An important tool that is available to supply nitrogen to cool season pastures is the addition of clovers. During earlier periods of inexpensive nitrogen fertilizer, clovers were considered a valuable addition to dilute infected tall fescue, improve forage quality and also supply nitrogen. More recently, the cost of commercial nitrogen has rearranged the priority list with clover’s ability to fix nitrogen perhaps being the most economically important.
Nitrogen is “fixed” in clovers through a symbiotic relationship with rhizobium bacteria that infect roots. The plant provides energy for the bacteria and bacteria provide the “machinery” necessary to convert atmospheric nitrogen to a form available to plants. Most people picture a ‘conduit’ that transports nitrogen directly from clover to grass. Unfortunately, almost no nitrogen is contributed in this mode. Essentially, nitrogen is supplied to grasses indirectly via the decomposition of the clover root nodules. Nitrogen must then be converted into a form available to plants. This conversion or ‘mineralization’ releases nitrogen slowly- more like a time release fertilizer than an application of ammonium nitrate or urea.
After perennial clovers are well established, nitrogen will be released to grasses at a relatively constant rate as nodules decompose. White clover can fix 50-125 pounds of nitrogen per year and red clover can fix 75-150 pounds depending on stand, soil and growing conditions. At current urea prices this translates to $25-$75 per acre in added nitrogen.
When clover makes up less than 15% of the stand, it may also be beneficial to fertilize with nitrogen. Under these conditions, clover is contributing little nitrogen to the system and overall forage production could be increased with nitrogen addition. Clover leaves and stems contain approximately 5-6% nitrogen by weight. This nitrogen can be made available to grasses through animal urine and manure. Grazing activity recycles a large amount of nitrogen to the pasture. However, when grazing distribution is uneven, nitrogen may be concentrated around water sources or shade where animals congregate.
Successfully adding clovers can be accomplished by broadcast seeding during the winter months (frost seeding). A study conducted at the Kentland research farm by Dr. Ben Tracy in 2009 compared frost seeding and no-till planting of clover. The pastures consisted of mostly tall fescue, bluegrass and some orchardgrass. Clover had not been sown into experimental pastures in recent years. Before establishment of seeding treatments, each pasture was heavily grazed by cattle to remove standing dead vegetation. Pastures were then fertilized with phosphorus (P) and potassium (K) as recommended by soil test. Soil pH was above 6.5 on all pastures so no lime was added. On February 4, 2009, one half each pasture was sown with a mixture of red (Juliet), ladino white (Pinnacle) and white clover (Kopu II) using a broadcast seeder. On March 10th, the remaining half of each pasture was planted with the same legume mixture using a no-till drill. The seeding rate was 4, 2 and 2 lbs/ac. for red, ladino and white clover, respectively. Table 1 contains the percent ground cover of clover from April – August.
Percent ground cover occupied by clovers during 2009 growing season. No significant statistical differences were found between seeding methods in any month.
April June August
Frost-seed Drill Frost-seed Drill Frost-seed Drill
Red clover 1 1 8 12 14 13
White clovers* 3 3 28 17 26 20
Total 4 4 36 28 40 33
*White clovers included both sown Ladino and white clovers as they could not be visually separated
The clover component in pastures increased more than 20 fold from April to August 2009. Broadcast frost-seeding and no-till drilling were equally effective for establishing clover. The scientist credited the successful clover establishment to a combination of factors: 1) heavy, mob grazing in winter that reduced standing dead vegetation and helped seedling emergence, 2) timely frost-seeding during 1st week February, 3) aggressive rotational grazing in spring that simultaneously reduced grass competition and allowed clovers to grow enough to establish, 4) good soil fertility (adequate P, K and pH) to stimulate clover growth, and 5) rainfall, which was abundant during the 2009 growing season. The exact factors that determine successful clover establishment still remain elusive; as there are probably combinations of events involved. The good news is that producers can control most of these variables through management (e.g., seeding time, stocking rate) and, by doing so, should increase their chances of successful clover establishment in permanent pasture. For more details refer to VCE publication No-Till Seeding of Forage Grasses and Legumes (http://pubs.ext.vt.edu/418/418-007/418-007.html). The addition of clover is an economical method to increase the productivity and quality of fescue pastures.
Source : Dr. Mark A. McCann, Extension Animal Scientist, VA Tech