.4 lb/acre) was applied on April 11, 2008 and Temik^® 15G (10 lb/acre) on June 3, 2008. Sugar beets were fertilized on June 13 using a special blend fertilizer to supply 180 lb nitrogen, 20 lb potassium, and 3 lb zinc. The study was irrigated based on a calendar schedule (April 16 to September 11, 2008) to maintain moisture in the top 12 inches of the soil profile. All volunteer potatoes and other weeds in the study area were controlled by applying glyphosate at 22 fl oz/acre and ammonium sulfate at 2 percent V/V as dictated by the application timing. The treatments were: spray once at 4-inch rosette stage, hooking (pre-tuber initiation), tuber initiation, early tuber bulking, and at mid-tuber bulking. The corresponding treatments intended to control new shoots or regrowth were: spray as needed starting at 4-inch rosette, tuber hooking, and tuber initiation. The 'spray as needed' treatments were evaluated weekly to determine when follow-up spraying was needed. Glyphosate was broadcast-applied using a CO₂-pressurized backpack sprayer fitted with a boom equipped with four Teejet 8002EVS nozzles calibrated to deliver 12 gal/acre. Hand weeding was used to control other weeds not controlled by the herbicide. Headline^® fungicide at 12 oz/acre was sprayed to manage powdery mildew on July 18 using a commercial aircraft. Follow-up fungicide applications included Enable^® at 8 oz/acre, Headline at 12 oz/acre, Topsin-M^® at 1 lb/acre, and Enable at 8 oz/acre on July 29, August 7, August 15, and September 3, respectively. Volunteer potato was harvested October 15 by digging four representative plants from each plot if plants were present. Tubers were sorted by size, counted, and weighed. Sugar beet yield was determined by mechanically harvesting the two center rows of each plot on October 16, 2008. The data were subjected to analysis of variance and means compared using the least significant difference (LSD) at P = 0.05.

Results and Discussion

The number and potato tuber yield at harvest time varied among treatments (Table 1). The vast majority of the tubers produced were less than 1 oz with treatment application timing up to hooking stage producing the greatest number. However, the greatest tuber yield in the under 1-oz category (9 and 10 cwt/acre) was observed when glyphosate was applied either once or as needed starting at potato tuber initiation stage. Similarly, a large number of tubers and high yield in the 1- to 4-oz category were observed when control timing commenced at tuber initiation or later. Likewise, the potato tubers in the 4- to 6-oz category were generally greater when control started at tuber initiation or later compared to earlier timings. The only treatments that produced greater than 6-oz tubers were the remove once at 4 inch and in plots where potatoes were not removed. Sugar beet root yield varied with timing of potato removal (Table 2). Sugar beet root yield ranged between 39 and 51 tons/acre with season-long competition producing the lowest yield. The root yield was generally lower (39 to 40 tons/acre) when potato removal was delayed to tuber initiation or later. Percent sugar content, sugar extraction, and estimated recoverable sugar per ton was not affected by timing of potato removal. However, timing of potato removal affected the estimated recoverable sugar (lb/acre). The reduction to estimated recoverable sugar was directly proportional to the observed root yield. In order to maximize sugar beet root yield, growers are advised to initiate volunteer potato control when sprouts are at 4- to 6-inch height and a sequential application following anytime before canopy closure.

Table 1. Potato tuber number and weight when Roundup PowerMax^™ was applied at different timings to control volunteer potato in Roundup Ready^® sugar beets at the Malheur Experiment Station, Oregon State University, Ontario, OR, 2008.

				Potato tuber number and yield ‡
Treatment †	Rate	Unit		<1oz								1-4oz							4-6 oz							>6 oz					TOTAL
				No./acre*				cwt/acre				No./acre			cwt/acre				No./acre			cwt/acre				No./acre		cwt/acre			No./acre		cwt/acre
No volunteer potato				0	e		0.0		d		0		b	0.0		b		0		b	0.0		b		0		b	0.0	b	0		c	0	c
Remove once at 4" tall				46,026	abc		2.0		cd		819		b	0.3		b		182		b	0.2		b		68		b	0.1	b	47,095		b	3	c
Roundup OriginalMax	21.3	fl oz/a
Remove as needed at 4" tall				49,757	ab		2.1		cd		0		b	0.0		b		0		b	0.0		b		0		b	0.0	b	49,757		b	2	c
Roundup OriginalMax	21.3	fl oz/a
Remove once at hooking				46,071	abc		2.3		cd		0		b	0.0		b		0		b	0.0		b		0		b	0.0	b	46,071		b	2	c
Roundup OriginalMax	21.3	fl oz/a
Remove as needed at hooking				52,828	a		3.1		bc		0		b	0.0		b		0		b	0.0		b		0		b	0.0	b	52,828		b	3	c
Roundup OriginalMax	21.3	fl oz/a
Remove once at tuber initiation				27,643	bcd		9.2		a		41,157		a	18.9		a		2,457		b	2.7		b		0		b	0.0	b	71,257		ab	31	b
Roundup OriginalMax	21.3	fl oz/a
Remove as needed at tuber at initiation				29,486	a-d		10.2		a		31,328		a	15.6		a		614		b	0.7		b		0		b	0.0	b	61,428		ab	26	b
Roundup OriginalMax	21.3	fl oz/a
Remove once at bulking				15,971	de		3.6		bc		31,328		a	16.0		a		1,229		b	1.3		b		0		b	0.0	b	48,528		b	21	b
Roundup OriginalMax	21.3	fl oz/a
Remove once at mid-tuber-bulking				25,800	cd		5.0		bc		29,486		a	13.4		a		1,843		b	2.1		b		0		b	0.0	b	57,128		ab	20	b
Roundup OriginalMax	21.3	fl oz/a
Volunteer potato not removed				23,343	cde		5.3		b		36,243		a	21.8		a		13,514		a	15.9		a		7986		a	15.2	a	81,085		a	58	a

^† Each Roundup PowerMax application included ammonium sulfate at 2 percent V/V.

^‡ Means within a column followed by the same letter do not significantly differ (P = 0.05, LSD).

*No./acre = Number of potatoes per acre.