Abstract

Worldwide, nitrogen use efficiency (NUE) for cereal production (wheat, Triticum aestivum L., corn, Zea mays L., rice, Oryza sativa L., barley, Hordeum vulgare L. sorghum, Sorghum bicolor, L. , millet, Pennisetum glaucum L., oats, Avena sativa L. and rye, Secale cereale L.) is approximately 33%.  The unaccounted 67% represents a $15.9 billion annual loss of N fertilizer (assuming fertilizer-soil equilibrium).  Loss of fertilizer N results from gaseous plant emission, soil denitrification, surface runoff, volatilization, and leaching.  Increased cereal NUE is unlikely unless a systems approach is implemented that uses varieties with high harvest index, incorporated NH₄-N fertilizer, application of prescribed rates consistent with in-field variability using sensor-based systems within production fields, low N rates applied at flowering, and forage production systems.  Furthermore, increased cereal NUE must accompany increased yields needed to feed a growing world population that has yet to benefit from the promise of N-fixing cereal crops.  The Consultative Group on International Agricultural Research (CGIAR) linked with advanced research programs at universities and research institutes is uniquely positioned to refine fertilizer N use in the world via the extension of improved NUE hybrids/varieties and management practices in both the developed and developing world.

Introduction

In 1996, a total of 82,906,340 metric tons of fertilizer-N were applied in the world, of which 11,184,400 were applied in the United States (FAO, 1996).  Cereal production accounted for approximately 49,743,804 metric tons of N fertilizer world-wide (60% of total, Table 1)(FAO, 1995).  Of that, only 16,572,232 metric tons were estimated to have been removed in the grain (Dale, 1997; Tkachuk, 1977; Keeney, 1982, Table 1).  The world cereal grain NUE would therefore be estimated at 33% (NUE = ((total cereal N removed)-(N coming from the soil+N deposited in rainfall))/(fertilizer N applied to cereals)), far less than the 50% generally reported (Hardy and Havelka, 1975).  Similar results in NUE for West German agriculture would have been found, had they considered N derived from the soil (Keeney, 1982; van der Ploeg et al., 1997).  Using the same references and assumptions in Table 1, developed and developing nation cereal NUE's are 42 and 29%, respectively.  Based on present fertilizer use, a 1% increase in the efficiency of N use for cereal production world wide would lead to a $234,658,462 savings in cost of N fertilizer (Table 1).  An increase in NUE of 20% would result in a savings in excess of $4.7 billion per year. 

Why are NUE's so low?

Not until recently have scientists documented that cereal plants release N from plant tissue, predominantly as NH₃ following anthesis (Harper et al., 1987; Francis et al., 1993).  Plant N losses have accounted for 52 to 73 % of the unaccounted N using ¹⁵N in corn research ( Francis et al., 1993), and between 21% (Harper et al., 1987) and 41% (Daigger et al., 1976) in winter wheat.  Gaseous plant N loss in excess of 45 kg N ha^-1 yr^-1 has also been documented in soybean (Stutte et al., 1979).

Reported gaseous N losses due to denitrification from applied fertilizer N include 9.5% in winter wheat (Aulakh et al., 1982), 10% in lowland rice (DeDatta et al., 1991), and 10 (conventional tillage) to 22% (no-till) in corn (Hilton et al., 1994).  Incorporation of straw and/or application of straw on the surface of zero till plots can double denitrification losses (Aulakh et al., 1984).

Fertilizer N losses in surface runoff range between 1% (Blevins et al., 1996) and 13% (Chichester and Richardson, 1992) of the total N applied, and are generally lower under no-tillage.  When urea fertilizers are applied to the surface without incorporation, losses of fertilizer N as ammonia can exceed 40% (Fowler, 1989, Hargrove et al., 1977), and generally greater with increasing temperature, soil pH, and surface residue.

When fertilizer N is applied at rates in excess of that needed for maximum yield in cereal crops, nitrate leaching can be significant (Olson and Swallow, 1984; Raun and Johnson, 1995).  In cooler temperate climates, nitrate losses through tile drainage have approached 26 kg N ha^-1 yr^-1 under conventional tillage corn when only 115 kg N ha^-1 was applied (Drury et al., 1996).  However, it should be noted that because past N balance work has failed to account for plant N losses, leaching losses attributed to unaccounted N have likely been overestimated (Francis et al., 1993; Kanampiu, 1997).

Many ¹⁵N recovery experiments have reported loss of fertilizer N in cereal production from 20 to 50%.  These losses have been attributed to the combined effects of denitrification, volatilization and/or leaching (Francis et al., 1993; Olson and Swallow, 1984; Karlen et al., 1996; Wienhold et al., 1995; Sanchez and Blackmer, 1988) when each was not measured separately.

Using today's management practices, low nitrogen use efficiencies in the world are compounded by both complacency and economics.  Depending on the source of fertilizer, N costs approximately $0.49 kg^-1.  Applying an added 40 kg N ha^-1 at planting when average cereal N rates are greater than 100 kg N ha^-1 will cost less than $20 ha^-1.  This affordability combined with the convenience of not having to apply N again during the growing season is attractive to farmers.  In this regard excess N is applied as insurance, and because farmers are often overly optimistic concerning expected yields and yield goals (Schepers et al., 1991).  Because of this, the affordability of N in the developed world has led to its misuse and over application.  In the developing world, the same does not always hold true as access to fertilizer is limited (Hubbell, 1995), especially for subsistence farmers in remote areas whose immediate goal is economic survival, not preservation of the environment (Campbell et al., 1995). 

How Can NUE's be Increased

Production practices that have resulted in increased NUE when compared to conventional or standard practices are those that will counter conditions, or environments, known to contribute to N loss from soil-plant systems. 

Rotations

In irrigated or high-rainfall production regions, soybean-corn rotations have high NUE and can reduce the amount of residual N available for leaching when compared to continuous corn (Wen-Yuan et al., 1996).  Also, precipitation use efficiency is greater for corn grown in rotation when compared to continuous corn (Varvel, 1994).  Unfortunately, rotations are not easily adopted by farmers who have become accustomed to monoculture production systems since a new crop often requires purchase of additional equipment and learning to integrate new cultural practices.  In irrigated agriculture, the use of high N rates as a substitute for more N use efficient rotation systems (corn-soybean) must be weighed against the increased potential for NO₃-N loss (Anderson et al., 1997). 

Nitrogen use efficiency for wheat following legumes is greater than that for wheat following fallow or continuous wheat (Badaruddin and Meyer, 1994).  Wheat-corn-fallow production systems are now promoted instead of popular wheat-fallow where only 420 mm precipitation is received per year (Kolberg et al., 1996).  The more intensive systems (growing more crops in a given period of time), require greater fertilizer N inputs but are higher in total yield and economically advantageous (Kolberg et al., 1996).  More intensive dryland cropping systems lead to increased water use efficiency and better maintain soil quality (Halvorson and Reule, 1994).  Alternative dryland systems proposed include spring barley (Hordeum vulgare L.) corn (Zea mays L.) and winter wheat (Triticum aestivum L.) grown in rotation with adequate N fertilization instead of continuous winter wheat-fallow (Halvorson and Reule, 1994). 

Forage Production Systems

Forage-only production systems have lower plant gaseous N loss and improved NUE because the plant is never allowed to approach flowering where N losses have been found to be greater (Altom et al., 1996).  Averaged over 3 years and 2 locations, forage-only NUE's for winter wheat were 77% compared to 31% for grain-only when 90 kg N ha^-1 yr^-1 was applied preplant (Thomason, 1998).  Total N removed in the forage-only production system was nearly double that found in grain, averaging 104 and 59 kg N ha^-1, respectively (Thomason, 1998).  Similarly, calculated NUE's for forage (silage) production in corn exceeded 70% and were greater than that reported for grain (O'Leary and Rehm, 1990).  However, it should be noted that substitution of forage for grain will ultimately place greater dependency on animal protein and decrease the supply of starch for human diets.

Improved NUE hybrid/variety

The early study of NUE was facilitated by identifying individual components that explained both uptake and utilization efficiency (Moll et al., 1982).  Differences among corn hybrids for NUE are largely due to variation in the utilization of accumulated N before anthesis, especially under low N supply (Moll et al., 1982).  Eghball and Maranville (1991) noted that NUE generally parallels water use efficiency (WUE) in corn.

Wheat varieties with a high harvest index (grain produced divided by the total dry biomass) and low forage yield have low plant N loss and increased NUE (Kanampiu et al., 1997).  Higher NUE has also been observed in rice varieties with high harvest index (Bufogle et al., 1997).  Other work by Karrou and Maranville (1993) suggests that wheat varieties that produce more seedling dry matter with greater N accumulation are not necessarily the ones that use N more efficiently.  Furthermore, N assimilation after anthesis is needed to achieve high wheat yields (Cox et al., 1985) and high NUE. 

Genetic selection is often conducted with high fertilizer N input in order to eliminate N as a variable, however this can mask efficiency differences among genotypes to accumulate and utilize N to produce grain (Kamprath et al., 1982).  This is consistent with Earl and Ausubel (1983), noting that high yielding varieties of corn, wheat, and rice released during the Green Revolution were selected to respond to high N inputs.  Consequently, continued efforts are needed where plant selection is accomplished under low N, often not considered to be a priority by plant breeders and uncharacteristic of agricultural experiment stations.

Conservation tillage

Conservation tillage systems have not been found to increase productivity of high-yielding corn genotypes; yet they have not resulted in yield reductions compared to conventional tillage (Al-Darby and Lowery, 1986).  The use of conservation tillage is based more on erosion control, the environment and operation costs, not yield potential (Al Darby and Lowery, 1986), where potential advantages in NUE would be seen. 

Under a no-tillage production system, grain yield was improved 32% when 60 kg N ha^-1 was banded 8 to 10 cm below the seed row, and 15% when banded between the rows compared to surface broadcast urea (Rao and Dao, 1996).  Adaptation of subsurface placement of N fertilizer for no-till winter wheat has the potential to significantly improve N availability to plants and thereby improve NUE and reduce environmental and economic risks (Rao and Dao, 1996).

NH₄-N Source

Because ammonium-N is less subject to leaching or denitrification losses, N maintained as ammonium in the soil should be available for late-season uptake (Tsai et al., 1992).  Increased N uptake during grain-fill, for N-responsive hybrids, indicates a potential advantage of ammonium nutrition for grain production (Tsai et al., 1992).

Wheat N uptake was increased 35% when supplying 25% of the N as NH₄⁺ compared to all N as NO₃^- (Wang and Below, 1992).  High-yielding corn genotypes were unable to absorb NO₃^- during ear development, thus limiting yields otherwise increased by supplies of NH₄⁺  (Pan et al., 1984).  Assimilation of NO₃^- requires the energy equivalent of 20 ATP mol ^-1 NO₃^-, whereas NH₄⁺ assimilation requires only 5 ATP mol^-1 NH₄⁺ (Salsac et al., 1987).  This energy savings may lead to greater dry weight production for plants supplied solely with NH₄⁺  (Huffman, 1989).  However, this has not been consistently observed, nor is it easy to carry out given N-cycle dynamics.

In-season and foliar applied N

Increasing protein content by applying higher rates of fertilizer is relatively inefficient, as NUE decreases with increasing N level, especially under dry soil conditions (Gauer et al., 1992).  In-season applied N resulted in more efficient fertilizer use in four of five years when compared to N incorporated prior to planting winter wheat (Olson and Swallow, 1984).  Pre-plant N must be carefully managed to optimize grain yield, but adding excess N at that time reduces NUE, whereas the late-season supplied N can be adjusted to increase grain protein and NUE (Wuest and Cassman, 1992a).  In-season N, with point injection or topdressing can maintain or increase NUE compared with pre-plant N in wheat (Sowers et al., 1994).

Nitrogen fertilization should take place early in the season to maximize winter wheat forage production (Boman et al., 1995a).  However, if grain production is the only goal, N fertilization can be delayed until much later in the season without significantly affecting wheat grain yields.  Injection of anhydrous ammonia into established winter wheat has produced significant stand damage, however, it has proven to be equally effective when compared with broadcast urea-ammonium nitrate in grain production (Boman et al., 1995b).  In general, placement of fertilizer N below the surface soil layer can decrease immobilization and increase plant uptake of N (Sharpe et al., 1988).

As early as 1957, foliar application of urea solutions at rates from 11 to 56 kg N ha^-1 at flowering were shown to increase wheat grain protein by as much as 4.4% (Finney et al., 1957).  Recovery of N applied at planting ranged from 30 to 55% while that applied at anthesis ranged from 55 to 80% (Wuest and Cassman, 1992b).  Foliar applied urea (6-10 days after awn emergence at a rate of 50 kg N ha^-1 applied in three sprayings to minimize leaf damage) to barley (Hordeum vulgare L.) increased grain protein more effectively than broadcast NH₄NO₃ (Bulman and Smith, 1993).

Irrigation

Work in corn has shown that maximum fertilizer use efficiency was obtained with the low N rates, applied in-season, and with light, frequent irrigation (Russelle et al., 1981).  Randall et al. (1997) reported that split N applications do not always result in increased NUE for corn production in cooler, wetter climates.  Freney (1997) indicated that supplying fertilizer in the irrigation water, applying fertilizer to the plant rather than the soil and use of slow-release fertilizers were useful for controlling losses of fertilizer N.  This work also suggested that urease and nitrification inhibitors have the capacity to prevent loss of N and increase yield of crops.  Wienhold et al. (1995) reported that supplemental irrigation appears to be a viable technology for growing corn in the northern Great Plains if care is taken to ensure that irrigation inputs are optimized to prevent nutrient leaching from the root zone.  On sandy soils, N fertilizer placement and timing, and effective irrigation management are important considerations in promoting efficient N use that will also maintain groundwater quality (Oberle and Keeney, 1990).   In this work, the principles of production related to increased NUE are considered to be similar under dryland and irrigated conditions since NUE decreases in relation to the amount of excess fertilizer N applied in both systems.

Precision agriculture and application resolution

Conventional application of N to cultivated fields is made at a single rate based upon perceived average needs of the field, usually areas more than 10 ha.  Natural and acquired variability in production capacity or potential within a field cause the average rate to be excessive in some parts and inadequate in others.  Alternatively, precision agriculture practices include the timely and precise application of N fertilizer to meet plant needs as they vary across the landscape.

In order to capitalize on any potential N fertilizer savings and increased NUE, management decisions need to be made at the appropriate field element size (Solie et al., 1996). Field element size is defined as that area or resolution which provides the most precise measure of the available nutrient where the level of that nutrient changes with distance  (Solie et al., 1996).  Random, field variability in soil test and plant biomass has been documented at resolutions less than or equal to one square meter (Solie et al., 1996).  When N management decisions are made on areas of one square meter, the variability present at that resolution can be detected using sensors (normalized difference vegetative index or NDVI), treated accordingly with foliar N (Solie et al., 1996; Stone et al., 1996), thus increasing NUE (Stone et al., 1996). 

It is important to note that soil testing (NO₃-N), irrespective of within field variability is a first approximation to refine field N rates.  A combination of soil testing, fertilizer N experiences of the producer, and projected N requirement (expected yield or yield goal) are the best management tools available for farmers to determine fertilizer N rates (Westfall et al., 1996)

Discussion

The best hope for reducing growth in N use is in finding more efficient ways to fertilize crops (Smil, 1997).  After five years of annually applied N (56 – 112 kg N ha^-1) in winter wheat produced under conventional tillage, only 27 to 33% of the fertilizer N had been recovered in the grain (Olson and Swallow, 1984).  Results like these are common, consistent with worldwide NUE and cause for initiating a collaborative global effort to increase NUE.

Organic farming methods that include legume cultivation and crop rotation are highly efficient, however, if all farmers adopted these methods, they could not feed today's population (Smil, 1997).  Also, the promise of N-fixing cereal crops by the turn of the century (Hardy, 1988), specifically corn and wheat have not materialized, compelling the present need for increased adoption of high NUE practices using commercial fertilizers.  Alternative N application strategies, specifically split applications (e.g., part pre-plant, part in-season) of N that are known to increase NUE, have not been widely adopted, largely because of the ease and affordability of applying more N than needed at or before planting.  Agriculture's focus in developed countries has been on maximizing yields per unit area, and not until recently have we considered the environmental consequences of over application of nutrients (Schlegel et al., 1996).  Improving NUE will decrease the risk of NO₃-N contamination of inland surface and groundwater supplies (Stone et al., 1996), as well as hypoxia in specific oceanic zones believed to be caused by excess nitrogen fertilizer (Malakoff, 1998). 

It should be noted that there are some benefits associated with practices that have low NUE's.  Increasing the N rate will increase crop production, especially in the developing world (Hardy and Havelka, 1975), where lower rates are applied, however, this will decrease NUE if not combined with recommended management practices.  Also, when N fertilizer is applied at rates greater than that required for maximum yield, plant biomass and long-term soil organic C increases (Raun et al., 1998), but NUE decreases.  Increasing soil organic C when high N rates are used could assist in removing atmospheric CO₂ widely believed to be responsible for global warming (Smit et al., 1988), but likely to increase N losses via denitrification (Aulakh et al., 1984). 

Similar to what took place in the auto industry when confronted with demands to increase fuel efficiency, approaches to increase NUE should integrate many known components of grain crop production into one system.  Foliar-applied N at 10 to 25 kg N ha^-1 is highly efficient, but it alone will not meet N demands for maximum yields.  Slow-release NH₄-N sources, forage production, improved NUE hybrids and varieties, and in-season applied N combined with an application resolution consistent with in-field variability is expected to lead to NUE's in excess of 85%.  Unfortunately, there is no published research today where scientists have designed a package of practices specifically for high NUE.  Some combinations of practices which optimize NUE may presently be unaffordable, nonetheless, agronomic sciences need to accumulate the knowledge of systems that will achieve an NUE for grain crop production in excess of 85%.   What may make sense for increased NUE may adversely impact our ability to maintain production and satisfy human needs. 

The overall impact of adopting increased NUE production practices in cereal production, suggests that the environment would be less at risk.  However, economic risk should increase substantially since short-term adoption would likely come with a cost.  The humanitarian risk or hunger incidence should decrease as these practices as a whole should increase production, reflecting the value of better stewardship.

Research and extension of production practices that would lead to a world wide increase in NUE should be implemented by a reorganized and formal association of the CGIAR centers with universities and research institutes that have advanced plant and soil science research programs. Although the principal focus of the CGIAR centers has been on developing improved varieties, they are uniquely equipped to extend management and fertilization practices, along with new seed, that are easily adopted by farmers.  In addition, the CGIAR network of regional programs, directly interfaced with the national programs of virtually every developing nation in the world provide needed access and credibility for both short and long-term adoption of new production practices.  Advanced research programs at universities and research institutes can provide the basic and strategic research underpinning to backstop NUE.  A 1% increase in NUE for cereal production world wide would cover 3/4 of the entire annual budget for the CGIAR that encompasses 16 international centers ($304 million, 1996 budget (Consultative Group on International Agricultural Research, 1996)).

So who would pay for such an effort? The international community should expand support to the CGIAR to enable the CGIAR centers to engage in and coordinate a worldwide effort on NUE.  Likewise, developed countries should provide funding for research to increase NUE.  The benefit-cost ratio to the U.S. government for contributions to the International Maize and Wheat Improvement Center (CIMMYT) in Mexico and the International Rice Research Institute (IRRI) in the Philippines were estimated at 190 to 1 and 17 to 1, respectively, (Pardey et al., 1996).  Both of these CGIAR centers focus on improved higher yielding genetic materials and have outreach programs in place to extend both new varieties and production practices to wheat, maize, and rice growing regions throughout the world.  With this kind of success and benefit to the U.S. economy from U.S. government support of CGIAR research centers, their involvement seems obvious.  Excess nitrogen flowing down the Mississippi each year is estimated to be worth $750,000,000 (Malakoff, 1998).  At an average value of $490 per ton of actual N, the $750,000,000 would comprise over 13.6% of the total value of N fertilizer ($5,480,356,000) applied in 1996 in the entire United States.  In light of this excessive waste, adoption of known practices which will improve NUE should be encouraged, and increased NUE should be a first priority.

References

Al-Darby A.M., and B. Lowery. 1986. Evaluation of corn growth and productivity with three conservation tillage systems. Agron. J. 78:901-907.

Altom, W., J.L. Rogers, W.R. Raun, G.V. Johnson, and S.L. Taylor. 1996. Long-term rye-wheat-ryegrass forage yields as affected by rate and date of N application.  J. of Prod. Agric. 9:510-516.

Anderson, Irvin C., Dwayne R. Buxton, Douglas L. Karlen, and Cynthia Cambardella. 1997.  Cropping system effects on nitrogen removal, soil nitrogen, aggregate stability, and subsequent corn grain yield. Agron. J. 89:881-886.

Aulakh, M.S., D.A. Rennie, and E.A. Paul. 1982. Gaseous nitrogen losses from cropped and summer fallowed soils. Can. J. Soil Sci. 62:187-195.

Aulakh, D.A. Rennie, and E.A. Paul. 1984. The influence of plant residues on denitrification rates in conventional and zero tilled soils. Soil Sci. Soc. Am. J. 48:790-794.

Badaruddin, M., and D.W. Meyer. 1994. Grain legume effects on soil nitrogen, grain yield, and nitrogen nutrition of wheat. Crop Sci. 34:1304-1309.

Blevins, D.W., D.H. Wilkison, B.P. Kelly, and S.R. Silva. 1996. Movement of nitrate fertilizer to glacial till and runoff from a claypan soil. J. Environ. Qual. 25:584-593.

Boman, R.K., R.L. Westerman, W.R. Raun, and M.E. Jojola. 1995a. Time of nitrogen application: effects on winter wheat and residual soil nitrate. Soil Sci. Soc. Am. J. 59:1364-1369.

Boman, R.K., R.L. Westerman, W.R. Raun, and M.E. Jojola. 1995b. Spring-applied nitrogen fertilizer influence on winter wheat and residual soil nitrate. J. Prod. Agric. 8:584-589.

Bufogle, A. Jr., P.K. Bollich, J.L. Kovar, R.E. Macchiavelli, and C.W. Lindau. 1997. Rice variety differences in dry matter and nitrogen accumulation as related to plant stature and maturity group. J. Plant Nutr. 20:1203-1224.

Bulman, Patrick, and Donald L. Smith. 1993. Grain protein response of spring barley to high rates and post-anthesis application of fertilizer nitrogen. Agron. J. 85:1109-1113.

Campbell, C.A., R.J. K. Myers, and D. Curtin. 1995. Managing nitrogen for sustainable crop production. Fert. Res. 42:277-296.

Chichester, F.W., and C.W. Richardson. 1992. Sediment and nutrient loss from clay soils as affected by tillage.  J. Env. Qual. 21:587-590.

Consultative Group on International Agricultural Research. 1996. www.cgiar.org

Cox, Michael C., Calvin O. Qualset, and D. William Rains. 1985. Genetic variation for nitrogen assimilation and translocation in wheat. II. Nitrogen assimilation in relation to grain yield and protein. Crop Sci. 25:435-440.

Daigger, L.A., D.H. Sander, and G.A. Peterson. 1976. Nitrogen content of winter wheat during growth and maturation. Agron. J. 68:815-818.

Dale, Nick. 1997. Ingredient analysis table:1997 edition. Feedstuffs. 69(30):24-31.

DeDatta, S.K., R.J. Buresh, M.I. Samsom, W.N. Obcemea, and J.G. Real. 1991. Direct measurement of ammonia and denitrification fluxes from urea applied to rice. Soil Sci. Soc. Am. J. 55:543-548.

Drury, C.F., C.S. Tan, J.D. Gaynor, T.O. Oloya, and T.W. Welacky. 1996. Influence of controlled drainage-subirrigation on surface and tile drainage nitrate loss. J. Environ. Qual. 25:317-324.

Earl, C.D., and F.M. Ausubel. 1983. The genetic engineering of nitrogen fixation. Nutrition Reviews 41:1-6.

Eghball, Bahman, and J.W. Maranville. 1991. Interactive effects of water and nitrogen stresses on nitrogen utilization efficiency, leaf water status and yield of corn genotypes. Commun. Soil Sci. Plant Anal. 22:1367-1382.

FAO. 1995. World agriculture: towards 2010 an FAO study. Nikos Alexandratos (ed.). Food and Agriculture Organization of the United Nations and John Wily & Sons, West Sussex, England. p. 190.

FAO. 1996. FAOSTAT. www.fao.org.

Finney, K.F., J.W. Meyer, F.W. Smith, and H.C. Fryer. 1957. Effect of foliar spraying of pawnee wheat with urea solutions on yield, protein content, and protein quality. Agron. J. 49:341-347.

Fowler, D.B., and J. Brydon. 1989. No-till winter wheat production on the Canadian prairies: placement of urea and ammonium nitrate fertilizers. Agron. J. 81:518-524.

Francis, D.D., J.S.  Schepers, and M.F.  Vigil. 1993. Post-anthesis nitrogen loss from corn.  Agron.  J.  85:659-663.

Freney, J.R. 1997. Strategies to reduce gaseous emissions of nitrogen from irrigated agriculture. Nutrient Cycling in Agroecosystems 48:155-160.

Gauer, L.E., C.A. Grant, D.T. Gehl, and L.D. Bailey. 1992. Effects of nitrogen fertilization on grain protein content, nitrogen uptake, and nitrogen use efficiency of six spring wheat (Triticum aestivum L.) cultivars, in relation to estimated moisture supply. Can. J. Plant Sci. 72:235-241.

Halvorson, Ardell D., and Curtis A. Reule. 1994. Nitrogen fertilizer requirements in an annual dryland cropping system. Agron. J. 86:315-318.

Hardy, R.W.F. 1988. Biotechnology for agriculture and food in the future. Amer. Chem. Soc. Symposium Series. 362:312-319.

Hardy, R.W.F., and U.D. Havelka. 1975. Nitrogen fixation research: a key to world food? Science:188:633-643.

Hargrove, W.L., D.E. Kissel, and L.B. Fenn. 1977. Field measurements of ammonia volatilization from surface applications of ammonium salts to a calcareous soil.  Agron. J. 69:473-476

Harper, L.A., R.R.  Sharpe, G.W.  Langdale, and J.E.  Giddens.  1987. Nitrogen cycling in a wheat crop: soil, plant, and aerial nitrogen transport.  Agron.  J. 79:965-973.

Hilton, B.R., P.E. Fixen, and H.J. Woodward. 1994.  Effects of tillage, nitrogen placement, and wheel compaction on denitrification rates in the corn cycle of a corn-oats rotation.  J. Plant Nutr. 17:1341-1357.

Hubbell, D.H. 1995. Extension of symbiotic biological nitrogen fixation technology in developing countries. Fert. Res. 42:231-239.

Huffman, J.R. 1989. Effects of enhanced ammonium nitrogen availability for corn. J. Agron. Educ. 18:93-97.

Kamprath, E.J., R.H. Moll, and N. Rodriguez. 1982. Effects of nitrogen fertilization and recurrent selection on performance of hybrid populations of corn. Agron. J. 74:955-958.

Kanampiu, Fred K., William R. Raun and Gordon V. Johnson. 1997. Effect of nitrogen rate on plant nitrogen loss in winter wheat varieties. J. Plant Nutr. 20:389-404.

Karlen, D.L., P.G. Hunt, and T.A. Matheny. 1996. Fertilizer 15nitrogen recovery by corn, wheat, and cotton grown with and without pre-plant tillage on Norfolk loamy sand. Crop Sci. 36:975-981.

Karrou, M., and J.W. Maranville. 1993. Seedling vigor and nitrogen use efficiency of Moroccan wheat as influenced by level of soil nitrogen. Commun. Soil Sci. Plant Anal. 24:1153-1163.

Keeney, Dennis R. 1982. Nitrogen management for maximum efficiency and minimum pollution. In Frank J. Stevenson (ed.)  Nitrogen in agricultural soils. Agron. Monogr. 22. ASA, CSSA and SSSA, Madison, WI.

Kolberg, R.L., N.R. Kitchen, D.G. Westfall, and G.A. Peterson. 1996. Cropping intensity and nitrogen management impact of dryland no-till rotations in the semi-arid western great plains.  J. Prod. Agric. 9:517-522.

Malakoff, David. 1998. Death by suffocation in the gulf of Mexico. Science 281:190-192.

Moll, R.H., E.J. Kamprath, and W.A. Jackson. 1982. Analysis and interpretation of factors which contribute to efficiency to nitrogen utilization. Agron. J. 74:562-564.

Oberle, S.L., and D.R. Keeney. 1990. Factors influencing corn fertilizer N requirements in the Northern U.S. corn belt. J. Prod. Agric. 3:527-534.

O'Leary, M.J., and G.W. Rehm. 1990. Nitrogen and sulfur effects on the yield and quality of corn grown for grain and silage. J. Prod. Agric. 3:135-140.

Olson, R.V., and C.W. Swallow. 1984. Fate of labeled nitrogen fertilizer applied to winter wheat for five years. Soil Sci. Soc. Am. J. 48:583-586.

Pan, W.L., E.J. Kamprath, R.H. Moll, and W.A. Jackson. 1984. Prolificacy in corn: its effects on nitrate and ammonium uptake and utilization. Soil Sci. Soc. Am. J. 48:1101-1106.

Pardey, Philip G., Julian M. Alston, Jason E. Christian, and Shenggen Fan.  1996. Hidden harvest: U.S. benefits from international research aid. International Food Policy Research Institute, Washington DC.

Randall, G.W., T.K. Iragavarapu, and B.R. Bock. 1997. Nitrogen application methods and timing for corn after soybean in a ridge-tillage system. J. Prod. Agric. 10:300-307.

Rao, Srinivas C., and Thanh H. Dao. 1996. Nitrogen placement and tillage effects on dry matter and nitrogen accumulation and redistribution in winter wheat. Agron. J. 88:365-371.

Raun, W.R., and G.V. Johnson. 1995. Soil-plant buffering of inorganic nitrogen in continuous winter wheat. Agron. J. 87:827-834.

Raun, W.R., G.V. Johnson, S.B. Phillips, and R.L. Westerman. 1998.  Effect of long-term N fertilization on soil organic C and total N in continuous wheat under conventional tillage in Oklahoma. Soil & Tillage Research 47:323-330.

Russelle, M.P., E.J. Deibert, R.D. Hauck, M. Stevanovic, and R.A. Olson. 1981. Effects of water and nitrogen management on yield and 15N-depleted fertilizer use efficiency of irrigated corn. Soil Sci. Soc. Am. J. 45:553-558.

Salsac, L., S. Chaillou, J.F. Morot-Gaudry, C. Lesaint, and E. Jolivoe. 1987. Nitrate and ammonium nutrition in plants. Plant Physiol. Biochem. 25:805-812.

Sanchez, C.A., and A.M. Blackmer. 1988. Recovery of anhydrous ammonia-derived nitrogen-15 during three years of corn production in Iowa. Agron. J. 80:102-108.

Schepers, J.S., M.G. Moravek, E.E. Alberts, and K.D. Frank. 1991. Maize production impacts on groundwater quality. J. Env. Qual. 20:12-16.

Schlegel, A.J., K.C. Dhuyvetter, and J.L. Havlin. 1996.  Economic and environmental impacts of long-term nitrogen and phosphorus fertilization. J. Prod. Agric. 9:114-118.

Sharpe, R.R., L.A. Harper, J.E. Giddens, and G.W. Langdale. 1988. Nitrogen use efficiency and nitrogen budget for conservation tilled wheat. Soil Sci. Soc. Am. J. 52:1394-1398.

Smil, Vaclav. 1997. Global population and the nitrogen cycle. Scientific Amer. 277(1):76-81.

Smit, B., L. Ludlow, and M. Brklacich. 1988. Implications of a global climatic warming for agriculture: a review and appraisal. J. Env. Qual. 17:519-527.

Solie, J.B., W.R. Raun, R.W. Whitney, M.L. Stone, and J.D. Ringer. 1996. Optical sensor based field element size and sensing strategy for nitrogen application. Trans. ASAE 39(6):1983-1992.

Sowers, K.E., W.L. Pan, B.C. Miller, and J.L. Smith. 1994. Nitrogen use efficiency of split nitrogen applications in soft white winter wheat. Agron. J. 86:942-948.

Stone, M.L., J.B. Solie, W.R. Raun, R.W. Whitney, S.L. Taylor, and J.D. Ringer. 1996. Use of spectral radiance for correcting in-season fertilizer nitrogen deficiencies in winter wheat. Trans. ASAE 39:1623-1631.

Stutte, C.A., R.T. Weiland, and A.R. Blem. 1979. Gaseous nitrogen loss from soybean foliage. Agron. J. 71:95-97.

Tkachuk, R. 1977. Calculation of the nitrogen-to-protein conversion factor. In Hulse, J.H., K.O. Rachie and L.W. Billingsley (ed.) Nutritional standards and methods of evaluation for food legume breeders. International Development Research Centre: Ottawa, p. 78-82.

Thomason, W.E., 1998. Winter wheat nitrogen use efficiency in grain and forage production systems. M.S. thesis. Oklahoma State Univ. Stillwater, OK. 74078.

Tsai, C.Y., I. Dweikat, D.M. Huber, and H.L. Warren. 1992. Interrelationship of nitrogen nutrition with maize (Zea mays) grain yield, nitrogen use efficiency and grain quality. J. Sci. Food Agric. 58:1-8.

van der Ploeg, R.R., H.Ringe, Galina Machulla, and D. Hermsmeyer. 1997. Postwar nitrogen use efficiency in West German agriculture and groundwater quality. J. Environ. Qual. 26:1203-1212.

Varvel, G.E. 1994. Monoculture and rotation system effects on precipitation use efficiency in corn. Agron. J. 86:204-208.

Wang, Xingting and Fred E. Below. 1992. Root growth, nitrogen uptake, and tillering of wheat induced by mixed-nitrogen source. Crop Sci. 32:997-1002.

Wen-Yuan Huang, D. Shank, and T.I. Hewitt. 1996. On-farm costs of reducing residual nitrogen on cropland vulnerable to nitrate leaching. Rev. Agric. Econ. 18:325-339.

Westfall, D.G., J.L. Havlin, G.W. Hergert, and W.R. Raun. 1996. Nitrogen management in dryland cropping systems. J. Prod. Agric. 9:192-199.

Wienhold, Brian J., Todd P. Trooien, and George A. Reichman. 1995. Yield and nitrogen use efficiency of irrigated corn in the northern great plains. Agron. J. 87:842-846.

Wuest, S.B., and K.G. Cassman. 1992a. Fertilizer-nitrogen use efficiency of irrigated wheat: ii. partitioning efficiency of preplant versus late-season application. Agron. J. 84:689-694

Wuest, S.B., and K.G. Cassman. 1992b. Fertilizer-nitrogen use efficiency of irrigated wheat: i. Uptake efficiency of preplant versus late-season application. Agron. J. 84:682-688.

Summary of Improved NUE Practices

1. Rotation: Because rotations in dryland production are dependent upon moisture availability, no specific rotation for improved NUE can be recommended. However, we believe that the development of an easily mineralizable organic N fertilizer would be consistent with the continued but low demand for N over the season in dryland production and/or slow release N that would come from an incorporated legume.

2. Production system: Forage production systems when compared to grain are much more efficient in their use of N, since harvest takes place prior to flowering after which gaseous plant N losses become significant. Plant N loss helps to explain why grain production systems are less N efficient.

3. Plant Breeding: Simultaneous selection for improved WUE and NUE, varieties with high HI, low forage yield and low plant N loss

4. Tillage: N fertilizer use efficiency is known to be lower in zero-tillage when N is applied to the surface. Decreased NUE for zero or reduced-tillage compared to conventional tillage can be expected. Although conventional tillage is recommended at this time for increased NUE, it should not be condoned at the expense of increased soil erosion. Surface application of N fertilizer in zero-tillage systems should be avoided.

5. N Source: Provide NH4 supply under low N inputs and NH4+NO3 under high N inputs. Inhibit nitrification under low N input (low yield potential) and stimulate nitrification under high yield potential

6. Preplant and in-season applied N: Applied N as NH3 split for forage production (½ at planting and ½ in February). For grain production, no N applied preplant, with NH3 applied once in late February, or UAN knifed in late February.

7. Spring applied foliar N: Rates of foliar applied N, post flowering in wheat should be within 10 and 25 kg N ha-1 and should effectively increase NUE when combined with preplant N rates less than the average required for estimated yield goals.

8. Precision Agriculture and Application Resolution: Sense and treat each 1m2 independently using NDVI at early vegetative stages of growth. N rates used for the first application will not be selected to obtain maximum yields, but rather a first approximation to adjust for the variability present (0- 50 lbs). The second application will apply a similar range of N, but at a time when yield potential can be better defined.

Return to