Today is the fourth in a series of posts outlining current University of California weed science research published in the California Weed Science Society (CWSS) Research Update and News (June 2013, Volume 9, #2).

Steve Fennimore (UC Davis, Cooperative Extension Specialist, stationed in Salinas) works mostly in cool season vegetables, strawberries, and other annual cropping systems.  Some of the crops he works in, like lettuce, have very few chemical weed management options and he spends a lot of his time working on developing chemical and non-chemical (tillage, steam, etc) weed management for these very high value, low acreage crops.

In this paper, Steve discusses some of his research with the herbicide Prowl H2O (pendimethalin) in transplanted lettuce.  In transplanted lettuce, he reports that the crop is very tolerant of this herbicide and should be explored further.  He  also shared some data on seeded lettuce as well but that does not look like a good option for these growers!

This report begins on page 5 of the CWSS Research Update

Brad

Prowl H2O a potential new herbicide for transplanted lettuce

by Steve Fennimore, UC Davis Extension Specialist

The lettuce herbicides available for California producers have not changed much since the 1970s. The last significant lettuce herbicide registration was Kerb, registered in 1972 – 41 years ago. Much of course has changed since 1972 – the higher costs of fuel, labor and land just to name a few. Additionally the costs and risks for registration of new herbicide active ingredients are much higher now than they were 41 years ago. Consumer preferences have changed too in the past 41 years. In 1972 head lettuce was the dominant lettuce type, now romaine lettuce is the more popular lettuce type. In 2009, a major setback occurred when the Kerb registration was lost on leaf lettuce. From 2009 until the present Kerb is only registered for use on head lettuce and is no longer labeled for use on leaf lettuce. As an aside the Kerb registrant is attempting to register that product on leaf lettuce, but that issue remains unresolved.

In 2012 pendimethalin (Prowl H2O) was granted a food use tolerance for lettuce. Prowl H2O does not have a lettuce label yet and it will be up to the registrant to make a final decision on whether to label or not. However, we have quite a bit of support data for Prowl H2O use on lettuce and the purpose of this article is to review what we know.

Kerb is applied post plant preemergence where lettuce is planted by seed and then Kerb is spray applied to the soil surface. The unique aspect of Kerb is that it is very safe to seeded lettuce. In contrast, Prowl H2O (pendimethalin) applied post plant preemergence (PRE) is very injurious to emerging lettuce seedlings. In 2005 we screened Prowl H2O at 0.75 lb ai/A PRE on head and romaine lettuce at Salinas, CA. The results were very clear; Prowl H2O is…. click here for the rest of the article and data