C 4MachineHarvesting

Rice Quality Workshop 2003

1

Machine Harvesting and Rice Milling Quality

Harvesting affects yield and milling quality of rice so growers must harvest rice as carefully as

they grow it. Value for most classes of rice in the United States is determined primarily by the

percent of whole milled grain (head rice) and total milled grain, although additional quality

characteristics related to appearance and eating quality may also be used in certain markets. For

example, the 1998 crop USDA loan values for medium and short grain rice average $.043/cwt.

for each point of head rice. At a yield of 80 cwt./ac, this is $3.44/ac for every point of head rice

gained or lost. Total rice is worth about $.049/cwt., or $3.92/ac for every point. As the overall

market value of rice changes, the value of milling quality changes proportionately. The point is

that high milling quality pays dividends. During harvesting, what can growers do to maximize

milling quality?

Harvest at the Optimum Moisture Content

Many studies show that head rice is highest when rice is harvested at moisture content greater

than what is required for safe storage. The optimum harvest moisture content differs with each

variety, as shown in Table 1. Any field of rice at harvest time is a mixture of wet and dry kernels.

Growth of the crop, cultural practices and genetics explain this and are discussed elsewhere in

this workshop. The dry kernels in this mixture, those below approximately 16% moisture, are

the problem. If they are rehydrated, from any source of moisture, they can crack. When these

cracked kernels are milled, they usually break, so you get lower head rice. Every field has some

dry kernels, which are subject to damage from rehydration. This relationship of moisture to

milling yield gives growers a degree of control over head rice by allowing them to time their

harvest according to moisture content. However, while moisture content is very important, there

are many factors determining milling quality so that it is possible to get high or low head rice over

a wide range of harvest moisture[4]. Dry fields dont always give low quality if they are not

exposed to rehydrating conditions.


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Milling yield for California medium and short grain varieties tends to drop as moisture goes

below 20% and both head and total rice decrease above 27- 30% moisture content. Long grain

rice is an exception and produces highest quality at lower harvest moisture (Table 1).

Table 1. California rice varieties and suggested optimum harvest moisture for maximizing milling

quality.

Variety Comment

CM-101 Dries rapidly during ripening, uniform maturity; best head rice at 23-26%

S-102 Watch carefully, pubescent variety that looses moisture rapidly; has high head potential; cut at 20-23%.

Calhikari-201 High milling yield potential but somewhat less than Japanese varieties; harvest in 22-24% moisturerange. Quality assessment for this and other Japanese-type varieties goes beyond milling yields

Akitakomachi High milling yield potential; harvest at 23-24% moisture to minimize fissuring.

Koshihikari High milling yield potential; harvest at 23-24% moisture; to minimize fissuring.

M-103 Very good milling quality; yield and quality may be reduced by early planting in warm areas; headrice stable over a wide range of harvest moisture, but total increases slightly at lower moisture.

M-202 Ripens uniformly, threshes easily; head rice fairly sensitive to declining moisture, best to cut at 22-24%.

M-204 Higher head and total potential than M-202; threshes easily; head more stable over wider moisturerange than M-202.

M-205 Slightly higher head and total potential than M-202 when harvested at similar moisture content;cool weather delays heading more than M-202 and this could affect milling quality.

M-401 Generally lower milling yields than other medium grains; head rice sensitive to low moistureharvest; cut in 22-25% range.

M-402 Average 5% higher head rice than M-401; harvest at 22-25% moisture.

L-204 Harvest at 16-17% moisture; avoid early draining; requires 40-45 days after heading to mature. Lessresistant to cold weather blanking than medium grains.

L-205 Harvest at 16-17% moisture but avoid letting moisture drop too low before harvest; dont beginharvest until grain moisture is below 19% and green grains on the panicles are less than 1%.

A-201 Poor milling yield; use slower cylinder speed; harvest at 18-20%

Calmati 201 Suggest handling similar to L-205.


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Because wetter rice is more expensive to dry, growers may compromise between cost and quality

and try to harvest at the economically optimum moisture content. No recent studies relate

head rice and drying costs to discover the optimum moisture, but the industry has generally

settled on 22+/- 1% moisture content as the best compromise of when to start cutting standard

short and medium grain rice. However, recent work demonstrated that high head yields at lower

moisture contents at harvest are possible under California conditions (Figure 1). Analysis of

receiving data from a major drying facility revealed that many growers harvested at moisture

contents as low as 20% while maintaining high head yields. In contrast, others harvesting at

optimal moisture produced lower mill quality.

Figure 1. Grain moisture content as received at the dryer and the corresponding head yield for

M-202 in 1996.

Growers should collect a sample prior to harvest to check its moisture. For best accuracy, use

a harvester to cut your sample. A combine sample will best represent the true moisture.

Alternatively, one can hand strip from random areas, but be sure to take some of the sample from

lower, less mature panicles. Avoid taking just the ripe grains from the topmost panicles.

30

35

40

45

50

55

60

65

70

14 16 18 20 22 24 26

Receiving Moisture (% wb)

Hea

d Yi

eld

(%)


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Avoid Delays during Harvest and from Field to Drier

Once harvest has started, a field should be completed as soon as possible unless there are

significant areas that have higher moisture; in such cases, leave them until they are ready.

Because stripper harvesters move so fast, they should theoretically be able to harvest all the

grain in a field at closer to the optimum moisture, compared to cutter bar combines. Over the

long run, this should lend itself to higher milling quality. Start harvesting each day when the dew

is mostly off the rice, to avoid false moisture readings and putting wet rice in the truck. Stop

harvesting when significant moisture accumulates on the plants in the evening and the plants

become tough.

Expedite handling after harvest and through the drying process to reduce head rice losses.

Ideally, rice should not sit in trucks overnight. A delay in aerating wet rice can reduce can reduce

quality characteristics, such as taste, which is important in some export markets (Figure 2).

Studies have shown that nonaerated wet rice begins to produce ethanol within hours of harvest.

The evolution of ethanol corresponds to the development off-flavors and odors in cooked rice,

designated as a sour/silage odor by an USDA taste panel (Figure 3).

Most sunchecking occurs in the field before harvest from dew and rain, but rehydration of dry

kernels can also occur after harvest as grain sits in harvester bins, bankout carts, trucks and

holding bins, as moisture transfers from wet to dry kernels.

Harvesting is to Maximize Profit

Harvesting is a compromise between loss of grain yield, reduction in quality, and speed. The

main routes of grain loss are dropping it on the ground or leaving it on the stalk. Physical damage

includes reducing milling quality by cracking or crushing kernels, shelling kernels and

unacceptable amount of trash (dockage) because of inadequate cleaning. Most adjustments in

combine operators manuals emphasize reducing losses first, which are usually worth more than


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quality. Acceptable losses are less than 5% (400 lbs/ac at 80 cwt./ac yield). California studies

found that the average rice field after harvest had about 300 lbs/ac grain on the ground. This

suggests growers are doing a good job of adjusting combines to minimize losses. After losses are

minimized, reduction in grain damage is achieved by adjusting the threshing mechanism.

Figure 2. Effect of delayed aeration after harvest on the taste quality of Akitakomachi.

Figure 3. Accumulation of ethanol and the development of off-odor in wet rice.

40

45

50

55

60

65

70

0 5 10 15 20 25 30 35

Time (hr)

Tast

e Sc

ore

SE=0.8

Sour/silage

Ethanol vs 1,3 butandiol: R2 =0.90


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Combine speed and variety (Figure 4) influenced grain recovery performance of cutter bar

headers as compared to stripper headers. Based on header performance studies and southern rice

varieties, there were no differences in efficiency between the two header types at speeds less

than 1 mph. However, operating a stripper header above 2 mph resulted in a substantial increase

in harvest loss. Stripper header performance was also decreased when harvesting the shorter

variety with a robust and prominent flag leaf projecting well above the head.

Figure 4. Relative harvest loss of a stripper header compared to a conventional header mounted

on IH 1680 combines in two southern rice varieties. Source: Bennett et al., 1993.

Environmental Effects on Head Rice

The differences in grain quality among varieties and at different harvest dates are sometimes

attributed to environmental factors that occur during grain fill and maturity. Among the many

possible factors only a few have been studied under California conditions. One of the important

Harvest Speed (mph)


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environmental factors is temperature. A five-year study from 1979 to 1984 based on over 1000

dryer receiving lots found that head yields are not generally affected by temperature immediately

before harvest (Figure 3-A). The exception to this observation is the very early varieties. In

which case, head yield declined with increasing temperatures. The head yield for the very early

varieties improved as the harvest progressed, presumably, due in part to cooler temperatures

(Figure 5 B). Grain moisture content trended downward with increasing temperatures and was

not generally associated with harvest date (Figures 5 C & D). Differences in day and night time

temperatures did not affect head rice yields during the 5-year course of the study.

Understanding Harvester Basics

Figures 5 - A to D. Relationship of % head,% moisture to air temperature and harvestdate. Averaged 1979 1984.p Very earlyn Early Intermediatel Late

Figure 5 E. Minimum and maximumtemperatures during the harvest season.Averaged 1979 1984.

After: Geng, S et al. 1985. HARVEST WEEK

Max temp

Min temp

E

DC

A B


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A combine is a machine that harvests (cuts and gathers the rice crop) and threshes (separates out

the grain from the plant and cleans the grain) the crop. All combines used in California rice

production are self-propelled, which means they have on-board engines, which propel the

combine and drive the combine mechanisms. Modern rice combines are powerful with 300+

horsepower and cut a wide swath, 18-22'. There are several types of combines. Cutterbar

headers cut the stems and separate grain and straw internally while stripper headers strip the

grain off the stalk without cutting. Both types are similar in other respects. Some combines

thresh the rice using a cylinder running against a fixed concave, which is set crosswise in the

travel direction. Axial flow combines have a long rotor which also turns against a fixed surface

but which is set on the long axis of the combine. Separators vary also, including those, which use

a straw walker to pass the grain along a series of fishbacks allowing the grain to fall onto

screens and sieves, and tine separators, which are a kind of secondary cylinder and concave.

Axial flow combines use the rear end of the rotor to separate the straw and grain. Figure 1 shows

the main sections of a cutterbar combine.

Figure 6. Cutter bar combine and its major systems and functions.


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Cutterbar headers. The parts of a cutterbar header include a reel, as siclebar and either a gathering

auger or cross draper. The reel parts a section of the crop and pushes it against the cutterbar,

which cuts the stalk. Rice reels have fingers that help pick up lodged plants. As the cut stalk

falls backward into the combine, helped by the reel, the draper helps move it toward the

gathering auger which in turn carries it to the center of the header into the feeding mechanism. See

Figure 6. Some aftermarket headers use cross drapers, which replace the function of the gathering

auger. Some feel they provide more even feeding. Reels can be adjusted forward and back and

up and down. In standing grain, the bats should strike the grain just below the lowest grain

heads. The peripheral reel speed should be 1.25 times greater than the ground speed. The

formula for reel speed, RPM in relation to ground speed is:

RPMreel = 366 X 1.25 X ground speed in mph reel diameter in inches

Augers and drapers can be adjusted for clearance from the bottom of the platform and rotational

speed. Auger speed should be sufficient to give a uniform flow of crop into the threshing

mechanism. If too slow, clogging could occur in the feed area of the platform.

Figure 7. Cutterbar type of header and key components.


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Stripper headers. This type of header uses a rotating drum equipped with stripping fingers

which operate under a cowl or shroud which deflects the crop under it and briefly traps the

heads so the stripping elements can comb off the grain. This header can be attached to standard

combines of all types. With stripper headers, the threshing and separation functions of the

combines are little used and mainly pass the grain on to the cleaning and handling systems (Figure

7). Rotation of rotor is upward so that grain is swept into the header. An auger takes it to the

feeder mechanism.

Figure 7. Stripper header showing rotor and stripper elements (cover not in place).

Feeding mechanism. As the cut material enters the throat of the combine, called the feeder house,

a feed conveyor feeds it into the threshing mechanism.

Cylinders and concaves. These remove the grain from the stems by rubbing and abrasive action.

Both spike tooth and raspbar types are used in rice. The former is more aggressive and may give

more complete threshing but with slightly more grain damage, while the latter is considered more

gentle but with slightly higher losses. California studies suggest that the virtue of one offsets

those of the other (Figure 8).


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Figure 8. Cylinder and concave threshing mechanism.

Axial flow or rotary threshers. These threshers can have either single or double rotors mounted

longitudinally in the combine. Impeller blades at the front draw the crop in from the feeder

house. Rasp bars work against concave sections at the front end of the rotor to provide threshing

action. The crop is threshed as it swirls between the periphery of the rotor and the concaves

(Figure 9).

Separation. Several devices are employed to separate the straw and grain. In cylinder and

concave type machines a beater located directly behind the cylinder deflects the grain onto the

straw walkers. (See Figure 5) The walkers hold up the straw and allow the grain to fall below

while they convey the straw to the rear of the combine. In axial flow machines the rear of the

rotor provides separation and conveyance of the straw, so there are no straw walkers. In all

machines the grain then falls through separator grates onto the cleaning mechanism.

Figure 9. Position and orientation of rotor in axial flow combine.


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Cleaning. A fan blows air up through the bottom of the chaffer and sieve to aerodynamically

separate the chaff and small stems from the grain. The chaffer and sieve together are called the

cleaning shoe. The chaffer is an oscillating frame, which suspends straw and chaff so it can be

carried out the end of the combine. Next, the sieve, which has smaller openings than the chaffer

and operates in a similar manner, does the final cleaning. See Figure 10.

Handling. Elevators and augers are used to convey the grain, clean grain going into the storage

tank at the top, or, in the case of unthreshed material from the end of the shoe called tailings,

back to the threshing cylinder where it passes through a second time. From the storage tank and

unloading auger moves the grain into bankout wagons driven alongside the combine.

Figure 10. Cleaning and handling mechanisms.

Adjust the Combine for the Conditions

Combines require settings unique to the crop, variety and maturity. Most harvester manuals give

initial settings for the crop, but you must be prepared to adjust these based on conditions in a

given field and the threshing characteristics of the particular variety.


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Proper adjustment = minimum loss and damage to grain

The initial settings in Table 2 are reasonable for long grains, which thresh easily. For more

difficult to thresh short and medium grain varieties, California rice growers should increase

cylinder speed to 550-700, decrease clearance to 19 mm (3/4") and decrease fan speed to 1050-

1150. Varieties that are difficult to thresh, such as Koshihikari, might require higher cylinder

speed and less clearance.

Table 2. Initial settings for long grain varieties recommended by manufacturers. Adjust for

medium and short grains.

Operation/Model JD CTS II Case/IH 2388

With Shelbourne

Reynolds header

Rotor speed, rpm -800 400-500

Cylinder speed 400-550- JD CTS - 450 to 700

Case/IH 2388 - 660

Concave setting, mm 28Indicator at 1 3-4 mm minimum

Fan speed, rpm 1200880 Full speed

Screens Variable - see operators manualFull open

Forward speed Depends on crop ripeness, moisture, standing or lodged, soil moisture, power available,combine capacity, crop yield

While harvester settings can affect milling yields, the magnitude is usually not as much as for

harvest moisture content unless they are way off. Settings that are most important to maintaining


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milling yields are cylinder speed and cylinder clearance, according to work in California.

Researchers working on common short stature varieties found a significant effect of cylinder

speed on milling yield. An increase of 100 rpm decreased head rice 0 .2, 1.0, 1.0 and 2.6 points

and losses decreased by 1.1, 0.5, 0.5, and 3.0 points for CalBelle, M9, M-201 and M-401,

respectively.

Generally, the reduction in head rice was offset by the increase in recovery as cylinder speed

increased. Speeding up the cylinder increased revenue for the long grain variety by about 8%,

in a range of 420 - 700 rpm but did not change revenue for M-201 between 500 and 890 rpm or

M9 between 900 and 1100 rpm. Cylinder/concave clearance did not make a great deal of

difference in this study. In comparison tests of rasp and spike tooth cylinder, the rasp bar was

gentler and produced slightly higher milling yields, but had greater losses which offset the head

rice gains. Both cylinder types produced lower head rice and lower losses as cylinder speed

increased in a range of 500-880 rpm on M-201. There was no significant advantage of one over

the other in respect to revenue. Generally, in this study, reducing losses was worth more than

increasing head rice. And, each variety needed its own cylinder speed and forward speed for

optimum performance.

In more recent work in Arkansas rotor harvester settings were studied for their effects on losses

and milling yield. Depending on the variety, the most important factors relating to losses were

feedrate (the amount of material in the harvester at any one time which is affected by forward

speed and cutter bar height) and the ratio of material other than grain (MOG) to grain (G).

MOG/G ratio is the composition of material in the harvester. Grain moisture content,

composition of the material, feedrate and rotor speed had only small impacts on quality in this

study. Field variability was more important than combine settings.


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Troubleshooting Combines

Grain loss and damage troubleshooting guides are usually included in the operators manual.

Identification of problems requires frequent checking for losses on the ground, at the reel and

behind and inside the machine, and for signs of damaged grain in the tank. The following tables

are adapted from operator manuals and provide suggested remedies for various problems.

Table 3. Troubleshooting cutterbar combines, including cylinder and rotor types.

Problem Possible Adjustments

Excessive free grain in straw Remove rear concave inserts

Remove tine grate covers

Increase cylinder speed by 30 rpm

Increase tine speed (change drive sprocket)

Unthreshed grain Close concave

Increase cylinder/rotor speed

Increase tine speed

Add concave fillers

Free grain at rear of shoe or under rear

axle

Change fan speed (could need more or less)

Open chaffer and/or precleaner

Reduce cylinder/rotor speed

Open concave

Decrease tine speed

Reduce ground speed

Excessive grain in lower elevator Open sieve, chaffer

Reduce fan speed

Damaged grain in tank Reduce cylinder/rotor speed

Open concave

Remove concave filler strips

Tank sample is not clean Close pre-cleaner, chaffer, sieve

Increase fan speed

Decrease rotor speed

Open concave

Free grain on ground behind header Reduce reel speed


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Table 4. Troubleshooting stripper combines.

Problem Possible Adjustments

Unstripped grain in heads Increase speed of stripper rotor

Replace stripping elements

Lower rotor

Reduce forward speed

Excessive shelling of seeds at header Lower header

lower cowl

Increase forward speed

Trash in sample Close concave

Close sieve

Cracked grainr Reduce cylinder speed


Grain loss over the sieves Increase fan speed if sieves are blocked

Decrease fan speed if blowing over


Stripper torque limiter operating

excessively

Raise header

Decrease forward speed

Crop too green

Increase rotor speed

Measuring Grain Losses

Visually.

1. Disengage the straw spreader or straw chopper at the rear of the combine.

2. Harvest a typical area in the field.

3. Stop forward motion of the combine and allow the combine to clear itself of material.

4. Back the combine a distance equal to the length of the combine and stop the combine.

5. Walk the area in front of the combine and observe the ground for rice kernels.

NOTE: California rice is generally heavier than Southern varieties so the operator manual tables

for loss calculations are not accurate. For California varieties, approximately 35 kernels/ft2 is

equal to 100 lbs/ac.


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By measurement.

First, follow procedures 1 to 4 above.

FOR PREHARVEST LOSSES:

1. Place a frame of known area (usually 1 foot square) at several locations in a typical area of the

unharvested field.

2. Count the number of kernels on the ground laying within the frame.

FOR HEADER LOSSES:

1. Place a frame at several locations in front of the combine.

2. Count the kernels and take an average.

3. Subtract the average found in the pre-harvest test.

FOR THRESHING LOSSES:

1. Place a frame at several locations directly behind the separator area of the combine.

2. Count the number of kernels remaining on partially threshed heads. Average the number of

kernels.

3. Count and separately record the number of loose kernels lying on the ground. These will be

used to determine straw walker and cleaning shoe losses.

4. Use the following Table 5 to determine losses attributable to the threshing system:


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Table 5. Loss table for preharvest, header and thresher losses.

Number of rice kernels (medium grain) per foot square to equal 100 lbs/ac

Cutting Width (ft.)Separator

width (in.)

15' 16' 18' 20' 22'

29" 57 -- -- -- --

38" 43 45 51 57 --

44" 37 39 44 49 54

55" 29 31 35 39 44

FOR STRAW WALKER AND CLEANING SHOE LOSSES

1. Determine the average number of loose kernels from #3 above (Threshing Losses)

2. Subtract from this result the average number of kernels from the pre-harvest field loss tests

as well as the average number of kernels from the header loss tests.

3. Use the Table 5 above to determine the losses attributable to the straw walker and cleaning

shoe.

Once the source of an unacceptable loss has been identified, the information in Table 6 below will

help you identify the cause and correct it.


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Table 6. Source, cause and remedies for rice grain losses.

Source of loss Cause of loss Remedy

Preharvest losses Wind and hail shatter, lodging Variety selection and cultural

practices

Header losses Grain heads missed by cutterbar

Grain shattered by reel

Grain thrown over in front of the reel

Lower cutter bar

Reduce reel speed

Raise reel, reduce reel speed

Threshing losses Unthreshed grain carried over the straw walkers

Cracked grain due to overthreshing

Cracked grain due to excessive tailings

Slow ground speed

Slow cylinder, open concave

Slow forward speed, increase

threshing action

Straw walker losses Caused by feeeding too much material over the straw

alkers

Slow ground speed to reduce

throughput

Shoe losses Grain blown out the rear by high fan speed

Too much material on the chaffer

Slow fan speed

Slow ground speed

Leakage losses Open inspection, cleaning and drainage doors

Torn seals, sheet metal holes

Close doors

Repair seals and holes

In summary, timing of harvest is the most important quality related decision rice growers make.

Harvesters have less impact on milling yield and should be adjusted to minimize losses first then

fine tuned to reduce damage. Growers should also expedite transport of harvested grain to the

drier.


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Bibliography

In: Andrews, S.B., T.J.Siebenmorgen, E.D. Vories, D.H. Loewer, and A. Mauromoustakos. 1993.

Effects of combine operating parameters on harvest loss and quality in rice. Amer. Soc. Ag. Eng,

vol 36(6): 1599-1607, Nov-Dec.

Bennett, KE, TJ Siebenmorgan, E Vories, and A Mauromoustakos. 1993. Rice harvesting

performance of Shelbourne Reynolds stripper headers. Arkansas Farm Research, 42(4):4.

Geng, S., J.F. Williams, and J.E. Hill. 1984. harvest moisture effects on rice milling quality. In

California Agriculture, Nov.-Dec. pp 11-12.

Jongkaewwattana, S. 1990. A comprehensive study of factors influencing rice milling quality.

Ph.D. dissertation. Dept. of Agronomy and Range Sci., University of California, Davis. 138 pp.

Kunze, O.R. 1985. Effect of environment and variety on milling qualities of rice. In: Rice grain

quality and marketing. International Rice Research Institute. pp 37-48.

Miller, G.E., B.M. Jenkins, S. Upadhyaya, and J. Knutson. 1985. Rice harvesting research, In

Comprehensive research on rice, annual report. California Rice Research Board. pp 91-98.

Xu, X.B. and B.S. Vergara. 1986. Morphological changes in rice panicle development. A review

of literature. International Rice Research Institute. Res. paper ser. 117. 13 pp.

Yoshida, S. 1981. Fundamentals of rice crop science. International Rice Research Institute.269

pp.

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