Nitrogen Fertilization

INTRODUCTION

Maintenance of a snow cover during the winter is necessary for successful winter wheat production in most areas of the Canadian prairies outside of southern Alberta. Direct seeding into standing stubble (no-till or stubbling-in) has proven to be the most reliable method of ensuring a uniform snow cover on winter wheat fields.

Most stubble fields are deficient in available soil nitrogen (N) and residual soil N levels are often less than 30 lb/acre in the surface two feet of soil. Low residual soil N levels usually result in crop responses to added N that are very dramatic making N fertilizer a highly profitable input. The large N requirements of stubbled-in winter wheat also makes N fertilizer a major cost factor, often exceeding 50 percent of the variable input costs. Consequently, N fertilization has an important influence on both the income and expense columns of the winter wheat balance sheet. For this reason, N fertilization of stubbled-in winter wheat has been the focus of numerous research studies in the past several years. These studies have identified a number of important factors that should be considered by producers when formulizing their fertilizer management strategies to produce Maximum Economic Yield of stubbled-in winter wheat.

Maximum Economic Yield
Nitrogen Form
Timing
Placement
Interactions I
Interactions II (continued)
Grain Quality


A COMPREHENSIVE MANAGEMENT PACKAGE IS NECESSARY TO OBTAIN MAXIMUM ECONOMIC N FERTILIZER RESPONSE

Winter survival and a healthy, vigorous spring stand are required for economic fertilizer response with winter wheat. The winter survival potential of the hardiest winter wheat cultivars is not sufficient to ensure overwintering without snow cover protection on most of the Canadian prairies. Consequently, strict attention must be paid to management factors that maximize the winter hardiness potential of the crop and maintain a uniform snow cover during the coldest part of the winter.

No amount of N fertilizer will salvage a crop that has been winterkilled or severely winter damaged. In addition, any management factor that limits the yield potential of a crop that survives the winter will also result in reduced N fertilizer response and lower Maximum Economic Yields. In the stubbling-in production system, the most important management decisions are made before winter wheat seeding moves into high gear. For example, effective trash management, seeding at the optimum date and depth, and correcting phosphate deficiencies have a major influence on the degree of success that can be achieved in the production of stubbled-in winter wheat.


EARLY SPRING BROADCAST AMMONIUM NITRATE (34-0-0) GIVES THE HIGHEST, MOST CONSISTENT N RESPONSE

Significant N losses have been reported for surface applied urea (46-0-0), especially when broadcast on snow. These losses arise primarily through ammonia volatization (loss as a gas) that occurs before urea moves into the soil.

Nitrogen losses of more than 50 percent have been reported for fall and spring broadcast applications of urea in the absence of snow. When both fertilizer price and potential yield are considered, the nitrogen loss from broadcast urea can often be very costly. However, significant yield reductions with broadcast urea are only observed about 1/3 of the time indicating that specific weather conditions are required for volatization losses to occur. This means that potential losses with urea are as unpredictable as the weather. Consequently, losses with broadcast urea often cannot effectively be corrected for by simply increasing application rates to compensate for average losses.

Application of nitrogen fertilizer immediately after the soil has thawed in the spring has provided the most consistent, predictable grain yield response. When applied in the early spring, average grain yield responses have been 100 percent for broadcast ammonium nitrate (34-0-0), 90 percent for broadcast urea (46-0-0), 89 percent for urea banded on the soil surface, 88 percent for surface dribble banded urea-ammonium nitrate solution (28-0-0), and 81 percent for urea-ammonium nitrate solution applied as a spray.

Banding beneath the soil surface has been the main method of minimizing losses for N forms that are vulnerable to volatization. These N forms include anhydrous ammonia, aqua ammonia and urea.

Attempts to band fertilizer in the stubbled-in production system have exposed several problems and variables such as:

  • precision of seed placement
  • horsepower requirements,
  • seedbed disturbance,
  • labour availability during seeding,
  • time priorities,
  • method of N application for other crops produced,
  • type of drill required to seed other crops produced,
  • ammonium nitrate availability,
  • climatic factors, and
  • relative cost of different N forms.

These and other factors must all be assessed before the best fertilizer management system can be identified for an individual producer.


POST-HARVEST OPERATIONS THAT REDUCE THE SNOWTRAPPING POTENTIAL OF A STUBBLE FIELD, DRY OUT THE SEEDBED, OR DAMAGE THE WINTER WHEAT STAND MUST BE AVOIDED

Height and density of stubble determine the snowtrapping potential of a stubble field and any post harvest operation, including fertilizer banding below the soil surface, that breaks down the stubble will increase the risk of winter damage. Soil moisture in stubble fields is also often limiting for germination and establishment of winter wheat. When soil moisture is poor, a fall banding operation prior to seeding may result in further moisture loss and poor seed germination. Excessive tillage associated with fertilizer banding during the seeding operation can also create similar problems.

If banding below the soil surface is done after emergence of the winter wheat there will be damage to the stand. This will result in greater susceptibility to winterkill, delayed maturity and increased weed competition.

Spring banding below the soil surface into established winter wheat stands will result in delayed maturity and increased weed competition.

SEED PLACEMENT MUST RECEIVE PRIORITY OVER FERTILIZER PLACEMENT

Shallow seeding (approximately one inch) into a firm, moist seedbed provides optimum seed placement for stubbled-in winter wheat. Improper seed placement can result in increased winterkill, later maturity and lower yields. For example, in the severe winterkill winter of 1984-85, a difference in seeding depth of one inch (2.5 cm) compared to two inches (5 cm) often meant the difference between a crop and no crop the following spring.

Accurate seed placement is often difficult to accomplish, especially when drill openers create considerable soil disturbance and seeding is followed by a rain or other factor resulting in furrow cave-in. Straw and chaff can present obstacles to proper seed placement with the stubbling-in production system. Consequently, most successful winter wheat producers invest in the equipment necessary to provide for chopping of straw and uniform spreading of straw and chaff to facilitate their seeding operations, thereby maximizing the opportunity for uniform stand establishment and winter survival. In addition, when selecting seeding equipment, they will remember that trash clearance and seed placement should not be sacrificed for fertilizer placement.


KEEP THE WINTER WHEAT PRODUCTION SYSTEM AS SIMPLE AND STRAIGHTFORWARD AS POSSIBLE

Stubbled-in winter wheat involves seeding directly into standing stubble with a no-till drill. Phosphate fertilizer, if required, should be applied with the seed. Winter annual weeds are sprayed for in the late fall or early spring. Nitrogen fertilizer should be applied by early spring at the latest. The crop is then harvested. While only minor changes appear to be required to accommodate winter wheat on a farm geared for spring crops, many producers have run into problems inserting winter wheat into their rotations. Most western Canadian farmers are not experienced in either the production of a crop with a winter growth habit or the no-till production system. Consequently, the production scheme for stubbled-in winter wheat presents a major change in management philosophy for most farmers. Unfortunately, this production system is often too simple for modern hi-tech agriculture and many of the production problems with winter wheat can be directly attributed to a tendency to make some of these operations overly complicated.

Most of the problems that producers have with the stubbling-in production system are associated with the seeding operation. Winter wheat is seeded in late August or early September. This often results in a conflict with harvesting of late-season, spring sown crops. Time means money during harvest and winter wheat seeding. Too much experimentation or excessive labor demands at this busy period are a sure formula for disaster. Forward planning and the postponement of operations that could be completed at a later date help to reduce these conflicts ensuring priority can be given to essential production steps, such as getting the seed into the ground properly at the optimum date.

Interest in fertilizer banding drills has accompanied the growth of winter wheat. While the concept of banding urea and urea based fertilizers during the seeding operation has merit because N losses with urea are reduced, most producers have had little or no experience with banding drills. Also, compared to conventional no-till drill openers, some types of banding drill openers increase the horsepower requirement per unit width of drill by 2 to 3 times, depending upon soil characteristics. This means that drill size has to be reduced by 1/2 to 2/3's, or tractor horsepower has to be increased by 2 to 3 times when the seeding of winter wheat is accompanied by N fertilizer banding with these openers. Therefore, the advantages gained from eliminating the need for a broadcast fertilizer application and reductions in urea N losses with banding during the seeding operation must be weighed against reduced horsepower requirements and quicker seeding when ammonium nitrate is broadcast later as a separate operation.


RISK OF WINTER DAMAGE IS INCREASED WHEN N IS SEED PLACED

The response of winter wheat to seed-placed N is dependent upon the N source, row spacing and opener type. Drill row spacing and opener type determine the concentration of fertilizer in the row and immediately adjacent to the seed. For instance, moving from a 6 to 12 inch row spacing has the effect of doubling the fertilizer concentration in each row while a disc opener places the fertilizer in a narrower band than a broad hoe opener.

Urea (46-0-0) and ammonium nitrate (34-0-0) are the two most common N forms that are seed placed. Urea has become the main form of granular N and many fertilizer distributors have little interest in stocking ammonium nitrate.

Field trials have indicated that, when placed in the seed row, both urea and ammonium nitrate can reduce seedling number and size, especially when the soil is dry at seeding. In the absence of winter damage, seed placement of 34-0-0 at 30,60 and 90 lbs N/acre in 3/4 inch wide seed rows spaced 8 inches apart has produced grain yields that are 100, 86 and 70% of comparable early spring broadcast N rates, respectively. The effect of seed-placed urea is more insidious and yield performance is often significantly lower than with ammonium nitrate. Placement of urea a minimum of one inch from the seed row will minimize seedling damage.

Table 1.The effect of seed-placed ammoniun nitrate (34-0-0)fertilizer on winter survival. The seed and fertilizer were placed in 3/4 inch wide rows spaced 8 inches apart in this study.


Seed placed N
(lb/acre)
0
30
60
90
Subtract
(FSI)
0
17
34
51
FSI = Field Survival Index.

Similar grain yield responses for 30 lb N/acre seed-placed and spring broadcast ammonium nitrate indicates that ammonium nitrate can be safely seed placed at low rates (see Chapter 12 ). However, even at low rates, increased damage to winter wheat stands has been observed following high stress winters ( Table 1 ). The importance of this seed-placed N induced reduction in winter hardiness is emphasized by the following example. The reduction in winter survival potential with 30 lb/acre seed-placed N is equivalent to the difference in winter hardiness potential between Norstar (FSI=514) and Sundance (FSI=496) winter wheat cultivars (514-496=18). In other words, the winter survival advantage of Norstar over Sundance is eliminated if 30 lb/acre N is seed placed with Norstar. The use of high rates of phosphate fertilizer will not counteract the effect that seed row banded N has in reducing winter hardiness.

NITROGEN FERTILIZER MUST BE APPLIED BY EARLY SPRING TO ACHIEVE MAXIMUM N RESPONSE WITH WINTER WHEAT

Under average conditions, 90 percent of the total N accumulated by the winter wheat plant will have been taken up by the start of heading, which normally occurs near the 3rd week in June. Therefore, N fertilizer must be applied early in the season if Maximum Economic Yields are to be achieved. For maximum yield response and minimum N loss, broadcast N fertilizer should be applied as early as possible after the soil thaws in the spring. This increases the probability of subsequent rainfall moving the fertilizer N into the rooting zone before the soil N reserves become insufficient to meet the plants' N demands for healthy growth.

Yield responses to N will be small if the plants' early season demands are not met because of delays in N fertilization. A three week delay in spring N application has produced average grain yield responses that are only 69 percent of those observed when fertilizer is applied as soon as the soil thaws in the spring. Delaying N fertilizer application until early June often results in no grain yield response at all.

Poor yield responses from late May or early June N fertilization are often associated with increased grain protein percentages. However, percent protein is simply the ratio of grain protein yield to total grain yield and the higher percent protein is normally a result of the reduced grain yield response with late compared to early N application dates. Consequently, if protein premiums are available, rather than attempting to improve percent protein with late N fertilization, it is much more profitable to obtain both higher percent protein and grain yield by increasing early season N fertilizer rates.

Stranding of fertilizer N at the soil surface, due to dry weather following early spring broadcast applications, has the same effect as delaying N fertilizer application date. Late fall N fertilization avoids this problem. However, reduced grain yield and percent protein, attributed primarily to denitrification losses and immobilization, have been observed for fall applied broadcast N in regions with cool, damp weather conditions in early spring. Therefore, late fall broadcast N fertilization of dryland stubbled-in winter wheat should be restricted to the brown and dark brown soil zones where the risk of spring surface stranding are greatest and losses from denitrification are lowest.


GROWING SEASON WEATHER CONDITIONS HAVE A LARGE INFLUENCE ON WINTER WHEAT N RESPONSE

Winter wheat grain yield response to N is highest for the first pound of N added (Figure 1). After the initial increment of N, the grain yield N response gradually decreases reaching zero when maximum yield is achieved. The response becomes negative at excessively high N rates and there is a loss in grain yield for N added beyond that required for maximum yield.

The grain yield N response for each pound of added N is larger and the N requirement for maximum yield is higher when weather conditions create a high yield potential (Figure 1). This strong interdependence of N fertilizer grain yield response and weather demonstrates the difficulty there is in predicting the N fertilizer requirements for winter wheat. Because N responses are so dependent upon growing season weather, N requirement predictions are only going to be as good as our ability to predict the weather.

The influence of weather on stubbled-in winter wheat grain yield has been studied in detail in Saskatchewan. Evaporation during the two week period immediately prior to heading, root zone extractable soil water at heading and evaporation during the last two weeks in July were found to be the primary weather factors determining grain yield in these studies.

Figure 1 Figure 1. Norstar winter wheat grain yield response to total available N for six levels of drought stress. Intersect lines identify the N rates that give 1) maximum grain yield, 2) Maximum Economic Yield when wheat prices are $180 and 3) $60/tonne and N fertilizer price is $0.66/kg ($0.30/lb). Total available N = Soil test N for the surface two feet + fertilizer N.

Evaporation rates during the growing season generally increase gradually from May to July and then drop off quickly in August. On average, evaporation rates are highest in the southwest and lowest in the north and east of the agricultural region of western Canada. Consequently, maximum potential grain yield and N fertilizer requirements should increase as we move from the brown soil zone to black and grey soil zones.

The growth and development of winter wheat is normally 10 days to two weeks ahead of spring wheat and therefore coincides more favorably with the mean temperature and precipitation patterns experienced in western Canada. However, stubbled-in winter wheat is by definition a stubble crop. This makes it highly dependent upon precipitation that occurs between the harvest of the previous crop and pre-heading, the critical period for moisture availability.

Snow trapped in the standing stubble provides additional spring moisture that can be especially valuable to crop production following dry years. Consequently, moisture availability from fall rains and the snow trap can provide useful guides to decisions on N fertilizer rates. However, field studies have demonstrated that soil water reserves only contribute approximately 20 percent to the total annual water use indicating that, unless irrigation water is available, the yield potential of stubbled-in winter wheat is very dependent upon growing season rainfall. These studies also demonstrated that stubbled-in winter wheat often exhausts most of its available soil water reserves by heading. This makes later season growth even more dependent upon growing season rainfall. Because N has to be applied by early spring at the latest, this strong influence of growing season rainfall makes it extremely difficult to accurately determine N requirements for Maximum Economic Yield of stubbled-in winter wheat.


GRAIN AND FERTILIZER PRICES INFLUENCE THE N RATES AT WHICH MAXIMUM ECONOMIC YIELDS OF STUBBLED-IN WINTER WHEAT ARE ACHIEVED

The last few years have seen dramatic reductions in grain prices, especially for winter wheat where the price of #1 Canada Western Red Winter Wheat (CWRW) fell from $194/tonne in 1981/82 to $109/tonne in 1986-87 (basis Thunder Bay). Because grain yield N responses gradually decrease with increases in total available N, the largest returns are realized on the first increments of fertilizer N ( Figure 2 ). For this reason price changes do not have a straight line influence on the "break-even" (marginal cost = marginal return) N fertilizer rates and as price per tonne of wheat increases the amount of N fertilizer required to achieve Maximum Economic Yield increases ( Figure 1 ).

With most farm chemicals, such as herbicides, rates cannot be modified without compromising the performance of the chemical. Consequently, rate cutting is usually not an economic option for farmers faced with increases in the price of farm chemicals. However, not only is N fertilizer rate cutting an option, it is a requirement if Maximum Economic Yield is to be achieved following an N fertilizer price increase ( Figure 3 ).

 

Figure 2
Figure 2.
Economic return on N fertilizer ($/kg N) for three levels of drought stress (see Figure 1 ) when wheat prices are $60, 100 and 180/tonne and N fertilizer price is $0.50/kg ($0.23/lb).

Figure 3
Figure 3.
Grain yield response to total available N for six levels of drought stress. Intersect lines identify the N rates that give 1) maximum grain yield, 2) maximum economic yield when N fertilizer prices are $0.44/kg ($0.20/lb) and 3) $0.88/kg ($0.40/lb) and wheat price is $100/tonne.

HIGH N RATES ARE NECESSARY TO MAINTAIN QUALITY

Cereal protein contains approximately 17.5 percent protein. Because N is obtained from the soil, plant-available soil N also has a direct influence on grain protein yield. The ratio of grain protein yield to total grain yield determines grain protein concentration (percent protein). Consequently, the influence that N fertilizer has on this ratio determines its influence on percent grain protein.

The following general grain yield, grain protein and grain protein concentration N response patterns have been observed for winter wheat grown in Saskatchewan. There is a minimum N level for plant growth that results in a constant ratio of total grain yield to grain protein yield and a minimum grain protein concentration of approximately 8.0 percent protein. Consequently, when conditions are favorable for growth, the correction of severe N stress by the addition of fertilizer N produces a lag phase in the protein concentration N response curves ( Figure 4 ).

Once cultivar yield potential or weather factors become limiting to growth and subsequent yield increases, excess N is utilized mainly for grain protein production and the protein concentration N response curve enters an increase phase. Maximum grain yield is achieved at N rates that coincide with the end of the increase phase of the protein concentration - N response curve ( Figure 4 ).

Figure 4 Figure 4. Percent grain protein response to total available N for three levels of drought stress (see Figure 1 ).

These observations indicate that high percent grain protein can only be achieved at N fertilizer rates that are in excess of those required for Maximum Economic Yield (Figures 1, 3, and 4). Under average to good weather conditions, the maximum N requirements of the winter wheat plant can be expected to have been met when the grain protein concentration N response curve reaches approximately 12.5 percent. The protein concentration N response curve will reach a maximum near this level unless spring environmental conditions favorable for plant growth and N uptake are followed by extreme drought that severely limits grain yield. Maximum protein concentrations ranging from 14.5 percent to 20 percent have been observed for Norstar winter wheat produced under these conditions.

It is clear from the above observations that, because most stubble fields are deficient in plant-available soil N, N fertilization is also required to maintain grain protein concentration at an acceptable level. Low percent protein (less than 11 percent) is reflected in a high frequency of "piebald", "yellow berry" or "starchy kernels" in a sample. If the frequency of piebald kernels is high, a sample will be degraded to No. 3 CWRW, which usually sells for the same price as feed wheat. Therefore, grain quality can become an important consideration in determining N fertilization rates required for Maximum Economic Yield (Figures 1, 2 and 3).

Identification of the N levels required for 11 percent protein in our examples has demonstrated that both reductions in grain price and increases in fertilizer price can shift the economic N rate curves below the N levels required for 11 percent grain protein concentration (Figures 1 and 3). Consequently, market opportunities and penalties for low protein concentration, such as degrading to feed wheat prices, or premiums for high protein concentration should receive attention when determining N fertilizer requirements for Maximum Economic Yield of winter wheat.