Glyphosate resistant ryegrass has been a persistent problem since appearing in almond orchards in the Sacramento Valley in the late 1990’s.  It is a winter annual weed, but under irrigation it can germinate any time of the year.  This year round germination pattern wasn’t noticeable in the Sacramento Valley when it was effectively controlled by glyphosate.  Summer and early fall germinating ryegrass plants are particularly difficult to control at the typical November to December herbicide timings because they are larger established plants at the time of application.

Pre-emergence herbicides are a good way to deal with this extended germination period.  June/July pre-emergence herbicide treatments with irrigation for incorporation were not effective in previous trials in several different locations.

A new herbicide, Alion, with longer lasting residual control than some of the other commonly used pre-emergence herbicides was evaluated in a large scale field trial.  Herbicides were applied in March and April 2012.  Perennial weeds and summer escapes from all treatments were treated with post-emergence herbicides to avoid the need for mowing weeds to prepare the orchard floor for harvest.  The orchard middle was most of the area treated since that is where most of the weeds were located.  Individual plots were 12 ft. wide to 150 ft. to 300 ft. long, with six replications.  A summary of the results is in the following table (MAT= months after last treatment, rounded to an integer):

Why was the weed control so long?  Bayer representatives state 5 plus months control is expected.  This control lasted 10 plus months.  It could just be statistical variation, where five months plus is the shorter time typical of most orchards, or are there other explanations?

This 12 ft. by 5,000 ft. long trial area had no mowing all season.  The floating of the middles after harvest was also skipped.  Floating of orchard floors looks like ideal seedbed preparation for small seeded weeds.  Could keeping all soil disturbances out of the pre-emergence treated tree row extend the effectiveness of a longer acting pre-emergence herbicide like Alion?  With Alion use, does the soil right in line with the tree row (least likely to be disturbed) have longer lasting weed control?

Good spring, summer, and fall weed control (10 plus months) is a valuable goal worthy of some additional research.  This trial may very well have been a statistical outlier, but if it wasn’t, it could be a very useful tool for glyphosate resistant ryegrass and other weeds.

Picture taken January 28, 2013.  Treated area is in middle from 3 ft. to 15 ft. left of tree row.  Control plot weeds seen in distance 150 ft. from sign.