|
Rising production costs and increased environmental concerns have led growers to become more in-tune with the concept of nutrient use efficiency (NUE). Several commonly used practices have been documented to improve NUE including selecting a variety or hybrid with a high harvest index (ratio of grain yield to total plant mass; Bufogle et al., 1997); selecting the proper nutrient rate through soil testing or realistic yield expectations and expected crop removal; matching application timing with crop uptake patterns (Bandel et al., 1990; Scharf and Alley, 1993); banding fertilizer sources (Eghball and Sander, 2001); and many others. Growers can easily recognize yield benefits or nutrient savings associated with implementing these practices, but how can they assess NUE on their farms?
As commonplace as the term is becoming, ask 10 different farmers to define NUE and you’ll likely get 10 different answers. This ambiguity is corroborated in a review paper by Ladha et al. (2005) who defined 18 different ways to calculate NUE. Growers seek a straightforward measure to describe the uptake and utilization of applied nutrients; however, these nutrients have a complex role in cropping systems. Thus, even the simplest forms of calculated NUE values need to be interpreted with care.
Estimating NUE can be as simple as measuring yield. The most basic calculation for NUE is units of crop yield divided by units of nutrient applied. This efficiency term is called the Partial Factor Productivity (PFP), which answers the question “How productive is this cropping system in comparison to its nutrient input?” (Snyder and Bruulsema, 2007). For example, applying N fertilizer at 90 lb/A to produce a 75 bu/A winter wheat crop would result in a PFP of 50 [(75 bu/A*60 lb/bu)/90 lb N applied/A]. This means that for each lb of N applied, 50 lb of grain were produced. The reciprocal of the PFP in this example also indicates that it took 1.2 lb N to produce a bushel of wheat, which is the type of information that serves as the foundation for N fertilizer recommendations for several crops (Donohue and Heckendorn, 1994).
Another easy strategy for estimating NUE considers nutrient removal in the harvested portion of the crop. The ratio of nutrient removed to nutrient applied can be called a Partial Nutrient Balance (PNB; Dobermann, 2007). Using the above example and the N removal estimate for winter wheat reported in Table 1, the PNB for this crop would be calculated to be 0.96 [75 bu/A*1.15 lb N removed/bu)/90 lb N applied/A]. A PNB equal to or very close to 1 suggests a sustainable system, while values well below 1 might suggest the need for improved NUE (Snyder and Bruulsema, 2007). Conversely, a PNB much over 1 could suggest a need for increased nutrient application. Partial nutrient balance should be interpreted with care as the behavior of particular nutrients in soils can affect NUE estimates. For example, let’s assume the winter wheat crop above also received 40 lb/A each of P2O5 and K2O according to a soil test recommendation. Using the removal estimates in Table 1, the PNB values for P and K are 1.03 and 0.64, respectively. Just going by the numbers, it appears that P efficiency is ideal and K use efficiency needs to improve. What these numbers don’t show is that the soil test levels for P and K were high and medium, respectively, in a build-up/maintenance program. So in the short-term K use efficiency is low, but long-term efficiency in this particular system will be improved. The point to remember when using this method of estimating NUE is that when a PNB is less than 1, it becomes important to understand the fate of the unaccounted-for nutrients and to determine if they are contributing to long-term increases in efficiency or creating a situation that may be harmful to the environment (Snyder and Bruulsema, 2007).
Table 1. Nutrients removed in the harvested portion of selected crops.
____________________________________________________________
Crop Harvested Unit N P2O5 K2O
____________________________________________________________
---- lb removed/harvested unit ----
Winter Wheat† bu 1.15 0.55 0.34
Spring Wheat‡ bu 1.50 0.60 0.34
Corn‡ bu 0.90 0.38 0.27
Rice† bu 0.55 0.29 0.18
Bermudagrass† ton 46.00 12.00 50.00
Soybean‡ bu 3.80 0.84 1.30
____________________________________________________________
† - For a more comprehensive list of nutrient removal estimates for crops grown in the southern U.S., see Snyder (2007); ‡ - for crops grown in the north-central U.S., see Murrell (2005). Also note that the reported coefficients may vary depending on growing conditions.
The PFP and PNB are easily calculated for any farm that keeps records of inputs and outputs, and provide useful information for growers. However, neither method considers inherent soil nutrient supplies; thus the true efficiency of fertilizer-derived nutrients is not known.
Estimating NUE in a manner that better reflects the impact of applied fertilizer is more complicated. To calculate the Agronomic Efficiency (AE), which answers the question “How much productivity was gained as a result of fertilizer application?” or the Recovery Efficiency (RE), which answers “How much of the applied nutrient was taken up by the plant?”, the grower needs to have knowledge of crop yield without nutrient input (Snyder and Bruulsema, 2007). The use of a “zero-fertility” or “check plot” has traditionally been limited to research settings, but could easily be established on the farm if a grower has interest in using one of the more complex methods to estimate NUE.
The AE, like PFP, is an estimate of production efficiency and is calculated in units of yield increase per units of nutrient applied. Going back to the winter wheat example, let’s assume that a check strip was left in the field and yielded 40 bu/A. The AE for this farm is calculated to be 23.3 {[(75 bu/A fertilized – 40 bu/A unfertilized)*60 lb/bu]/90 lb N applied/A}. Agronomic efficiencies for N in a well-managed system will usually exceed 25, with the typical range averaging 10 to 30 units of yield increase per unit of N input (Dobermann, 2007).
Recovery Efficiency (RE) is the NUE calculation often preferred by scientists and measures the increase in nutrient uptake by the plant in response to fertilizer application. While the PNB only considers nutrient content in the harvested portion of the crop, RE also takes into account nutrient uptake in above-ground parts of the plant remaining in the field after harvest. The portion of nutrients remaining in crop residue is important to overall system sustainability as the conversion of residue to soil organic matter depends largely on nutrient content (Snyder and Bruulsema, 2007). Recovery efficiency is calculated as [(total nutrient uptake in fertilized crop-total nutrient uptake in unfertilized crop)/nutrient applied]. Using the nutrient uptake values in Table 2, the RE of the winter wheat crop that yielded 75 bu/A using 90 lb N/A and yielded 40 bu/A without fertilizer would be 0.74. Typical values for RE of N in cereal crop production fall between 0.3 and 0.8, with well-managed systems usually having an RE greater than 0.5 (Dobermann, 2007).
Table 2. Total nutrient uptake in the above-ground and harvested portions of selected crops.
____________________________________________________________
Crop Harvested Unit N P2O5 K2O
____________________________________________________________
---- lb uptake/harvested unit ----
Winter Wheat† bu 1.90 0.68 2.00
Spring Wheat‡ bu 2.20 0.76 1.54
Corn‡ bu 1.35 0.54 1.37
Rice† bu 0.71 0.38 1.08
Bermudagrass† ton 46.00 12.00 50.00
Soybean‡ bu 4.90 1.08 2.30
____________________________________________________________
† - For a more comprehensive list of nutrient removal estimates for crops grown in the southern U.S., see Snyder (2007); ‡ - for crops grown in the north-central U.S., see Murrell (2005). Also note that the reported coefficients may vary depending on growing conditions.
Conclusion
Estimating NUE on the farm is a good way for growers to identify possible leaks in the system that may require their attention. However, high NUE does not necessarily indicate that the cropping system is operating most efficiently. Nutrient use efficiency calculated using any of the four described methods will increase as nutrient rates are reduced. However, reductions of nutrient application rates that lead to yield sacrifices are often neither economical nor sustainable. Practices implemented to increase NUE must always be evaluated in the context of the total cropping system and its ability to meet production needs for the world’s growing population.
References
Bandel, V.A., H.M. Kunishi, J.J. Meisinger, and F.R. Mulford. 1990. No-till corn production: achieving maximum nutrient efficiency. Maryland Coop. Ext. Fact Sheet 514.
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.
Dobermann, A. 2007. Nutrient use efficiency- measurement and management. 22 pp. Proc. of International Fertilizer Industry Association (IFA) Workshop on Fertilizer Best Management Practices. Brussels, Belgium. March 7-9, 2007.
Donohue, S.J. and S.E. Heckendorn. 1994. Soil test recommendations for Virginia. Virginia Coop. Ext. Publ.
Eghball, B., and D.H. Sander. 2001. Does variable distribution affect liquid P-use efficiency?. Fluid J. 9:18-21.
Ladha, J.K., H. Pathak, T.J. Krupnik, J. Six and C. van Kessel. 2005. Efficiency of Fertilizer Nitrogen in Cereal Production: Retrospects and Prospects. Advances in Agronomy 87: 85-176.
Murrell, T.S. 2005. Average nutrient removal rates for crops in the northcentral region. [Online]. Available at http://www.ipni.net/ppiweb/usanc.nsf/$webindex/E71D7CA9BD24A18D86257060007A8EB3!opendocument. (verified 6 August 2007).
Scharf, P.C. and M.M. Alley. 1993. Spring nitrogen on winter wheat: II. A flexible multicomponent rate recommendation system. Agron. J. 85:1186-1192.
Snyder, C.S. 2007. Nutrient uptake and harvest removal for southern crops. [Online]. Available at http://www.ipni.net/ppiweb/usams.nsf/$webindex/6603E5603BCA967586256966004DAEEC. (verified 19 July 2007).
Snyder, C.S., and T.W. Bruulsema. 2007. Nutrient use efficiency and effectiveness in North America: indices of agronomic and environmental benefit. International Plant Nutrition Institute. Publ. No. 07076.
|
