M

Recording-2025-09-17T14:31:49.181Z

Photosynthesis and carbon fixation (C3 vs C4)

  • Plants fix carbon dioxide and water into sugars via photosynthesis; two main pathways are C3 and C4 photosynthesis.
  • The number in the pathway name refers to the number of carbon atoms in the first molecule formed: C3 has 3 carbons, C4 has 4 carbons.
  • Cool-season grasses use C3 photosynthesis; warm-season grasses use C4 photosynthesis.
  • This difference in carbon fixation pathways is linked to how plants adapt to environmental conditions (light, temperature, water availability) and helps explain seasonal growth patterns.

Seasonal growth patterns: cool-season vs warm-season grasses

  • Warm-season grasses begin growth when temperatures rise; cool-season grasses begin in fall or early spring and grow most during the coolest months.
  • Temperature trigger for warm-season grasses (example in this area): Bermuda grass starts growing after five consecutive nights of 60^\u00b0\text{F} or warmer weather.
  • Frost tolerance and light requirements:
    • Warm-season grasses typically have lower frost tolerance and higher light/temperature requirements.
    • Cool-season grasses tolerate frost, low light, and lower temperatures.
  • Water needs:
    • Warm-season grasses generally require less water than cool-season grasses.
  • Daylight effects (Canada example): longer daylight in summer and shorter daylight in winter influence growth; in some areas, winter pastures are minimal (Fall Rye mentioned as an example in colder climates).
  • Seasonal pasture context for this area:
    • Warm-season grasses listed include bahiagrass, Bermuda grass, dallisgrass, pearl willow, sorghums, crabgrass, and native grasses.

Warm-season grasses in this area: key species and traits

  • Bahia grass (bahiagrass)
    • Seed head described as a pill-like head with dark seeds; a light-hearted aside about taste.
    • Grows on a wide range of soils; not very drought-tolerant; greens up earlier in spring and stays greener later in fall.
    • Strengths: thick sod reduces weed incursions; tolerates continuous grazing; moderate dry matter yield on low fertility soils; established by seeds (seed establishment can be slow); recommended in higher rainfall areas east of I-35.
    • Weaknesses/cons: drought tolerance is limited; fast growth can lead to maintenance issues in yards (mowing and seed-head spread).
    • Management note: can be sprayed to selectively kill Bahia without harming Bermuda.
  • Bermuda grass (Cynodon spp.)
    • Most common grass in this area; spreads via rhizomes (underground stems) and stolons (aboveground stems).
    • Highly adaptable to a wide range of soils and climate; limited cold tolerance; early cultivars and hybrids improved.
    • Generally, Coastal Bermuda is the most widely planted in the South/Texas; very productive with good drought tolerance and tolerance to heavy grazing; long-lived and responsive to irrigation and fertilization.
    • Coastal characteristics: excellent longevity, good drought tolerance, tolerant of heavy grazing; good for varieties with broad climate tolerance.
    • Tifton 85: a hybrid (South African x Tifton 68) with longer stems and fewer rhizomes than Coastal; superior forage quality.
    • Digestibility: ext{DM} yields and digestibility are higher than Coastal; estimates: about ext{26 ext{ ext%}} more dry matter and ext{11 ext{ ext%}} more digestible; animal gains are approximately ext{
      ≈ 25 ext{ ext%}} better on Tifton 85 hay.
    • Planting and establishment methods:
    • Sprigging: using rhizomes and/or stolons; requires specialized equipment (sprigging machine, accumulator) to plant live pieces into the ground.
    • Planting tops: planting only the above-ground green tops; historically done by baling green tops and spreading them, then discing and packing.
    • Modern practices use sprigging machines that disseminate sprigs or tops and push them into the ground, followed by packing.
    • Alternatives included discing or box blade methods to create windrows for hand-piling seed/stems.
    • Other Bermuda varieties discussed:
    • Tifton 44: cross-hardy plant from Germany; dry matter yield and disease resistance similar to Coastal; stronger nutritive value and greater cold tolerance than Coastal.
    • Jigs (Jigs Bermuda): bred for tops; better performance on poorly drained and tighter soils than other Bermudagrasses; more common in South Louisiana due to wet soils.
    • Alicia: good nutritive value, very pretty hay but less winter-hardy; susceptible to rust; slower nutrient value; often used for aesthetically pleasing hay.
    • Dallis grass: robust, tall (broad leaves), similar DM yields to Coastal; good ground cover; spring recovery may be slower; susceptible to rust.
    • Coast Cross One: Georgia origin; taller, broader leaves; more disease-resistant; rapid stolons but few rhizomes; higher crude protein and digestibility; not as winter-tolerant as Coastal.
    • Tifton 78: rhizomes; less cold tolerance than Coastal; very adaptable to southern areas.
      -