UREA HYDROLYSIS MODEL HELPS PREDICT N LOSSES

As more producers reduce tillage on their fields, concern about the loss of preplant nitrogen increases. In the past anhydrous ammonia application covered more acres and since anhydrous is knifed in, nitrogen losses were not a concern. More acres are being covered by either urea or UAN solutions. Without incorporation nitrogen loss is a potential. The loss mechanism is that urea is broken apart by an enzyme, urease that is found in soils and on crop residue. There is a commercially available urease inhibitor that retards the function of this enzyme and conserves the urea. The idea is to conserve the urea long enough for a rain, or tillage to move the urea into the soil. Cooperative Extension publishes an Extension Circular (EC96-144-C): “Fertilizer Management For Conservation Tillage,” that explains this process and gives the potential loss rates under various scenarios of temperature, pH and incorporation.

The EC does not help predict loss under specific conditions found in a field. The company, Agrotain International LLC that makes a commercially available urease inhibitor has developed a computer program that models the loss potential from a field fertilized with urea or UAN solution. The model is based on Bulletin Y-206, Ammonia volatilization from urea fertilizers, edited by B.R. Bock and D.E. Kissel (1988). The model is available on the Internet at: http://agrotain.com/calculator/calc2.html

Once downloaded and installed the program is fairly straightforward and easy to use. The user needs to provide 24 pieces of information, but most of the needed information can be estimated using knowledge about the field. For example, soil moisture is estimated as wet, medium, dry, very dry. The program, based on the inputted texture and the estimated moisture calculates, the actual inches of moisture. The output is given in lbs of nitrogen lost and percent N applied lost. There is also an estimate of the yield effect of this nitrogen loss.

While I have not been able to find the actual equations used in the calculations, and the documentation for the scientific basis of the model are not given, the model seems to conform to the generally recognized principles given in the Bock and Kissel publication.

In response to a producer question, I ran several scenarios for nitrogen loss. The table below shows the output for the specifics of this field. The field is in northeast Nebraska and has a pH of 7.6. I varied the initial soil moisture condition and the wind speed and had the model predict losses over 10 days. The situation presented contains a worse case scenario since the producer wanted to know the range of loss potential. The results show that starting with dry surface soil, the warm daily temperatures, and high winds did not cause much nitrogen loss. As the soil surface became wetter the losses increased. Interestingly, with the wind speed reduced from 20 MPH to 10 MPH the nitrogen losses were actually higher (25 vs 24 lbs N/acre). The model does not explain these results, but it is likely that the slower winds increased the time the soil surface stayed wet and therefore the urease acted for a longer period causing slightly more nitrogen loss.

The value of using a model that has interaction effects in it is that the cumulative effect of several factors can be estimated. As with the wind example above, the explanation may not be apparent initially. Using this model with the known weather conditions after nitrogen is applied will allow producers to make informed decisions about whether more nitrogen may need to be applied later in the season. (CS)

Volatilization Losses from Agrotain calculator:
Initial soil conditions:		Wet	Medium	Very Dry		Wet	Medium	Very Dry
soil moisture (in)		1.0	0.5	0		1.0	0.5	0
Wind Speed: (Mph)		20	20	20		10	10	10
Day Max Temp: F		80	80	80		80	80	80
Evening Temp: F		50	50	50		50	50	50
Texture:		Sil Clay L	Sil Clay L	Sil Clay L		Sil Clay L	Sil Clay L	Sil Clay L
Tillage:		Minimum	Minimum	Minimum		Minimum	Minimum	Minimum
Residue:		40%	40%	40%		40%	40%	40%
N form:		UAN	UAN	UAN		UAN	UAN	UAN
N applied: (lbs/acre)		195	195	195		195	195	195
Days to rain:		14	14	14		14	14	14
Total N loss: (lbs/acre)		38	24	0		29	25	0
Percent loss: (%)		19.7	12.4	0		15.3	13.2	0

INSECTS CAN INFLUENCE THE TIMING OF WEED MANAGEMENT IN SOYBEANS

Insects and weeds are common pests that can cause major expenses to crop producers. In order to help producers make reasonable judgments concerning pest control and pesticide use, scientists developed a concept of i ntegrated pest management. Basic philosophy of this concept is to utilize a combination of cultural, mechanical, biological, genetic and chemical methods for effective and economical pest control. In most crop production fields there are many species of weeds and insects with different life cycles and survival mechanisms, and it is not likely that they can be managed by a single control measure. In reality, weeds and insects interact and affect not only each other, and the crop, but also the efficacy of their respective management tactics. For example, insect induced defoliation can significantly delay soybean canopy development, which in turn provides more sunlight for weeds to grow and compete with the crop, directly affecting the subsequent weed management plans. Learning how the insects and weeds interact with each other and the crop is essential in developing IPM strategies.

Therefore, we are conducting a study of the interactions between weeds, insects, and soybean. The main objective of the study is to de termine the critical time for weed removal as influenced by the three levels of simulated insect defoliation (0%, 30%, and 60%). This study was done in the summer of 2003 and will be repeated in 2004 as part of the MS thesis for Travis Gustafson, as a collaborative project between the weed science (myself) and entomology programs (Tom Hunt).

Preliminary data suggest that the insect damage to the soybean leaf area indeed resulted in a need for earlier weed management. For example, with no insect damage to the soybean canopy, weeds could remain in the crop up to the V4 stage (third trifoliate) or about 20 days after crop emergence without significantly affecting the yields. However, at the 30% and 60% defoliation level, weeds should be removed by the V3 (17 days) and V1 (10 days), respectively.

From a practical standpoint, this indicates that soybeans with 30-60% of insect damage do actually have a shorter weed control window and potentially less weed control options. Soybean leaf damage by for example bean leaf beetles, affects not only the final yield but it also affects the timing of when weed control needs to be initiated in the growing season in order to prevent further yield losses. This research also shows that the producer may now have another tool to fight weeds in his soybean field – that being a good insecticide. If there is a particularly bad infestation of bean leaf beetle, spraying an insecticide to control the bean leaf beetle may actually widen the herbicide application window and increase weed control options for the producer. Funding for this project is provided by the Nebraska Soybean Board. (SK)

CUTTING HEALTHY ALFALFA EARLY OFFERS ADVANTAGES

The first alfalfa cutting often is the most important cutting of the year. It usually produces the most yield and its forage quality changes fastest from day to day. Many growers plan to cut soon after first blooms appear. But weather can cause long delays and sometimes alfalfa doesn't bloom very aggressively during spring. Plus, waiting until alfalfa begins to bloom often results in hay that is too low in quality for dairy use.

So what about cutting before plants bloom — or even before they form buds? Is this a realistic option? Being ready to cut healthy, vigorously growing alfalfa after it gets about fifteen inches tall has several advantages. Weather might be better than later in spring. You begin the harvest sequence early rather than waiting until all the alfalfa is ready at once. Some insect and disease problems can be reduced by early harvest. Most importantly, feed value can be very high. Plus, second cutting probably will be ready before summer heat lowers its forage quality.

One disadvantage, however, is that yield will be lower from this early cut, although much of it will be made up in later harvests. Also, regrowth for second harvest probably will be slower than if alfalfa had been cut at a more advanced stage of growth, especially if your alfalfa experienced winter injury this year. And you must be sure to allow a longer than normal recovery after either the first or the second cutting if you want to maintain long-term stands. Bruce Anderson Extension Forage Specialist