Why?
What are some common species combinations?
Factors affecting the legume balance competition for N Animal selectivity Grazing Management competition for light P and patchiness Seasonality and climate

Why are we interested in legumes?

	Legumes fix N
	Complementary growth patterns – being more active in summer and less active than grasses in spring/fall, results in a more uniform pasture growth pattern
	Greater forage quality

What are some representative species combinations?

  Factors affecting the legume balance

	Competition for N
	Selective grazing
	Grazing Management
	Competition for light
	Seasonality (temperature optimum)
	Legume growth pattern – conducive to a patchy distribution
	Annual cycles – climate?

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  Competition for N

Legumes have an advantage for N uptake – due to rhizobia fixing atmospheric N The 6-12 month delay in the pathway for N transfer from legume to grass results in cycles of the relative balance between these species Cycling through manure and urine Death and decay of roots, nodules Death and decay of leaves and stems Direct plant-plant transfer and through soil are trivial At low N, clover has a natural advantage compared to grasses, and will be more abundant The legume population increases over a period of several years - fixing nitrogen and building the soil N status The improved soil N status shifts the relative advantage back to the grasses Because of the delay in N availability the legume population “overshoots” the equilibrium thus resulting in a natural cycle in the population on a 5-7 year pattern

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Animal selectivity

Animals are selective and have a higher amount of legume in their diet than in pasture. due. Cattle and goats are less selective than sheep – sheep pasture will have less legume What’s the reason for this selectivity? Higher quality of legumes Nutritional basis – each component of the animal diet provides something required by the animals (protein vs energy) Related to sward structure – clover grows where animals like to graze

Grazing Management

	The proportion of white clover is increased by frequent intense grazing (light quality and plasticity of growth) The opposite for alfalfa Most legumes (white clover, red clover and alfalfa) are sensitive to treading and will be lost in high traffic areas. There will a tendency for high stocking rates to lower legume population

  Competition for light

It was once considered that clover could be “shaded-out” by grass. This has largely been discounted. Many legumes (red clover and alfalfa) are tall and thus competitive for light White clover can adapt its petiole length to lift its leaves high in a dense canopy to compete for light. Clover leaves are more horizontal than grasses and have better light harvesting characteristics. Clover leaves have a larger ratio of lamina:stem than grasses (0.5 vs 0.7)

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P and patchiness

	In general grasses have an advantage for P uptake – due to their larger and finer root system Under grazing, P is redistributed in manure and N is redistributed in urine. Clover and grass have a patchy distribution in pasture and this may be driven by the relative patchiness of nutrients (and reinforced by grazing and plant growth)

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Seasonality and Climate

	The higher temperature optimum of most legumes than C3 grasses results in their relative greater summer growth than grasses (and slower spring/fall growth) – this results in seasonal variation in legume abundance Legumes and grasses vary in their drought response – so annual variation in legume amount may be related to events from previous years

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