Don’t Break Up that Old Pasture Yet

Cathie Erichsen Arychuk, P.Ag.
Bison Production Specialist
AAFRD, Fairview


Forage production on many pastures starts to drop after a few years.  Soil fertility levels are important in maintaining long term forage production and preventing ‘sod-bound’ stands.

Don’t Break Up that Old Pasture Yet

After three to five years in production, forage production in many tame forage stands starts to drop off.  Traditionally, these old, ‘sod-bound’ stands are worked up, and reseeded to increase production.  For a couple of years after seeding, forage yields are exceptional because of the nutrient release from breaking up the old sod.  However, this apparent gain is short-lived.  Over time, the field reverts to its original state and has to be broken again. Is this practise economically sustainable?  In east central Alberta, many forage stands are seeded on marginal land, not well suited to annual crop production.  Many of these fields are susceptible to erosion when the soil is disturbed.  The cost of breaking up and reseeding a forage stand is fairly significant.  Is there a better way to maintain forage productivity?

Recent work done by the Agriculture Canada Research Centre in Lacombe, Alberta Agriculture and Western Co-Operative Fertilizers Limited may provide another option.  Their work suggests  the application of fertilizer can increase both the annual production from a forage stand, and the productive life span of tame forages.

Traditionally, forages receive almost no added fertilizer.  This is particularly true for the drier part of the province, the Brown and Dark Brown soil zones.  New research shows that adding nutrients to forage stands will accomplish as much as breaking up the stand.  It also costs less.  Research in the Dark Brown soil zone found the potential to increase tame forage yields by ½ to 1 ton per acre.  In areas receiving more moisture, yield increases are even greater.

Large amounts of nitrogen (N), phosphorus (P), potassium (K), and sulphur (S) are required for high forage yields.  When forages are harvested as hay or silage, these nutrients are removed from the field, and are not returned to the soil.  Several years of forage production can seriously deplete a soil’s ability to nourish a crop.  Few soils can supply adequate nitrogen or phosphorus to meet a forage crop’s full nutrient demand.  Although potassium and sulphur deficiencies are less common, they are often observed on poor quality land and on soils depleted by successive cropping with limited nutrient additions.

Most soils require the addition of nitrogen to obtain  profitable yields from grass stands.  Soil nitrogen is slowly released during the crop year by mineralization, but this process provides only a fraction of the nitrogen most crops require.  Available soil nitrogen is high following summer fallow, but is rapidly used up by a forage crop.  Most years, available stored soil nitrogen and nitrogen mineralized during the growing season do not meet the crop’s nitrogen needs.  Nitrogen fertilizer is needed to make up the difference.    Grasses require fairly large nitrogen application rates to maximize yield and protein levels.  The research indicates that economical N rates in the brown and dark brown soil zones range from 30 to 90 pounds per acre, for grass stands with up to 20% legumes.  Nitrogen rates for grass-legume mixtures drop as the percentage of legumes in the mix increases.  Straight legume stands should not require nitrogen fertilizer, although added N will help stretch the productive life span of old stands a couple more years.  These amounts generally should be applied annually.  In a dry year the higher rates may provide a residual effect into the next year.

Phosphorus is also very important to forage crops.  Most soils cannot supply enough phosphorus to meet the requirements of a high yielding forage crop.  Adequate phosphorus will increase yields, reduce disease, and extend stand life.   Both grasses and legumes require P, with legumes needing more  than grasses.  Fertilizer recommendations for forage grasses in the brown and dark brown soil zone are 15 to 40 pounds P2O5 per acre annually.  For legumes and mixed stands, use 20 to 60 pounds per acre.  Phosphorus applications to mixed stands can cause a shift in the plant mixture in favor of the legumes.  P seems to give legumes a competitive advantage over grasses.

Deficiencies are most common on sandy soils, but they are becoming more widespread as soil nutrient levels are depleted by crop removal.  A soil test is the best way to decide if your field will need additional potassium.  Forage legumes are more responsive to K than grasses.  Legumes will often show a yield response to added K even when soil test levels are in the medium to high range.  Potassium affects protein content, winter survival, and stand longevity in legumes.  In mixed stands, addition of K does not seem to affect stand composition significantly.  If a deficiency exists, grass stands respond well to 30 to 60 pounds K2O per acre.  Legumes may need 40 to 100 pounds K2O per acre.

Forage crops use large amounts of sulphur, and deficient soils are unable to supply adequate amounts to the crop.  Not all soils are deficient in sulphur.  Soils with low organic matter, or sandy soils are more likely to require added S.  A soil test is the best way to determine if your field is deficient in Sulphur.  Forage grasses do not require large amounts of sulphur.  Forage legumes do need sulphur for maximum yields.    If soil tests indicate a sulphur deficiency, apply 20 to 40 pounds per acre of sulphur for legume production.

Recent research suggests that the low productivity from old, sod-bound forage stands may be mainly due to a lack in nutrients.  Breaking up the stand will increase nutrient availability by increasing the rate of mineralization from the soil.  This will increase nitrogen and phosphorus level, at least in the short term.  However, it may be more economical to improve forage stand productivity and increase stand life by applying fertilizer  to provide nutrients.  Both tame grasses, and legumes use large amounts of both nitrogen and phosphorus.  Even in the dry soil zones of east central Alberta, forages will respond to added nutrients with increased production and longer stand life.

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