Forage Crops

HCS412 Forage Crops
Forage Physiology - Chapter 4 (pg 73-97)

Introduction

Carbon cycle

C3 vs C4 plants

Farm Management

	Physiology Greek "phusis" = nature + "ology" = study
	the study of plant function
	the mechanistic basis by which plant processes occur
	a knowledge of plant physiology allows us to understand how forages respond to management and their environment
	nutrient uptake, drought tolerance, cold tolerance, disease resistance and photosynthesis all have a physiological basis

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Carbon cycle

	Energy is "trapped" and transported as soluble sugars (glucose, fructose, sucrose)	The carbon cycle click to enlarge (120kb)
	Energy is stored in the chains of carbon skeletons of organic molecules (fructans and starch)
	Energy balance of forages is synonymous with carbon balance
	The carbon cycle in forages is the basis of plant and animal production.
	Major carbon forms · Sugars (glucose, fructose, sucrose + others) · Starch/fructans · Hemicellulose/pectin · Cellulose · Lignin Total non-structural carbohydrates (TNC) = sugars+starch/fructan = all TNC are completely animal digestible

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C3 vs C4 (Fig 4.2)

C3 Forages

C4 Forages

characteristics

grow better in cool seasons, lower temperature optimum (20 C)
photorespiration - reduces the efficiency of photosynthesis
extend the grazing season in spring and fall
high protein content due to the abundance of Rubisco (typically 15-30%)
can be planted earlier in spring than C4's
higer digestibility by livestock
higher nutrient status than C4's (P, K, Mg, S)

characteristics

higher WUE - use less water for equal plant size
tolerate higher heat, higher temperature optimum (30 C)
fill summer slump
lower protein - typically <10%
higher fiber content than C3's
typically have a later planting date

C3 examples

alfalfa
all legumes (red clover, white clover, crimson clover, birdsfoot trefoil)
tall fescue
orchard grass
ryegrass
timothy
smooth bromegrass
Kentucky bluegrass
all dicotyledonous weed species
brassica
sugar beet
cereal grains (oats, wheat, triticale, rye)

C4 examples

corn
big bluestem
switchgrass
bermudagrass
summergrass
sorghum sudangrass
summer annual grass weeds are typically C4

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Implications for Farm Management

Photosynthesis is maximized by a leafy crop with uniform and complete ground cover. Gross photosynthesis will be maximized at canopy closure, and net photosynthesis may decrease if senescence of shaded leaves gets too high.

Crop quality is maximized by the highest concentration of animal available carbohydrates, and the minimum amount of fibre.

following winter - initial plant growth is from root reserves (carbohydrates stored in the crowns, stolons, rhizomes and roots of plants). Defoliation during this phase will deplete these reserves and weaken the stand. The forage crop will take several months to reach a positive energy balance

for an establishing pasture, it takes time for the crop to build root reserves (energy reserves for regrowth after cutting/grazing). Defoliation during this phase can deplete these reserves and weaken the stand.

In fall, plants will be translocating carbohydrates below ground as reserves for subsequent regrowth. The standard recommendation for alfalfa is to allow 4-6 weeks between the last harvest (August/Sept) and the 1st killing frost

After any defoliation (grazing/cutting) plants will use stored carbohydrates for regrowth. It will take 2-4 weeks for the crop to rebuild its reserves. Repeated defoliation will weaken plants and stands.