Bradley C. Venuto, Edward K. Twidwell and Jerry D. Ward
Annual ryegrass forage is grown on approximately 300,000 acres in Louisiana each year. It is planted over the entire state on widely diverse soils. Significant variation in ryegrass performance occurs among these diverse production areas, and reduced forage yields on some soils can limit the benefit of ryegrass for livestock producers. In addition, plant nutrient uptake is an important component of forage diet quality and is greatly affected by soil type and soil mineral status. The objectives of this study were to 1) evaluate variation in soil nutrient status among a wide range of soils where ryegrass is commonly grown, 2) evaluate relative dry matter yield production on these soils and 3) compare mineral uptake and relative livestock nutritional value of ryegrass grown on these soils.
Eight soils from annual ryegrass production sites were collected from throughout Louisiana with the aid of the Louisiana Cooperative Extension Service parish agents. The soils and the regions were: Ben Hur (Sharkey clay), Crowley (Vidrine silt-loam), Jennings (Crowley silt loam), Jonesboro (Bowie silt loam), Lafayette (Coteau silt loam), Many (Latonia silt loam), Rosepine (Guyton silt loam) and Ruston (Sacul- Bowie silt loam). Each soil was analyzed for pH, organic matter and macro and micro nutrients at the LSU Agricultural Center’s Soil Testing Laboratory on campus.
Greenhouse studies were designed to compare two annual ryegrass varieties, Gulf and Rio, grown on each soil for at least two months. The first greenhouse study was done in the spring of 1997, and the second was completed in the fall of 1997. Soils were completely unamended and received no nutrient applications during the studies.
Five ryegrass plants were established in each of seven containers for each soil type. The first test was seeded Feb. 16, 1997, and the second test was seeded Sept. 8, 1997. Dry matter yields were taken for ryegrass harvested from each container and averaged to provide yield values for each replication. Each test was harvested twice: test 1 on April 10, 1997, and May 16, 1997, and test 2 on Oct. 17, 1997, and Nov. 17, 1997. A composite of harvested plant material from all four replications for each variety and each soil type was made at each harvest and used for plant tissue analysis. Plant samples were analyzed for mineral content at the Forage Quality Laboratory at the Southeast Research Station in Franklinton.
The average values for yield, soil pH, organic matter and nutrient analysis for the eight soils are presented in Table 1. Yield was negatively correlated with potassium and pH, but it was positively correlated with sodium and aluminum. These results contrast with conventional wisdom and other research. The discrepancies are primarily the result of the Crowley soil, which had the highest aluminum and sodium content and the lowest pH. This soil, however, produced more than 1.5 times as much forage as the next closest soil. This indicates that some soil types in Louisiana deserve more research. It should be noted that these soils had no fertilizer added to them during this experiment, and more research is needed to determine the effects of fertilization on these soil types.
Significant differences in ryegrass yield performance and nutrient uptake were observed among soils, but there was no overall soil by variety interaction. Contrary to expectations, yields were positively correlated with aluminum levels and negatively correlated with pH. Mineral concentration in the harvested ryegrass varied significantly among soils but not between varieties. This research has implications for future efforts to develop site specific management strategies and to identify or develop varieties that perform well under less than ideal soil conditions.
The average mineral content of the ryegrass varieties grown on the eight soil types along with the recommended mineral concentrations for a lactating dairy and lactating beef cow diet are shown in Figures 1 and 2. None of the soils produced ryegrass with sufficient calcium to meet the requirements of a dairy cow, even though several of the soils had adequate to high soil levels of calcium. Two of the soils could not meet the calcium requirement of beef cows; one soil produced ryegrass that barely met the requirements of a beef cow. All of the soils had adequate magnesium for beef cows but only half provided adequate magnesium for dairy cows. All soils produced forage that met the requirement for zinc of beef cows but only six provided adequate zinc for dairy cattle. None of the soils produced ryegrass that contained enough copper to meet the requirements of beef or dairy cattle. All soils produced ryegrass with enough sodium and manganese to meet the requirements of both beef and dairy cattle, but there were large variations among the forages grown on the eight soils.
The differing amounts of mineral uptake by ryegrass among these eight soils would have profound effects on the required mineral supplementation programs needed while grazing these forages. The differences would have more of an impact for beef cattle grazing only ryegrass without mineral supplementation. Ryegrass grown on Jennings soil, however, would need to be supplemented only with trace mineralized salt to provide adequate trace minerals, assuming adequate phosphorus content.
Phosphorus uptake was not measured in this experiment because of the small sample of ryegrass available for analysis. Even though mineral analysis indicates adequate manganese and zinc content of the ryegrass produced on Jennings soil, some zinc and manganese should be added to the mineral supplement to provide a safety net. It is not well understood exactly how much of the trace minerals found in forages are available for use by ruminants. Research is under way in the Louisiana Agricultural Experiment Station to answer that question.
The differences in mineral concentration in ryegrass would have less impact on the mineral supplementation programs of dairy cattle because ryegrass rarely contributes more than half the intake of dairy cattle and frequently provides less than 10 percent of the diet, especially in once-a-day grazing systems. The results of this study indicate that forage mineral content can be highly variable. Therefore, the mineral content of the ryegrass should be taken into account to make sure mineral deficiencies, excesses or imbalances do not occur in the dairy ration.
Bradley C. Venuto, Assistant Professor, Department of Agronomy, LSU Agricultural Center, Baton Rouge, La.; Edward K. Twidwell, Extension Forage Specialist, LSU Agricultural Center, Baton Rouge, La.; and Jerry D. Ward, Assistant Professor, Southeast Research Station, Franklinton, La.
(This article was published in the winter 1999 issue of Louisiana Agriculture.)