UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460

OFFICE OF PREVENTION, PESTICIDES AND TOXIC SUBSTANCES

MEMORANDUM

SUBJECT: Estimates of Crop Irrigation for Endothall Registration (DP# 369862, 369863)

FROM: Bill Phillips, II, Ph.D., Agronomist

Biological Analysis Branch 

Derek Berwald, Economist

Economic Analysis Branch

Biological and Economic Analysis Division (7503P)

THRU: Timothy Kiely, Chief

Economic Analysis Branch

Arnet Jones, Chief

Biological Analysis Branch

Biological and Economic Analysis Division (7503P)

TO: Sidney Jackson, Risk Manager Reviewer

Barbara Madden, Risk Manager

Risk Integration Minor Use Emergency Response Branch 

Registration Division (7505P)

Product Review Panel: October 7, 2009 
Summary

EPA is evaluating a proposed use of the aquatic herbicide endothall for weed control in irrigation canals. 

This memo provides estimates for the percentage of crops that may be exposed to endothall that EPA is 

reasonably certain will not be exceeded. Crops will be exposed to endothall by irrigation water, so the 

maximum percent that could be exposed is the share of production from irrigated land. 

Estimates, summarized in Table 1, are derived using two methods. Where data on irrigated and dryland 

production are available, the estimates are based on the share of production from irrigated land, which is 

the maximum percent of the crop that could be exposed to endothall. Estimates using the share of 

production that is irrigated are provided for barley, corn, dry edible beans, peanuts, oats, rice, sorghum, 

soybeans, sugarbeets, sugarcane, and wheat. For crops of concern that do not have available data on 

irrigated and dryland production, BEAD uses the share of production grown in 17 western states where 

irrigation is used extensively and endothall use will be concentrated. Estimates based on production in the 

17 western states are provided for apples, grapes, oats, green peas, wheat, watermelon, and strawberries. 

In addition, BEAD reviewed the protocol for the residue trials and concludes that the resulting data are 

conservative. The trials used applications of treated irrigation water applied through overhead sprinklers, 

which would result in high exposure to the edible portion of the crop. Many fields are irrigated with drip 

lines or a flood system, which would result in less of the treated water, and thus less pesticide, deposited on 

the crop. In the registrant’s field trials plant samples were taken for residue testing from crops irrigated 

overhead using water that was treated the day of harvest; this is not common practice under actual 

production conditions. 
Table 1. Estimated Percent Crop Treated with Endothall for Dietary Risk 

Assessment

Share of 

Irrigated

Production

Share of 

Production

in West 1

Estimated 

Percent Crop 

Treated for 

Dietary Risk 

Assessment

Apple n/a 65 65

Fresh market n/a 78 78

Processing n/a 44 44

Juice n/a 49 49

Canned n/a 14 14

Barley for grain 36 - 36

Corn for grain 19 - 19

Dry Beans 232 - 32

Grape n/a 94 94

Fresh market n/a 99 99

Processing n/a 94 94

Green Peas n/a 11 11

Oats for grain 7 37 7

Peanut for nuts 42 - 42

Rice 100 - 100

Sorghum for grain 15 - 15

Soybean for beans 9 - 9

Strawberry n/a 91 91

Fresh market n/a 89 89

Processing n/a 100 100

Sugarbeet for sugar 37 - 37

Sugarcane for sugar 54 - 54

Watermelon n/a 39 39

Wheat for grain 14 70 14

Source: USDA 2009a, 2009b, 2009c, 2009d, EPA calculations.

1 Arizona, California, Colorado, Idaho, Kansas, Montana, Nebraska, Nevada, 

New Mexico, North Dakota, Oklahoma, Oregon, South Dakota, Texas, 

Utah, Washington, and Wyoming. Estimates are only provided for certain 

crops.

Background

Endothall is an herbicide registered for control of plants in water bodies. It is currently registered for weed 

control in irrigation canals, but with the requirement that water cannot be released from the system or 

applied to crops for seven days after treatment. 

The Agency is currently evaluating a petition for tolerances on many irrigated crops to allow registration of 

endothall for use in irrigation canals to control aquatic weeds without the requirement that the treated water 

be held or contained prior to being used to irrigate a crop. Because this is a new use pattern for endothall, 

dietary risks must be estimated and analyzed. Normal methods for estimating the percent crop treated for a 

new pesticide are inappropriate in this case, because the pesticide is not directly applied to the crop. 

Exposure occurs via irrigation water. 

Endothall will be used to control weeds in irrigation systems, not as treatment for the water in the systems. 

Most of the water that goes through a canal to irrigate crops will not be treated. Instead, endothall will be 

periodically injected into irrigation water to keep weed populations down. Because the water in the canal is 
rapidly turned over, after several hours most if not all of the endothall will no longer be in the canal 

systems. In addition, the use directions for endothall restrict applications to irrigation canals to once every 

seven days, and a maximum of six times per year. 

Due to the rate of endothall used, the number of applications, and the timing of applications the residue 

trials for endothall seem to be conservative to BEAD. This supposition is supported given the protocol 

used where crops were overhead irrigated six times with an inch of water containing the maximum 

concentration of endothall. After the final treatment the crop was harvested the same day, resulting in 

maximum endothall residue concentrations on the crop. From one of the registrants field protocols: 

“[d]uplicate control and treated samples of each commodity were harvested from the 

respective tests. Samples of field corn forage, sorghum forage and wheat forage and hay 

were harvested 0 days after the second or third application (0 DAT). Samples of sweet 

corn forage, kennels plus cob with husks removed (K+CWHR) and stover, field corn 

grain and stover, sorghum grain and stover, and wheat grain and straw were harvested 

following the sixth application at 0 DAT (or at 1 DAT in one wheat test).”

1

Under actual field production conditions, crop harvest is conducted only after a specified number of days 

after treatment. Due to standard agronomic practices this duration is extended when, for example, wheat is 

left to dry in the fields for some time before harvest, to reduce the moisture content. While in other cases 

crops such as grapes are not overhead irrigated when fruit are present, because additional moisture on the 

bunch can lead to fungal diseases. There is an additional consideration that should be given here and that is 

that much of the crops irrigated from surface water sources are done so by using some type of delivery 

system other than overhead sprinklers, systems such as flood or drip irrigation.

Method for Estimating Percent Crop Treated

For some crops, the estimates in this memo are based on yield and acreage data for irrigated and nonirrigated crops from the 2007 Census of Agriculture (USDA 2009d). The estimates provided will be an 

upper bound on the percentage of crops that may have been irrigated with water from canals that were 

treated with endothall, because some of the irrigated crops are not grown in areas where endothall will be 

used. 

Other crops that are important to the dietary risk assessment do not have data on irrigated and non-irrigated 

production available. For these crops, this memo provides estimates of the amount of the crop grown in 17 

western states where endothall use is expected. These are states where substantial amounts of water are 

transported through open canal systems to be used for irrigation. It is these systems where weed control is 

important, and endothall is likely to be used. 

Estimates from the 2007 Census of Agriculture

For select crops, the 2007 Census of Agriculture (USDA 2009d) provides estimates of the acres and yield 

from farms with irrigation, farms without irrigation, and farms where some of the crop is irrigated. Using 

this information we provide estimates of the share of production irrigated for those crops where the data are 

available. These crops are barley, corn, dry edible beans, oats, peanuts for nuts, rice, sorghum, soybeans, 

sugarbeets, sugarcane, and wheat. 

Table 2 shows the estimated percent of the crop that has been irrigated for the food crops for which data are 

available. To estimate the production from the irrigated acres, the first step is to estimate the crop 

production from irrigated land. Using the data in Table 2, we multiply the irrigated yield by the irrigated 

acreage, which is the acreage from farms where the entire crop is irrigated plus the irrigated acreage from 

 

1 Registrant submission: 47520713. Arsenovic, M. (2008) Endothall (Hydrothol 191): Magnitude of the 

Residue on Grain Cereal Group (Except Rice): Lab Project Number: Z9768. Unpublished study prepared 

by Interregional Research Project No. 4. 590 pages. 
partially irrigated farms. To get an estimate of the share of the crop irrigated, we divide this by an estimate 

of total production. To estimate total production from the information in Table 2, we multiply the yield by 

the acreage for each type of farm, and add these together. Taking wheat as an example, estimated 

production from irrigated farmland is (1,806,902 + 1,557,177) × 80.3 = 270,135,544 bushels, which is the 

numerator for our percentage calculation. Total estimated production is 1,806,902 × 80.3 + (1,557,177 + 

3,703,599) × 42.7 + 43,865,291 × 37.0 = 1,992,745,133 bushels. Dividing estimates for production from 

irrigated land by the estimate of total production yields an estimate of the share of production that has been 

irrigated, in this case 14%. The estimates for all the food crops with available data in the census are in the 

last column of Table 2. 

Note that the data available in the Census of Agriculture only provides overall yields for farms where only 

part of the crop is irrigated, although it does provide total production and irrigated and dryland acreage 

data. For this analysis, BEAD assumed the yield from the irrigated acreage on partially irrigated farms to 

be equal to the average yield per acre from irrigated farms. This is a conservative assumption, because the 

implied non-irrigated yield is substantially lower than from dryland farms in all but one case2, which tends 

to inflate the estimates of the share of crop irrigated. 

These estimates are based on only one year of data. However, it is unlikely that the amount of acreage 

irrigated will change a large amount from one year to the next, because irrigation equipment is an 

expensive capital investment. 

Note that these are estimates of the amount of the crop irrigated, not the amount of the crop that may have 

endothall residues. Not all of the crop that is irrigated will be grown in areas where endothall is likely to be 

used, and even in the West, where endothall use for irrigation is expected, not all systems will necessarily 

be using endothall, a new method of control. Therefore, the share of crops that are actually exposed to 

endothall are likely to be lower than the estimates of irrigated acreage in Table 2. 

 

2

The exception is peanuts, where the implied yield estimate is slightly higher than the dryland 

yield. The difference is less than 3%. 
Table 2. Share of Crop Production Irrigated

Entire crop irrigated Part of crop irrigated None of crop irrigated EPA 

Estimated 

Share of 

Production 

Irrigateda

Average 

Yield 

Per 

Acre

Acres Acres not

Average 

Yield 

Per 

Acre

Average 

Yield 

Per 

Crop Farms Acres Farms irrigated irrigated Farms Acres Acre

Barley for grain 

(bushels) 3,566 605,340 100.0 675 136,135 155,262 53.3 15,607 2,625,220 49.9 36%

Corn for grain (bushels) 17,927 6,103,769 180.0 20,984 7,053,000 6,435,486 150.0 308,849 66,656,287 144.3 19%

Dry edible beans, 

excluding limas (cwt) 2,275 317,193 22.6 181 36,310 50,432 16.5 3,780 1,051,614 15.9 32%

Oats for grain (bushels) 1,304 67,948 83.7 316 10,266 15,560 63.4 40,938 1,415,375 58.1 7%

Peanuts for nuts 

(pounds) 1,301 251,564

3722.

5 1,080 167,225 186,723

3234.

1 3,801 595,052

2725.

8 42%

Rice (cwt) 6,084 2,758,792 72.0 - - - - - - - 100%

Sorghum for grain 

(bushels) 2,092 443,599 86.9 2,391 401,615 670,030 74.7 21,759 5,254,590 69.2 15%

Soybeans for beans 

(bushels) 7,007 2,175,069 45.3 13,326 3,062,006 3,396,716 40.8 258,777 55,282,030 40.2 9%

Sugarbeets for sugar 

(tons) 1,535 387,224 30.0 60 7,451 20,424 24.2 2,427 838,718 23.4 37%

Sugarcane for sugar 

(tons) 210 431,796 38.7 9 3,409 3,373 33.3 473 408,088 34.7 54%

Wheat for grain, all 

(bushels) 7,695 1,806,902 80.3 7,518 1,557,177 3,703,599 42.7 145,597 43,865,291 37.0 14%

Source: USDA 2009d and EPA Calculations

a Assumes irrigated acres on farms where only part of the crop is irrigated obtain the same yield as acres on farms where the entire crop is irrigated
Crop Estimates 

Endothall residues on some crops are of particular concern to EPA based on residue trials and dietary habits. These 

crops are peas, watermelon, strawberries, apples, oats, grapes and wheat. For these crops BEAD provides estimates 

of the proportion of the crop that is grown in the 17 western states, which are the states expected to be users of 

endothall in irrigation water. According to the Farm and Ranch Irrigation Survey (USDA 2004), almost 70% of the 

irrigated acreage in the United States is in the 17 western states, and about 87% of the irrigation water applied to 

crops is in these western states, which correspond to the states served by Bureau of Reclamation water projects. 

The share of the crop grown in the 17 western states can be used as a crude measure of the share of the crop that 

may have come in contact with endothall. These estimates are the best that can be made with the available data, and 

they likely overestimate the crop that will encounter endothall treated water, because not all production in the 

western states will be irrigated with water from canals that have been treated with endothall. Not all irrigation 

systems will use endothall, some crop production is irrigated with wells that do not require weed control, and some 

crops such as wheat and oats may be farmed without irrigation, even in the arid West. When possible (wheat and 

oats), the estimates provided in Table 2 above, based on the 2007 Census of Agriculture, are preferable. 

The 17 western states are Arizona, California, Colorado, Idaho, Kansas, Montana, Nebraska, Nevada, New Mexico, 

North Dakota, Oklahoma, Oregon, South Dakota, Texas, Utah, Washington, and Wyoming. It is in the West that 

large scale water delivery systems are important to agriculture; these are the systems that will use endothall for 

irrigation, as they are the systems that currently use acrolein or xylene, aquatic herbicides for which endothall may 

substitute. Agriculture in the East, where it is less arid, does not require large scale water transfers. BEAD does 

expect to see endothall use in the East, but primarily for drainage systems rather than irrigation water delivery 

systems where water will be applied to crops. 

Where available, estimates of the crop grown in the 17 western states are provided for different end-uses of the 

particular crop. This information can be useful for dietary risk estimates, which are based on consumption of 

specific food products. The estimates of the amount grown in the 17 western states are provided in Table 3. 

Peas

Only two western states are important producers of green peas for processing, which can appear in baby foods. In 

2008, about 103,000 tons were produced in Washington State, and about 33,000 tons in Oregon. Production in these 

two states was about 11% of US production of almost 412,000 tons (USDA 2009a). 

Watermelon

Four western states produce enough watermelon to show up in the USDA data, Arizona, California, Oklahoma, and 

Texas. California and Texas produced about 6,100,000 cwt. and 6,000,000 cwt. in 2008, respectively. Arizona 

produced almost 2,900,000 cwt., and Oklahoma only produced about 360,000 cwt. Together, the almost 15,400,000 

cwt. of watermelon represented about 39% of US production of about 39,600,000 cwt. in 2008 (USDA 2009a). 

Strawberries

California is the major strawberry producing state in the United States. In 2008, California produced almost 23,000 

cwt. of strawberries, and Oregon another 238 cwt., the two together totaling about 91% of the total US production. 

About 89% of the fresh market strawberries come from the western states of California, Washington and Oregon, 

and almost all the processing strawberries come from the same three states, accounting for almost all strawberries 

used for processing (USDA 2009b). 

Apple

Available data on apple production covers the production for the western states of Arizona, California, Idaho, 

Oregon, Utah, and Washington, with Washington being by far the largest producer. Combined these states 
accounted for about 65% of total US apple production in 2008, with over 59% of the total coming from Washington 

alone (USDA 2009b). 

Oats

Data on oat production is available for 13 of the seventeen western states, with South Dakota being the largest 

producer in the West, and 3

rd in the country, with about 8.8 million bushels. In all, the western states produce about 

37% of the nation’s oats (USDA 2009c). 

Grapes

California was the largest producer of grapes in 2008, accounting for over 89% of total production. The 17 western 

states combined accounted for about 94% of total production. Looking at the end uses of these grapes, the picture is 

similar. About 99% of fresh grapes were from the western states in 2008, and about 94% of the processed grapes. 

Of the grapes used for processing, California was the most important producer of wine grapes (about 99%) and 

seems to account for all dried grapes (USDA 2009b). 

Wheat

Wheat is widely grown in western states, with all 17 states appearing in the available production data. Together, the 

17 western states account for about 70% of the total wheat production. 

Table 3. Share of Production of Specific Crops in the 17 Western States, 2007

US Production

Western 

States 

Production

Western 

States Share

Green Peas (Tons) 411,780 43,666 11%

Watermelon (1,000 cwt.) 39,551 15,387 39%

Strawberries (1,000 cwt.) 25,317 22,913 91%

Fresh Market 20,911 18,653 89%

Processing 4,406 4,404 100%

Apple (Million Pounds) 9,769 6,394 65%

Fresh Market 6,304 4,911 78%

Processing 3,372 1,500 44%

Juice 1,534 750 49%

Canned 1,191 170 14%

Oats (1,000 Bushels) 88,635 32,565 37%

Grapes (Tons)

7,303,26

0 6,887,000 94%

Fresh Market 985,200 976,050 99%

Processing 

6,304,35

0 5,944,650 94%

Wheat (1,000 Bushels)

2,499,52

4 1,752,810 70%

Source: USDA 2009a, 2009b, 2009c, EPA Calculations

The 17 Western states are Arizona, California, Colorado, Idaho, Kansas, Montana, 

Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, Oregon, South Dakota, 

Texas, Utah, Washington, and Wyoming.

Suggested Federal Register Language 
EPA is establishing tolerances on multiple commodities to support the application of the aquatic herbicide endothall 

to be used in irrigation canals without a holding period. 

For a new agricultural pesticide use, EPA typically estimates percent crop treated by comparison with the amount of 

use of other pesticides for the same crop or site. That approach is inappropriate for the new use for endothall, 

because the use is on irrigation canals rather than crops and EPA does not have data on the frequency of use of 

aquatic herbicides on irrigation canals. 

Instead, EPA has estimated percent crop treated for endothall by estimating the percent crop irrigated which serves 

as an upperbound for crops that may be exposed to endothall in irrigation water. EPA used two methods to estimate 

percent crop irrigated. The preferred method, used where data on irrigated production are available, is an estimate 

of the share of total production that is irrigated. Estimates from this method are provided for barley, corn, dry edible 

beans, oats, peanuts, rice, sorghum, soybeans, sugarbeets, sugarcane, and wheat. Where data on irrigated production 

are not available, EPA estimated the percent crop irrigated by determining the percentage of United States 

production of a crop that is grown in 17 western states where endothall may be used. The 17 western states are 

Arizona, California, Colorado, Idaho, Kansas, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, 

Oregon, South Dakota, Texas, Utah, Washington, and Wyoming. These states are the states where large scale water 

projects predominate, and where other chemicals are used in canals for weed control. These types of irrigation 

projects are relatively rare in other parts of the country.

Use of these estimates in the exposure assessment is conservative, because it is the equivalent of assuming 100% of 

irrigated crops have irrigated with water from endothall-treated canals.

In fact, even in areas with surface water delivery systems, all irrigation canals may not be treated with endothall. 

Additionally, some crops, even in the heavily irrigated areas of the West, are not irrigated, such as dryland grain 

production. 
References

USDA 2004. US Department of Agriculture, National Agricultural Statistics Service, The Census of Agriculture, 

Farm and Ranch Irrigation Survey (2003) Volume 3, Special Studies Part 1. November 2009. Available 

here: http://www.agcensus.usda.gov/Publications/2002/FRIS/index.asp

USDA 2009a. US Department of Agriculture, National Agricultural Statistics Service, Vegetables 2008 Summary. 

January 2009. Available here: http://usda.mannlib.cornell.edu/usda/current/VegeSumm/VegeSumm-01-28-

2009.txt

USDA 2009b. US Department of Agriculture, National Agricultural Statistics Service, Noncitrus Fruits and Nuts 

2008 Summary. July 2009. Available here: 

http://usda.mannlib.cornell.edu/usda/current/NoncFruiNu/NoncFruiNu-07-08-2009.pdf

USDA 2009c. US Department of Agriculture, National Agricultural Statistics Service, Crop Production 2008 

Summary. January 2009. Available here: 

http://usda.mannlib.cornell.edu/usda/current/CropProdSu/CropProdSu-01-12-2009.txt

USDA 2009d. US Department of Agriculture, 2007 Census of Agriculture United States Summary and State Data 

Volume 1, Geographic Area Series, Part 51. February 2009, updated September 2009. Available here: 

http://www.agcensus.usda.gov/Publications/2007/Full_Report/Volume_1,_Chapter_2_US_State_Level/st9

9_2_032_032.pdf