Crop Advisory - June 13, 2005

Figure 1 Diagram showing Heat Unit accumulation from planting date in relation to cotton growth stage

Heat Units

a) Yuma valley:

Heat unit (HU) accumulated since Jan 1 = 1824, last year for the same period 2051. HU accumulated since January are running 7 days behind normal. Last week HU were running 7 days behind normal

b) Roll

Heat unit (HU) accumulated since Jan 1 = 1819, last year for the same period= 2029. HU accumulated since January are running 2 days behind normal. Last week HU were running 2 days behind normal

Estimated water use last week

Planting Date 2/1 2/15 3/1 3/15 4/1

Location Water use, inches

Yuma 1.80 1.69 1.56 1.34 1.12

Roll 1.88 1.78 1.64 1.44 1.20

Agronomy

Cotton development is progressing toward its most active stage. Early planting reached squaring stage. In general, cotton planted earlier is doing well, but cotton that was planted late is still growing behind schedule. At this stage, under the current weather conditions, the management of cotton fields will impact the final results. Therefore, field work should focus on plant nutrition, irrigation management, pest control conducive to good balance between vegetative growth and fruit set by observing cotton height, number of nodes and height/node ratio.

Research conducted by the University of Arizona showed relationship between heat unit accumulated since planting and the number of nodes. Based on that information the number of nodes per plant of unstressed cotton should be as follow for the selected planting date and the corresponding accumulated heat units.

Table planting date and expected number of node per cotton plant in Yuma and Roll areas, Yuma County, 2005

Planting date Nodes for Yuma Nodes for Roll

01-Feb 17 17

15-Feb 17 17

01-Mar 16 16

15-Mar 15 15

01-Feb 13 14

For additional information visit Arizona web site http://ag.arizona.edu/crops or call Cooperative Extension 928 726 3904

1) Plant Nutrition

The assumption that vegetable crops leave sufficient residual nutrient to sustain cotton growth during the early stage or even the entire season may or may not be true. If you planted cotton after produce, it is recommended to test soil and /or plant nitrogen level and make management decisions accordingly. The application of nitrogen (side dressed or water run) should be decided based on petiole tissue testing.

The accuracy of field nitrogen value depends very much on the quality of sampling strategies and sample handling. Most soil and plant laboratories are equipped with modern instrumentation capable of running a large number of samples with minimum error margin. Therefore, the true value of your field sample depends on the quality of sampling and sample handling. Inspect your cotton field uniformity and design sampling strategies. Before entering the field we need to make a sketch that divides the field into uniform and practical size that could be managed when making fertilizer adjustment. Samples from each zone can be collected as cluster, Zigzag, or random sampling.

1) if the cotton field is uniform then randomly collect several petiole samples, thoroughly mixed to form a composite sample This composite sample should be oven dried at 150 F for 24 hours. Fresh petiole could be sent immediately to laboratory for analysis if packaged in a disposable ice chest. In any case sample should not be left in the office or truck for several days.

2) If the cotton field is divided into sections with different levels of nitrogen levels and each section is big enough to be managed separately with nitrogen adjustment then samples should be collected from each section as described in case of uniform field.

How many samples per field are needed? Fewer samples may under or overestimate the real nutrient value of soil or plant level of the cotton field. A larger number of samples may cost you unnecessary charge.

Based on petiole nitrate nitrogen (NO3-N) guidelines developed for Arizona upland cotton (Silvertooth), plant tissue between 15000 to 20000 PPM NO3-N is considered adequate at this stage.

A rapid and inexpensive method of determining nitrate status in cotton plant is often used based on Sap nitrate (NO3) or Nitrate Nitrogen (NO3-N). Sap nitrate nitrogen can be determined using a specific nitrate electrode developed by Horiba (Cardy meter). Retail price is less than $400 for one time purchase. Sensors are replaced periodically based on the use. Sap nitrate nitrogen can also be determined on the field, at the office or at home with a fraction of the cost charged by commercial laboratories. The method is quick, easy and relatively accurate. Notice that Sap nitrogen is lower than dry petiole samples. Calibration is therefore necessary. It could be determined using the equation:

Y=10.21*x-1007

Y= dry petiole NO3, X= sap nitrate

This method is widely used in vegetable industry and cotton production. If you have any question about purchasing, calibrating and interpreting results using the SAP nitrate determination method, please contact your extension agent.

2) Insect control. Cotton field planted next to current cucurbit fields are more exposed to whitefly infestation. Scouting these fields allows early detection and treatment decisions to avoid any damage. For more information on pest evaluation visit web site http://ag.arizona.edu/crops, or call Mohammed Zerkoune at 726 3904, Zerkoune@ag.arizona.edu.