Rangeland Ruminant Nutrition
General
A grazing ruminant selects a diet that is higher in nutrients and lower in
toxins than the average of the plant community where they are foraging.
Provide animals choice - Animals, like people, have different dietary preferences.
They also have different nutritional needs and ability to detoxify toxins. Providing
choice allows animals to meet nutritional needs and avoid detrimental levels of
toxins. Availability of plant species varies with time of year, elevation and
precipitation. Healthy rangelands are diverse eco-systems.
Match genetics with available forage. Optimum range beef production is achieved
by matching the animal’s genetic potential to the nutritional environment (both
forage quality and quantity). Large, growthy cattle that produce lots of milk may not
do well in environments with marginal forage growth or quality. Limited forage
availability is more detrimental to animals with high rather than low genetic growth
potential.
What do ruminants need to meet their nutritional requirements?
Sources of Rangeland Nutrients – Grasses, Forbs, Browse (shrubs and trees)
ENERGY
Ruminants derive energy from carbohydrates, fats and protein. In general, digestible
carbohydrates in forages are highest in the spring and decline over the growing
season. Microbes in the rumen can readily breakdown cellulose and hemicellulose in
forage early in the growing season. As forages grow and mature, carbohydrates in
forage become lignified and become less digestible but ruminants can derive
adequate energy from most forages provided animals have adequate protein,
vitamins and minerals in the diet.
Carbohydrates (Energy) – Simple sugars and starch, cellulose and hemicellulose
Fats and Oils (Energy) – Gram for gram fats provide 2.25 times the energy of
carbohydrates. Ruminants normally consume very low levels of fat.
PROTEIN
Protein content of forage changes dramatically over the growing season. In early
spring, protein levels are in excess of nutritional requirements for most ruminants.
As the season progresses, protein value declines but is still adequate to meet
nutritional needs of ruminants most of the year. In late fall and winter, protein is
often lacking in the diet because mature forage, especially mature grasses, is too low
to meet nutritional needs.
VITAMINS
Vitamins needed for survival and growth of ruminants are 1) manufactured in the
rumen by the rumen microbes (all B vitamins and vitamin K), 2) in the body from
sunlight (vitamin D), 3) are synthesized in the liver and kidney (vitamin C) or 4) are
stored in the liver or contained in adequate amounts in feed (vitamin A and E). Even
under drought conditions, grazing animals almost never require expensive vitamin
supplements or injections.
Vitamin A: Green plants don’t make vitamin A instead they make carotene, which is
converted in an animal's body to vitamin A. Green pasture, tree or shrub leaves, hay
with good green color and yellow corn have sufficient carotene for animals to
produce adequate vitamin A. Adult ruminants can store enough vitamin A in the
liver to prevent deficiency even if they don’t ingest any green plant material for over
a year. As little as 28 g/d (1 oz) of green material will meet an animal's vitamin A
requirements almost indefinitely. Signs of vitamin A deficiency are night blindness,
eye discharges, poor hair coat and weight gain.
Vitamin C: Animals synthesize vitamin C in the liver and kidneys.
Vitamin D: Animals make their own vitamin D from sunlight and fresh or dry feed.
The body also stores sufficient vitamin D to overcome dietary deficiency for several
months.
Vitamin E is contained in green plant material, hay and grain; the concentration in
grain decreases during storage. It is not stored extensively in the body
B Vitamins and Vitamin K: The microbial population in the rumen will synthesize
adequate B vitamins to meet requirements. Vitamin K also occurs in plants.
Required Minerals
Calcium
Phosphorus
Sodium
Chloride
Magnesium
Potassium
Sulfur
Micro Minerals
Copper
Zinc
Manganese
Cobalt
Iodine
Iron
Selenium
Fluorine
Molybdenum
How do ruminants digest forages?
To feed ruminants, feed their microbes. Both microbes and ruminants need all
classes of nutrients, energy, nitrogen, vitamins and minerals.
Rumen microbes digest cellulose. Ruminants by themselves cannot digest all the
compounds in forages. Some of the nutrients are in the fibrous portion of the plant
(hemicellulose and cellulose). Ruminants can utilize hemicellulose and cellulose
because of the microbes in their rumen. The microbes help to supply the energy and
protein needs of the animal by breaking down the cellulose and other complex
compounds in forage to simpler compounds. Microbes breakdown carbohydrates
and produce VFAs (volatile fatty acids) as waste products. VFAs then pass through
the rumen wall. Ruminants can use VFAs to make glucose and as energy for
metabolic cellular activity.
Microbial numbers increase with adequate dietary nitrogen and minerals.
Microbial numbers are proportional to the dietary nitrogen supplied by protein
(cottonseed meal, soybean meal, feathermeal, etc.) or urea. Microbial numbers
depend on adequate supplies of nutrients. If feed is low in nitrogen (protein) and
minerals, microbes numbers decline. Fewer microbes result in lower forage
digestibility. Protein supplementation improves the digestibility of low quality
forage by as much as 10%.
Sustained nitrogen release in forage diets is important. The rate at which
nitrogen and energy are released in the rumen affects microbe numbers. As nitrogen
is released in the rumen, adequate energy is needed to build microbial protein
(more microbial protein produced so the animal gets more protein). A sustained
release of nitrogen or amino acids from protein ensures availability of nitrogen as
the microbes work to digest forage—a process that usually takes longer than
digesting concentrates (grains).
Intake
Plant factors affecting animal intake: availability, composition of plant community,
nutrient and toxin content of plant, ratios of energy to protein in the plant, structure
of the plant.
Animal factors affecting intake: body size, body condition, rate of intake (foraging
skills and plant structure), grazing time, digestibility, levels of supplementation, and
experience with foods.
Supplementation
Energy must be supplemented on a daily basis. The amount fed determines whether
supplementation or substitution occurs. If 10% or more of the diet is fed, then
animals begin substituting forage for supplement. This may be necessary if available
forage is limiting and is insufficient to meet nutritional needs.
Protein may be supplemented as infrequently as once or twice per week. Urea may
be supplemented as a source of nitrogen but only if the animal has access to readily
available sources of carbohydrates (grain, molasses). Microbes can make protein
from urea and readily available carbohydrates.
Too much grain eaten at one time will result in acidosis (grain overload).
When an animal eats more starch than its rumen can handle, a large amount of lactic
acid is produced and rumen pH drops below 5. As a result of these conditions, the
animal goes off feed (acidosis) 2—3 days later. This could happen when ruminants
are not started on grain slowly or if feeding management is not handled properly.
The end result could be just sore feet (laminitis) or death.