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**Conservation Tillage Fact Sheet**
**Taking Advantage of the Benefits of    No-Till****with Rainfall Probability    Distributions** **Merle F. Vigil, David Nielsen, Randy    Anderson and Rudy Bowman - USDA-ARS, Akron, CO****The Fact: Rainfall probability    distributions can help reduce the risk in making a wrong decision of crop choice in a    given year, for no-till and reduce-till dryland production systems.**
**Abstract**

**No-till and reduce-till (commonly referred to as conservationtillage) have become important soil-crop management systems on dryland farms of theCentral Great Plains. These systems are important because they protect soil from erosion,increase soil-organic matter, improve precipitation-storage efficiency, increasebiological yield, provide fuel and tractor-life savings of 25 to 50 percent, and increasethe number of crop options for dryland rotations. In our region of the Great Plains, 12 to20 inches of precipitation is typical. The traditional crop production system has beenwinter wheat-fallow. Using our best management practices, we might store in the soil 50percent of the precipitation that falls during the fallow period for use by a subsequentcrop. Improved precipitation storage efficiency (PSE) with no-till, has enabledresearchers and farmers to look for more intensive crop rotations. However, theserotations require careful consideration of the probability of receiving adequateprecipitation for crop growth and development. Long-term weather data andwater-use-production functions can help guide farmers in making management decisions withrespect to crop choice in a given year for conservation tillage systems.**

****Introduction: Dryland farming inthe West Central Great Plains- a short history****

**The dominant dryland-crop-production system in this region forthe last 50 years has been winter wheat-summer fallow (Haas et al. 1974). Summer fallow isthe practice where: no crop is grown and all weeds are killed by cultivation or herbicidesduring the summer when a crop is normally grown. Summer fallow is done to storeprecipitation during the fallow year in anticipation of better yields and reduced risk ofcrop failure from drought the subsequent year. The average annual precipitation fornortheastern Colorado is minimal for annual crop production and varies greatly from yearto year, requiring producers to rely on stored water for crop success (Fig. 1).**
******Figure 1. Long-term precipitation record at Akron,    Colorado. The dashed line is the long-term average of 16.5 inches.**
**In the "dirty thirties" summer fallow was conductedusing moldboard or one-way disk plows that invert the soil and bury weeds. Unfortunately,this kind of tillage greatly increases soil water loss by evaporation. Researchers(USDA-ARS) at Akron, Colorado estimate that 0.4 inch of water is lost through evaporationwithin two days after tillage with a disk or chisel (Table 1). Precipitation storageefficiency (PSE) during the 30's was only 24%, mainly because of the kind of tillage used(Fig 2). In the 1950's and 60's, the "stubble mulch" system was developed.Stubble mulch relies on the use of sweep-plows, consisting of v-shaped blades, pulled atshallow depths to undercut weeds. Sweep-plow tillage causes the loss of about 0.1 inch ofwater within 2 days after tillage (Table 1). Using sweep-plows in stubble mulch increasedfallow PSE to 33%.**

**Herbicides have allowed us to develop reduce-till and no-tillfallow. With reduce-till, residual herbicides are used to control weeds after wheatharvest. Tillage with sweep-plows are then used after the herbicide has begun to loseactivity. In reduce-till PSE is near 40% (Fig 2). With no-till, both residual and contactherbicides are used. The soil surface and crop residues are never disturbed increasing PSEto near 50%, In this respect, no-till is our best management practice for storing soilwater during fallow.**
**Table 1. Effect of    tillage on residue reduction and soil water loss 1 through 4 days after tillage (from Good    and Smika 1978).****Soil    water loss (inches) in the 0-5 inch depth****Tillage implement****Residue reduction****1 day****2 day*****4 day****Tandem disk****75****------******------****------****One-way disk****50****0.33****0.39****0.51****Chisel****10****0.29****0.35****0.48****Sweep-plow****10****0.10****0.11****0.14****Rod-weeder****15****0.04****0.10****0.22***** Values are linear interpolations    between measured values at 1 and 4 days.***** * data not collected for tandem disk in this    study**
 
****
**The increased water    savings with no-till has prompted researchers to examine more intensive nontraditional    crop rotations (Peterson et al. 1992, Halvorson et al. 1994). These include:    wheat-corn-fallow, wheat-millet-fallow, wheat-corn-millet-fallow,    wheat-millet-sunflower-fallow etc. That research indicates that we can grow successfully    more than one crop in two years. An examination of the response of nontraditional dryland    crops to no-till reveals an advantage for dryland corn (Fig 3)****Fig 2. PSE and stored soil water for various fallow    methods (from Nielsen and Anderson 1993)** 
 
**Fig 3. The percentage increase in yield using    no-till over conventional-till in a dryland cropping system where the previous crop is    winter wheat.**
******Fig 4. Rainfall-probability distribution for the    growing season at Akron.**
****
**However, success of more intensive rotations depends onconservation-tillage management. Moisture is valuable and tillage not needed to controlweeds can be costly. For the production of dryland wheat, at least 7 inches of total waterare needed before any yield will be harvested. That 7 inches can come from stored soilwater or precipitation. After the first 7 inches of water, any additional water (whetherit be stored or fall as rain) produces about 6.5 bushels of grain per inch of water. Fordryland corn the relationship is about 10 bushel per inch of stored soil water orprecipitation after the first 9 inches. Using the tillage-water-loss relationships intable 1 we can calculate that two one-way disk operations result in the loss of 1 inch ofsoil water. This translates into a 10 bushel yield loss costing as much as $20-$30.**

**We have established that more intensive rotations will work.However, with our variable climate farmers are cautious about adopting these systems.Their caution is justified because such schemes require additional management, capital,knowledge and some additional risk.**

**Reducing the risk with productionfunctionsand probabilities, distributions**

**In the preceding paragraph, we mention that wheat will produceabout 6.5 bushels of grain per inch of water after the first 7 inches of water. Thisrelationship between wheat yield and available water is a water-use-production function(Nielsen, 1995). Production functions have been developed for other dryland crops grownunder field conditions at the USDA-ARS Central Great Plains Research Station in AkronColorado. Some of these are given below:**- **Proso millet (lbs/acre) = 237 x (inch of water)    -818** 
- **Corn yield (bushels/acre) = 10.4 x (inch of    water) -95** 
- **Wheat yield (bushels/acre) = 6.5 x (inch of    water) -44** 
- **Sunflower (lbs/acre) = 161 x (inch of water) -843** 

**How to use these functions can be explained by an example: Let'sassume a farmer in Washington County (near Akron) wishes to plant dryland corn. The farmerprobes the soil the last day of April to assess soil available water to a 6 foot depth.For our example, the farmer finds 8.0 inches of available water in the profile.1 The farmer knows that the total long term-averageprecipitation for the months of May, June, July, August and September is 11.5 inches.**

**1 (On April 25th, 1995, we measured 8 inches of available water in the top 6 feetof the soil profile, in wheat stubble to be planted to corn.)**

**If our field were to receive the averageprecipitation, the yield potential could be calculated as:**

**Corn yield = 10.4 x (8.0 inches available in profile + 11.5inches as rain) -95 Corn yield = 108 bushels**

**That looks dandy, but how do we know if our farmer will get theaverage rainfall? How can we even guess if he/she will even get *50% of*the average,or 75% of the average? This is where long term weather records can help. At Akron we haveprecipitation records since 1908 (Fig 1). However, if we take the last 30 years (whichwere drier than average) we can plot a rainfall-probability distribution for the corngrowing months of May through September (Fig. 4). In Fig. 4. we see that 80% of the time(8 *out of 10*years based on the last *30 years of*data) we have *at least*9inches *of precipitation for May* through September. Our function can then be used todetermine yield on an 80% probability:**

**Corn yield = 10.4 x (8.0 available in the *profile*+ 9inches 80% of the time) -95 Corn yield = 82 bushels**

**This is a very good yield for dryland corn, but is notunreasonably high. However, we need to be cautious. Many years available water in the *soilprofile*the last day *of April*may be *less than 8 inches.*Also,precipitation that falls during silking and pollination, influences corn yields more thanthat which falls at other growth stages.Timing can be as important as total amount ofprecipitation received. Silking and pollen shed (depending *on*the year and thevariety) generally occur in Washington county the last week of July or the first two weeksof August. Pollen shed lasts about 9 days. During this period water use is about 1/3 of aninch per day. So, for maximum pollination, we need at least 3 inches of water available tothe crop during this period. Again, some of this water can fall as rain and some can comefrom soil water storage. A rainfall-probability *distribution for*this three weekperiod would be *helpful.***

**But soil water available during this time is justas important. So with respect to dryland corn we can't speculate any more about theprobability of success.**

**For crops like *proso millet*(hershey) andsunflowers, water stress at bloom or during seed development is not as critical and theuse of rainfall-probability distributions can be more predictive.**

**Since proso doesn't root as deeply as corn (onlyabout 3.5 feet) we have less soil profile to extract from and therefore less wateravailable.**
- **Proso-millet yield = 237 x (5.4 available in the    profile + 9 inches 80% of the time) -818** 
- **Proso-millet yield = 2595 lbs (about 50 bushel) 8** 

**Sunflower will grow roots as deeply as corn, buthas a lower wilting point and will extract more water from the same soil profile than willcorn. For sunflower we might have 10 inches available:**
- **Sunflower yield =1 60.5 x (10 available in the    profile + 9 inches 80% *of the time) -843*** 
- **Sunflower yield = 2207 lbs of sunflower.** 

**The grain yields predicted above represent an *educatedguess*based on long-term weather records and yield-water-use relationships developedover the years at Akron. The actual yields realized depend on management, variety, climateand the soils being farmed. The approach used in the above discussion doesn't include heatunits, hail damage, and/or extremes in temperature that could dramatically effect **final**yield. On the other hand, these relationships are better than guessing and *providean estimate of*what to expect for a certain rainfall probability.**

****Summary****

**Tillage practices that invert the soil (disking and moldboardplowing) disrupt the protective surface-residue barrier. This opens and exposes moistsubsoil to drying winds and direct sunlight. The greater the tillage depth, usually thegreater the *moisture loss.*Non-traditional rotations are being studied that takeadvantage of the additional water, fuel and tractor savings with no-till. The use ofrainfall-probability distributions for a given region, combined with water-use-productionfunctions and soil sampling for available-soil water can be used to help a producer decidewhich crop to plant in a given year.**

****Literature cited****

**Haas H.J., W.O. Willis and J.J. Bond. 1974. Summer fallow in thewestern United States. Conservation Research Report No. 17. published by USDA-ARS.**

**Halvorson A.D., R.L Anderson, S.E. Hinkle, D.C.Nielsen, R.A Bowman and M.F. Vigil. 1994. Alternative crop rotations to winter wheatfallow. Proceedings Great Plains Soil Fertility Conference. Vol 5:6-1 1.**

**Good L.G. and D.E. Smika. 1978. Chemical fallow for soil andwater conservation in the Great Plains. Journal of soil and water conservation.33(2):89-90.**

**Nielsen D.C. 1995. Water use/yieldrelationships for Central Great Plains crops. Conservation Tillage fact sheet #2-95.**

**Nielsen D.C. and R.L. Anderson. 1993. Managingresidue & storing precipitation. Conservation Tillage fact sheet #2-93.**

**Peterson, Gary and D.G. Westfall. 1992. Economics of drylandcrop rotations for efficient water and nitrogen use. Proceedings Great Plains SoilFertility Conference. vol 4:47-53.**
- Conservation Tillage Fact Sheet #4-95. published by the USDA-ARS,    and USDA-NRCS.
- Merle Vigil, David Nielsen, and Rudy Bowman    are USDA-ARS Research Scientists at the Central Great Plains Research Station, Akron, CO.    Randy Anderson is a USDA-ARS Research Scientist at the Grain Insect Research Lab in    Brookings, S.D.

**[Top][ 2002 Annual Report ][ Fact Sheets ][ Feed Back ][ Field Day ][ NRCS-Akron ][ Our Research ][ Publications ][ Related Links ][ Staff Pages ]****Questions? Comments? Contact forWeb Info RequestLast edited:****Wednesday April 14, 2004** |
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