The root of the problem: Is it disease, nutrients or weeds? -

Too often that thought process extends beyond the confines of the cultural landscape and into our forages. It just needs to rain, or stop raining, right? However, the root of the problem goes much deeper, and the solution might not have anything to do with the three essential nutrients (hydrogen, oxygen and carbon) plants need to stay alive. Let’s take a look at the roots of an alfalfa cool-season grass mix versus those found in rangeland and the thought process that goes into renovating or replanting an ailing stand.

Melissa A. Bravo

Livestock Herd Health Extension Agent / Rutgers Cooperative Extension

Email Melissa A. Bravo

Taproots

Plants and weeds that have a long tapering taproot have the ability to tap into soil nutrients and water stored deep beneath the plow or planter line. So, while the shallow-rooted grasses beneath that oak tree are obviously suffering from lack of water, the oak is not. That’s because her taproot goes beyond the 18-inch depth of her lateral roots. The taproot of an alfalfa plant, also a long-lived perennial under ideal conditions, can reach depths of 5 feet the first summer of planting. Two years later, that same root can extend 12 feet into the soil profile.

It’s logical to assume that any precision excavation that severs the taproot below the laterals is a death sentence. So in instances where utility disturbances are expected, it’s a safe bet that replanting is necessary. What is more difficult to perceive is whether or not the taproot has been damaged from disease.

Root rot

Phytophthora, aphanomyces and brown rot are examples of fungal diseases that attack the taproot of alfalfa plants. If these pathogens are responsible for the loss of your stand, renovation by planting into the same stand with another deep-rooted alfalfa is not recommended unless there is a known variety with resistance to that particular pathogen.

Cultivars resistant to these root rots and other diseases are listed in the “Alfalfa Variety Ratings, Winter Survival, Fall Dormancy & Pest Resistance Ratings” booklet on the National Alfalfa & Forage Alliance website. On the other hand, incorporating shallow-rooted legumes (clovers and trefoils) and adding more grasses into a failing alfalfa stand is a renovation option as long as seed-to-soil contact is achieved. But remember, if the existing grass sward is sufficient to fill in the gaps, a “let it be” approach just might be the best solution of all.

Nutritional deficiencies in your burnt up, burnt out forage roots are more easily addressed by renovation. Plants, after all, need 16 essential nutrients – and yes, water is one of them. But the three craved the most by plants like alfalfa and alfalfa-grass mixes are nitrogen, phosphorous and potassium. We know alfalfa roots can fix nitrogen, but in an ailing system, that might not be the case. The nitrogen-fixing nodules might have drowned or succumbed to disease. The nutrient removal by roots from the soil of as-fed alfalfa is quite high because the harvesting mechanism (in this case, the cow) is removing all of the above ground biomass repeatedly throughout the growing season.

So, if a formulated ration that provides 2 to 3 pounds average daily gain contains 6 pounds of as-fed alfalfa (33 percent legume) along with 18 pounds of corn (67 percent grass), you can transpose that need into a grazing system and the alfalfa portion of that ration equivalent needs upwards of 200 to 300 pounds of nitrogen during the grazing season. Tack on another 300 pounds or so of potassium and 50 pounds of available phosphorus, and it’s not hard to figure out you’ll need to supplement your stand with fertilizer to optimize and maintain season-long root health.

Don’t go overboard on the micros though. While boron, calcium, chloride, copper, iron, magnesium, manganese, molybdenum, sulfur and zinc are also needed to sustain a healthy alfalfa plant, we’re only talking a few pounds per acre per year. Too much micronutrients added and livestock toxicities can occur.

Undesired roots

While nutritional deficiencies can be addressed to improve any type of grazing system, invasive species complicate restoration efforts considerably. It’s not generally feasible to replant a million acres – oh, I mean, 20 million acres of infested grazing lands.

Joseph M. DiTomaso, professor of weed science at University of California – Davis, noted in the 2000 Weed Science Society of America invasive weeds of rangeland publication that there are more than 300 rangeland weeds in the U.S., with the most problematic listed as Bromus tectorum (cheat grass), Euphorbia esula (spurge) and several centaurea species (knapweeds).

While cheat grass is at least edible, it’s only a godsend in early spring when protein is high. After that it’s a fire hazard and a hindrance to edible forage growth. But removing cheat grass to allow desired range species to proliferate means dealing with a fibrous root system with lateral roots concentrated in the top foot of soil. It’s that well anchored, finely divided root system that makes it such a fierce competitor for available soil moisture.

Leafy spurge, on the other hand, has a tremendous root system that is part taproot, part fibrous root, and while not a true rhizomatous system, it’s certainly clonal in nature. The U.S. Forest Service summarized research on the vegetative nature of leafy spurge roots that revealed spurge roots can reach depths of 8 feet and even 15 feet over time. The enormous storage capacity of these types of weedy root systems means restoration will be a challenge.

So, what can you do to get to the root of the problem? Don’t dig too deep, feed or seed, and above all, don’t let those aliens get all the water!