Linda Benedict, Morgan, Donna S., Gurie, Jeffrey A., Gentry, Glen T.
Donna Morgan, Jeff Gurie and Glen T. Gentry
A large percentage of reported sources of water quality impairments in Louisiana are related to what is collectively known as nonpoint-source pollution. This source of pollution is a result of stormwater runoff from land uses including agricultural fields, construction, urban areas, forestry and any other source that can’t be directly correlated with the impairment.
A large percentage of reported sources of water quality impairments in Louisiana are related to what is collectively known as nonpoint-source pollution. This source of pollution is a result of stormwater runoff from land uses including agricultural fields, construction, urban areas, forestry and any other source that can’t be directly correlated with the impairment. As reported by the Louisiana Department of Environmental Quality, only 32 of the 1,074 reported impairments were determined to be related to point-source activities. In that report, beef cattle forage systems, such as managed grazing and rangeland grazing, negatively affected 12 rivers across the state. Soil type, topography, excessive rainfall events, and more than 10,000 named water bodies can make Louisiana uniquely vulnerable to impacts from agricultural runoff.
Cattle are ruminants and require large amounts of forage as a source of protein and energy. For cattle producers, providing this forage is more easily accomplished during the warmer months due to the availability of summer perennial pasture grasses such as bermudagrass, dallasgrass and bahiagrass. However, most warm-season grasses begin to lose nutritive value as they mature in late summer into early fall and become dormant by first frost. Therefore, winter forages play a key role in most livestock grazing systems found in Louisiana and the Southeastern United States. Annual ryegrass (Lolium multiflorum) is by far the most popular accounting for an estimated 2.5 million acres of planted winter forage in the U.S. each year. If managed correctly a good stand can provide 3-5 tons of dry matter per acre of high quality roughage extending the grazing season and meeting nutritional requirements during a critical time of year.
Ryegrass planting methods vary, depending on factors such as available resources, location and conditions, but may prove to have an effect on the success of a winter grazing program. Conventional planting methods require soil disturbance and may cause possible erosion and nutrient loss, but may facilitate seed emergence and stand establishment sooner than other methods. However, because of increasing concern over environmental impacts and water quality, conservation tillage is now a widely used means to protect soils from erosion and compaction, while reducing production costs. Additionally, winter-planted forages may also help to maximize soil protection during the heavier, seasonal rainfall months.
Soil tillage and erosion
Soil tillage involves the physical disturbance of the upper soil layers for a variety of purposes, including seedbed preparation, weed control, and incorporation and mixing of crop residues, fertilizers or other amendments. Tillage methods vary widely depending on climate and soil type, crop management objectives, and availability of technology, tradition, and the personal preference of farmers. These methods may include conservation tillage such as no-till and minimum till. Conservation tillage is an agricultural practice that reduces soil erosion and water runoff, increases soil water retention, and reduces soil degradation. By definition, conservation tillage is any tillage and planting system that leaves 30 percent or more of the soil surface covered by crop residues after planting. No-till leaves 50-100 percent of the soil surface covered from harvest to planting, depending on the crop residue. Until the mid-1940’s, conventional tillage was historically the method of choice for seed bed preparation and weed control. It was during WWII that the first herbicide, 2,4-D, was used, and this changed the way farmers managed their crops. Over time, new technologies also proved that conventional tillage reduced organic matter and increased erosion dramatically.
Planting methods for winter forages may include overseeding, no-till drilling and aerating the soil. Overseeding and no-till drilling are common planting methods in cool-season forages, and both minimize soil disturbance. Overseeding, or sod seeding, is a practice that approximately 65 percent of Louisiana cattle and forage producers use. Though typically the most economical and practical, the overseeding method does not always ensure good seed to soil contact. Once grasses have emerged, they also may be competing for moisture and nutrients with already established forages. Prepared seedbed requires cultivation and, thereby, exposes topsoil to rainfall and other environmental conditions. Drill seeding (no-till drilling) is not a common practice by Louisiana forage producers mainly because of the high costs of equipment. Drill seeding allows a pre-determined amount of seed to be accurately placed, while minimizing soil disturbance and possibly the amount of sediment that runs off during rainfall events.
Determining environmental impacts from fields, regardless of cropping system, is critical in reducing agriculture’s contribution to nonpoint-source pollution. To help determine the impact planting methods may have on water quality, a study was conducted at the LSU AgCenter Dean Lee Research Station in Alexandria from Oct. 8, 2013, to May 8, 2014 (Year 1) and from Sept. 25, 2014, to April 27, 2015 (Year 2). The goal was to evaluate the effect of winter forage planting methods on nutrient runoff and forage production.
Ryegrass planting methods were compared, and tillage treatments included: 1) prepared seedbed; 2) overseeded; 3) no-till drilled; and 4) unplanted/untreated/ungrazed control. All plots were treated pre-plant with 1 quart per acre of glyphosate to reduce competition from summer annuals. Each plot received 175 pounds diammonium phosphate per acre and 90 pounds ammonium sulfate per acre, according to soil test recommendations at the beginning of the study. Plots were then grazed with 16 mature cows for four to six hours, once forage reached an average height of 8-10 inches, to reduce forage height to 2 inches. Prior to grazing, forage samples for dry matter determination were collected.
Additional post-grazing urea applications were applied at 30 pounds nitrogen per acre throughout the grazing season. Composite water samples were collected and then analyzed for total solids (sediment), total phosphorus, phosphates, potassium and nitrates. In addition to sediment and nutrient runoff data, forage growth and forage production on a dry matter basis were recorded. Forage samples were collected prior to each grazing, with sample weights and dry matter calculations recorded.
Sample collections in both years began shortly after planting, following initial fertilizer applications in late September and early October. Although 153 rain events were documented throughout the study for both seasons, only 29 resulted in sufficient volume to cause runoff. The total rainfall amount for Year 1 was 21 inches and for Year 2, 36 inches. The average volume of each rainfall event across years was different. In Year 1 the average was 0.68 and in Year 2, 0.94 inches (Figure 1).
Effects on Water Quality
Results show the type of planting method did not affect concentrations of total solids, total phosphorus, phosphates or potassium in the runoff. However, there were higher concentrations of nitrates in runoff from the prepared plot compared with the untreated, but concentrations of nitrates from the prepared plot were not different from the other planting methods (Figure 2). This may, in fact, be related to the number of post-grazing applications of urea placed on the prepared plot compared with other planting methods, where 100 percent of all prepared plots were top-dressed four times, 50 percent of drilled were top-dressed four times and 50 percent three times, and the overseeded plots were top-dressed only three times. However, over 80 percent of the total nitrates collected during the study ran off during the first 66 days when forage mass was minimal, and almost all nutrient and sediment runoff showed different runoff patterns over time (Figure 3).
Runoff patterns of these substances may be different from year to year and month to month depending on environmental conditions and the substance itself. Regardless of treatment, results show that more total solids and nitrates ran off all the plots in Year 2, while more total phosphorus and phosphates left the field in Year 1. Potassium showed no difference from year to year. Rainfall events were heavier in Year 2 compared with Year 1, indicating that increased concentrations of total solids and nitrates in runoff may be correlated with volume of rainfall. Because nitrates were applied multiple times to all plots post-grazing across the grazing season, with the exception of untreated, the finding that more nitrates ran off in a year with heavier rainfall events is not surprising. Likewise, because phosphates are applied only once at the beginning of the grazing season, the expectation is that runoff would not be correlated with volume of rainfall across the season but only to rain events that occur early in the season.
Effects on Forage Production
Forage production results from this study show the planting method did not affect the average yields of dry matter, which were 5106, 4750 and 5808 pounds per acre pre-graze for sod-seeded, drilled and conventional, respectively. At each grazing, average pounds of dry matter per acre and average plant height were not different across treatments. However, the average number of days after grazing was lower for the prepared plot compared with the sod-seeded and drilled plots, indicating that the average day of each grazing occurred earlier in the grazing season. Figure 4 illustrates the grazing interval across planting methods and shows that it is lower for the prepared and drill-seeded plots compared with the sod-seeded plot. These findings show that ryegrass drilled or planted by prepared seedbed had the ability to recovery from grazing sooner than if planted by sod seeding. This scenario allowed the prepared plots to be grazed an average of five times over the two-year study while the drilled and sod-seeded plots were grazed four times.
After two years of very different environmental conditions and rainfall, there was considerable variability within the treatments or replications for both years. The first year of the study included very little rainfall early in the season, snow and freezing temperatures, and heavy rainfall in the spring of 2014. However, the second year encompassed more evenly distributed, though heavier, rainfall events and milder winter temperatures. Regardless of the variation, there was little difference in runoff across treatments, with the exception of nitrates from the prepared plots. The prepared plot was the most productive of methods evaluated with a shorter grazing interval allowing more grazing than other methods.
The data collected are not conclusive enough to state that these specific conservation tillage treatments do not have any beneficial effects on water quality, nor do they negatively impact them. Slope, soil type, rainfall intensity, time between rainfall events, grazing and nutrient uptake can all have an effect on edge-of-field runoff and, therefore, it is difficult to make broad recommendations based on this study. To that end, the objective of this study was to evaluate conservation tillage and conventional tillage planting methods and their effect on water quality and forage production. The data show that even though sod-seeding and drill-seeding are considered conservation practices and may reduce sediment and nutrient runoff in some situations, there was little difference on nutrients or sediment concentrations in collected runoff, while the prepared plot offered more grazing opportunities. Therefore, decisions on the ryegrass planting method used by a producer must be made on a case-by-case basis, keeping in mind topography and forage management needs.
Donna Morgan is an associate area agent for the Louisiana Master Farmer Program and county agent for Rapides and Grant parishes; Jeff Gurie is a research associate and livestock farm manager for the Dean Lee Research Station in Alexandria; Glen T. Gentry is an associate professor and interim coordinator at the Bob R Jones–Idlewild Research Station in Clinton. Special thanks are given to J Stevens, associate professor and soil fertility specialist, for his support and expertise.
The summer 2015 issue of Louisiana Agriculture magazine includes articles on a variety of topics that affect Louisiana’s agriculture industry and the environment – water management at Catahoula Lake, 4-H youth wetland programs, artificial reefs for water conservation, corn nitrogen management in saturated soil conditions, and more. 36 pages