A crop’s rooting depth is a major factor influencing its water-use efficiency.
One of the most important aspects of forage production is rarely ever seen. In fact, it is literally the root of the issue. Roots are not given much thought until drought or other stresses strike. With proper management, root growth and development can be optimized and the impact of stresses will be minimized.
Nearly every producer can list “the drought years.” For example, the drought of 2011 will be permanently seared in the minds of many Southern forage growers. In most areas, however, some amount of drought stress occurs each year. A crucial forage management principle is to insulate one’s production system from that risk.
During a drought, forage crops can exhibit stress tolerance by being more efficient with the available water; they produce relatively high amounts of dry matter (DM) for every acre-inch of rainfall received. In fact, the table above shows our most common perennial forages are typically very efficient at using water. This is critical when the region is frequently subjected to drought stress or is dependent upon irrigation.
A crop’s rooting depth is a major factor influencing its water-use efficiency. The rooting depths reported in the table are impressive. But these values come from plots that had deep soils, excellent fertility and were rarely cut or grazed. With the exception of sites with shallow bedrock or impenetrable layers, rooting depths can still be optimized in less-than-ideal situations using proper management.
Maintaining soil pH within recommended ranges fosters a well-developed root system, ensuring that plant nutrients are readily available and that other elements don’t become toxic. For example, maintaining a rather neutral soil pH prevents aluminum (Al) from becoming dissolved in the soil moisture. Soluble Al is toxic to the plant and a common cause of poor root development. Neutralizing the soil pH causes the Al to return to a solid form. In some cases, like when subsoils become extremely acidic, it may be necessary to add gypsum. Although gypsum doesn’t alter the soil pH, it can infiltrate the soil profile and reduce the solubility of Al.
Cutting and grazing management also affects a forage crop’s root system. When forage is cut or grazed, its root growth stops and some roots begin dying back. This allows plants to redirect those carbohydrate and protein reserves toward foliage regrowth. If the forage is quickly cut or grazed again, roots will die back even more. Eventually, this repeated defoliation will result in root systems that essentially mirror topgrowth. When drought stress becomes severe, these areas are the first to stop growing, turn brown and go dormant or die.
In pastures that are continuously stocked (i.e., no rotation or rest period), areas become preferentially grazed and individual plants may be grazed several times in the same week.
What effects such frequent defoliation can have on forage crops is demonstrated in the photo. University of Florida Extension soil scientist Cheryl Mackoviak had grown bahiagrass in 3’ lengths of PVC pipe and clipped them every two, seven or 21 days for a couple of months leading up to a recent field day. Note how well-developed the root system was when harvested relatively infrequently.
Allowing for adequate rest ensures that root systems become deep and well-developed. Many similar field and grazing experiments have established how much rest is required by plants to regenerate their root systems.
But it is not an exact matter of days. Growing conditions vary and the rate of topgrowth influences the rate at which roots redevelop. Most university Extension services and conservation districts provide recommended rest periods for forage species in their regions.
As plans are made for the upcoming growing season, take a few minutes to consider how current management is affecting root system development. If there is a chronic problem, get at the root of it.