Time Interval Needed Before Rain To Maximize Weed Control

Rainfall shortly after application often reduces weed control from postemergence herbicides, because the herbicide can be washed from the leaf surface. Herbicides vary in absorption rate and ease of being washed off the leaf.  The rainfall effect can also vary depending on rainfall amount and intensity. The approximate time between application and rainfall needed to maximize weed control is given in the table below.

Rain Free Period Needed After Herbicide Application Herbicide Time Between Herbicide Application and Rainfall Aim 1 hour Ally 4 hours Assert 3 hours Avenge 6 hours Banvel 6-8 hours Buctril 1 hour Bronate` 1 hour Cheyenne 4 hours Curtail 6-8 hours Curtail M 6-8 hours Express 4 hours Harmony Extra 4 hours Hoelen 1 hour Roundup 6 hours Puma 4 hours Starane 4 hours Stampede Cm 4 hours 2,4-D or MCPA     amine 4 hours 2,4-D or MCPA     ester 1 hour Source: University of Minnesota Extension Service

Estimating Grain Yield  
Grain yield can be estimated prior to harvest at several stages during the development of the crop. The earliest possible time to make an estimate is when stem elongation has started.  During stem elongation, the growing point develops what ultimately will become the spike.  At this point, the number of spikelets can be determined by counting the number of so-called “double ridges.” The daily maximum temperature just prior to jointing and during jointing heavily influences the number of spikelets that will form.  Depending on the variety, the number of spikelets can decrease by as much as 33% if the daily maximum temperatures increase from 60 to 90 °F during that period.

To roughly determine grain yield early, multiply the number of spikelet formations times 3 for wheat or times 6 for barley, if the stand is close to optimum. This estimate is not very reliable as conditions later in the growing season will determine how many kernels will form in each spikelet. As well, the weight of each kernel also will change, depending on the growing conditions during grain fill.

A more accurate estimate of grain yield can be made when the crop is fully headed.  The general formula to estimate grain yield is:

Wheat: Grain yield (bu/acre) = (kernels per spike x spikes per 3 ft row) x 0.0319

Barley:  Grain yield (bu/acre) = (kernels per spike x spikes per 3 ft row) x 0.0389

Oats:  Grain yield (bu/acre) = (kernels per spike x spikes per 3 ft row) x 0.0504

The formulas are for 7” row spacing and three feet of row. To adjust for other row spacing, multiply the estimated grain yield with the factor in the table below to calculate the correct grain yield estimate.

Factors to Correct Grain Yield Estimates for Different Row Spacing Spacing Factor 6” 1.17 10” 0.70 12” 0.58 Source: University of Minnesota Extension Service