Crude protein accounts for 90% of forage N – typically measured as total N*6.25

Typical levels

• Range 3-30%
• 12-15% adequate
• 15-20% is required for lactation
• >20% for total diet can impair reproduction

80% of forage protein is Rubisco – hence protein levels are lower in C4 species

Higher in legumes than grasses

Rumen protein digestion

1. Protein is degraded to ammonia and is excreted in urine (poor N efficiency)
2. The largest proportion is converted to bacterial protein which is digested in the lower digestive tract
3. A small proportion escapes breakdown and is digested in the lower digestive tract,
   “By-pass protein”

Lower digestive tract digestion

• Protein is degraded to amino acids and is absorbed by the animal (high N efficiency)

Most forages (alfalfa, red clover and grasses) have low by-pass protein and low efficiency of absorption of N

Tannin containing legumes (Birdsfoot trefoil & sainfoin) binds to protein, increases by-pass and the efficiency of protein absorption

Some supplementary proteins have high bypass protein

note: not all tannins are the same, excess of some tannin (e.g. in wood and bark) can decrease intake

In general plants reflect the levels of soil minerals – soil mineral deficiencies appear as plant mineral deficiencies (soil or herbage testing can be used)

Animal requirements can be greater than plant requirements, so plants can grow adequately but be inadequate for livestock (e.g Cu, Mg)

some minerals are not required by plants e.g. Na, I, Co and Se

In Only in rare cases do plants have excess mineral levels for livestock (e.g mine reclamation)

Some weeds can have high mineral levels and be sought by stock (e.g. plantain, chicory, dandelion)

Example

• Mg grass tetany or hypomagnesaemia
• Caused by Mg deficiency or replacement of Mg by K or Ca
• Occurs in most ruminants but most common in cattle because of their heavy milk production (and consequent loss of Mg)
• Cows respond rapidly to Mg treatment
  

Mare Reproductive Loss Syndrome (MRLS)
initially endophyte was suspected - now is attributed to eastern tent caterpillar (the exact mechanism is uncertain)

Bloat (pg 421) – foaming of legume protein (saponins) in the rumen which prevents passage of digesta
(prevented by tannins in birdsfoot trefoil and sainfoin)

Endophyte (pg 431) – alkaloids produced by fungi in ryegrass and tall fescue

Estrogens produced by old red clover varieties (hay appears to be safe) (pg 429)

Molds and other mycotoxins in hay (pg 437)

bleeding of the nose from an
anti-coagulant that can occur
in spoiled sweet clover hay
(click to enlarge, 56kb)

• Coumarin (harmless) in sweet clover is converted to dicoumarol during heating & spoilage of hay
• Dicoumarol is the active ingredient of wafarin – an anti-coagulant used in a rat poison and heart surgery

Nitrate poisoning & Prussic acid (pg 424-429)

• Warm season grasses (sorghum sudangrass, pearl millet, corn) and in rare cases wheat and cereal rye, can have toxic levels of nitrate (>0.5-1.5%)
• Nitrate converts to nitrite and interferes with hemoglobin
• Especially regrowth after rain or following drought
• Remains in hay
• Remedy is restrict intake to 1-2 hrs of intake, dilute with other feeds

regrowth of sorghum sudangrass can have levels of nitrates toxic to livestock
(click to enlarge, 31kb)

old varieties of
reed canargygrass
can produce alkaloids
which are harmful to
livestock
(click to enlarge, 56kb)

Reed canarygrass alkaloids (pg 435)

• Reduced palatability, diarrhea, death
  especially if grazed as a pure stand
• Remedy - dilution with hay or other species
  - shorten the grazing period
  - new low-alkaloid varieties