Knotroot foxtail: A new bermudagrass challenge

The author is a professor and extension weed management specialist at the University of Tennessee.

To successfully compete in the high value, small bale market, producers must be in a position to produce virtually weed-free bermudagrass hay for their customers, many of whom are horse owners. Horse owners are willing to pay a premium for clean hay and they are becoming more selective buyers.

The registration of Pastora (nicosulfuron + metsulfuron) has greatly improved the ability of producers to effectively manage many grass and broadleaved weeds in bermudagrass hayfields. In particular, it has proven effective on troublesome annual grass weeds such as barnyardgrass, fall panicum, broadleaf signalgrass, and annual foxtails. Foxtail control is critically important in production of horse hay in that the seed head bristles can cause serious problems with mouth ulcers in horses.

A perennial foxtail

We became keenly aware of this problem a few years ago when we started receiving reports of Pastora failures on foxtail in Tennessee and other states in the Mid-South. After investigating these reports, we found that the majority of cases of insufficient foxtail control were in hayfields infested with knotroot foxtail, a perennial species, rather than annual foxtails. Horse owners are becoming increasingly aware of this problem and many avoid purchasing hay that contains foxtail seed heads. Because of this, hay infested with knotroot foxtail seed heads is often harvested as large round bales for the cattle market rather than small bales for the horse market.

In Tennessee, clean hay in small bales averages over $200 per ton, whereas large round bales sell for approximately half that amount. Clearly, this weed presents a serious economic challenge for many producers.

We have conducted research during the past three years to identify effective herbicide options for controlling knotroot foxtail in bermudagrass hayfields. While no herbicides tested produced anything approaching complete control, the most effective option for suppression of knotroot foxtail (reduction or elimination of seed heads in harvested hay) was Pastora plus glyphosate at first cutting followed by a second application of Pastora alone two to three weeks later.

This program will cause substantial bermudagrass injury but it will recover. Injury can be reduced if the first herbicide application can be made within seven days following hay harvest.

While on farm visits in 2013 and 2014, we observed that there appeared to be a relationship between nitrogen (N) and potassium (K) applications and severity of knotroot foxtail infestations. Fields where Pastora was applied that were fertilized with recommended N and K rates and timings appeared to be less severely infested than those where Pastora was applied but were receiving low N and K.

Accordingly, we conducted research in 2014 to investigate this apparent relationship and a possible interaction of fertility management and Pastora plus glyphosate applications on knotroot foxtail biomass and seed head production. High N and K rates resulted in a lower knotroot foxtail seed head density at the second harvest as compared to low N and K rates. No herbicide by fertility interaction was observed. As expected, the effect of herbicide was stronger than that of fertility.

Take home message

Intensive management of bermudagrass hayfields includes following recommended N and K rates and timings, combined with a properly timed, sequential herbicide program is critical in high-quality bermudagrass hay production where knotroot foxtail is present.

Likewise, producers are strongly encouraged to scout fields often in order to detect knotroot foxtail invasion. This will allow spot treatment before the entire field is infested, and this will pay big dividends down the road. •

This article appeared in the August/September issue of Hay & Forage Grower on page 28.

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