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  • Lancaster / Wi
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How Does Grass Grow?

This title may seem a bit elementary, but likely fitting for those advancing the art of grazing and those looking to harvest high quality forages. We believe that if everyone understood plant growth, harvesting forages would be done differently, with a much higher success rate for the farmer, forage, and animal.


Plants, whether grass, legume, deciduous, or coniferous, grow the same way.They of course look quite different; trees, for example, have woody stems due to secondary thickening of the cell walls, while alfalfa does the same as it matures, but to a lesser degree. Photosynthesis is the common denominator.


The purpose of leaves and green stems is to capture sunlight, starting a series of photochemical reactions. These reactions cause the water molecule (H2O) to be split and ultimately combined with carbon dioxide (CO2) from the air (via stomata) to create sugars, carbohydrates, and with the addition of nitrogen from the air (via roots) protein. So the energy is higher in the top part of the plant (due to the sun), and the protein is higher closer to the ground (due to nitrogen uptake by the roots).

Photosynthetic Reactions

Another ongoing process within plants is called respiration or the "dark" reactions. This is how plants maintain their metabolic needs. During respiration, the plant takes oxygen (O2) from the air and uses it, just as humans do, to metabolize sugars and carbohydrates. During times of darkness, plants utilize just enough from their reserves to sustain metabolism until the sun shines again allowing replenishment of reserves. The overall per day rate of respiration (energy expended) is normally less than the energy gained by photosynthesis or the "light reactions" so the plant does not have a negative energy balance.

Photosynthesis and respiration are controlled by factors other than just light and dark. The rate at which "light" and "dark" photochemical reactions are carried out will be determined by the slowest step or rate limiting factor.

  1. Sunlight.

    Sunlight is obviously quite important for "photo"synthesis. We all know the importance of day length upon the varieties of forages planted in our particular geographical area. Several things also determine the availability of sunlight to the plant. Of the total solar energy falling on the leaf, 60% represents unabsorbed wavelengths, 8% is reflected or passes through the leaf, 8% is lost through heat dissipation, and only the remaining 24% is actually utilized by the plant. Of that 24%, 19% covers metabolic expenditures and 5% goes towards making carbohydrates.
  1. Temperature.

    Plants can photosynthesize in a broad range of temperatures from 0 oC in alpine areas to 50 oC in Death Valley (USA). There is a different optimum temperature for all plant species with decreased rates of CO2 fixation below or above the optimum temperature.
  1. Water.

    On average, 75% of the plant is composed of water (thus water's importance!). A deficiency of water can be devastating to plant growth and maintenance.
  1. Air Filtration.

    Plants not only breathe through their leaves but must also breathe through their roots. Gases exchange through the roots, as well as the leaves, emphasizing the importance of good soil aeration.
  1. Mineral Availability.

    Mineral availability is no different than in an animal. The rules of pH and excess/deficiency still apply. Energy increases towards the top of the plant, protein increases towards the ground. This makes sense because all nitrogen comes through the roots (for more information on nitrogen, check out our Nitrates page).



Plant Cell Diagram

If any of the above 5 factors are in excess or deficient, photosynthesis and/or respiration will be limited to some degree. Of course daily and weekly changes occur in weather patterns, etc. so plants actually store many items just in case of a deficiency. It's a little hard to keep the sun on hand, but many minerals can be stored with the vacuole of the cell waiting for a sunny day or a rain. Whether plant or animal, metabolic requirements will always be met before growth or production requirements.