I once heard a cow described as an animal with built-in locomotion, a harvesting mechanism, a fermentation vat, a calf incubator and fertilizer dispenser. I thought this was a pretty good explanation of the assets of a cow and why cattle are so effective at converting low-quality feedstuffs into high-quality proteins for human consumption.
Modern grazing livestock production practices strive to balance available forage with the nutrient requirements of the animal.
The science related to this task has been researched for many years in many parts of the world; the art comes in evaluating and integrating all of the factors related to nutrition for grazing livestock.
Let’s review a few of these considerations. I will address four that I feel are key components of this balancing act.
First: Quantity of forage
I am writing this article in mid-April; by the time you read it, the drought situation will be clearer.
Hopefully, the next month will have brought enough moisture to the rangelands of the West, Midwest and Southwestern regions of the U.S. so spring grass will have grown. Yet the prospects of this outcome are not promising.
The reason I begin with quantity is that when there is enough grass and other plants available for the animal to select from, they generally do a good job of finding high-quality plants and parts of plants to meet their needs. This is particularly true in native range or mixed-grass pastures.
For example, they will select leaves that are high quality rather than stems that tend to be lower quality.
Plant diversity in the grazing lands is also important since plants grow and mature at different times during the growing season, thus offering a longer, broader selection base for animals to meet their nutrient requirements.
Range and pasture management practices play a pivotal role in the longevity and diversity of grazing lands.
One of the primary challenges during a drought is allowing adequate rest and recovery time for pastures. Rotational or cell grazing systems have generally worked well for this; however, without sufficient rainfall, even this approach may not be enough.
Second: Quality of forage
A key question when determining quality of forage is: “Is it green or brown?” While this may sound like a simple question, the color of the forage actually goes a long way toward prediction of forage quality.
Why is this? When grazing plants are green, photosynthesis is typically occurring at an active pace. This means that soluble proteins and simple carbohydrates, like sugars, are being generated within the plant cells, making them readily available for the animal when consumed.
As the plant matures and changes from green to brown, photosynthesis slows, fewer new nutrients are being produced by the plant and existing nutrients are stored in a less available form such as cellulose, hemicelluloses and lignin, and a reduction of soluble proteins.
A higher proportion of plant proteins become bound in more complex chemical arrangements, making them difficult for the animal to utilize.
As the plant matures and prepares for seed production and dormancy, the nutrients are stored in the plant cell walls, stems and roots, rather than in the plant cells themselves.
In most cases, when grazing plants are green, and there is sufficient quantity of plants, the grazing animal can meet its nutrient needs without additional supplementation. This is particularly true for the primary nutrients of energy and protein.
However, in temperate climates, plants do not remain green yearround and undergo a dormant period during times of colder temperatures when hours of sunlight are reduced or when insufficient water reduces plant growth.
Fortunately, even brown plants contain nutrients that can provide nutrients for grazing livestock, especially for ruminants that have the built-in fermentation vat spoken of earlier.
The rumen promotes the growth of microbes that work in concert with the animal to break down the more complex carbohydrates like cellulose into glucose that is easily used by the animal as an energy source.
The microbes themselves also become a good source of protein for the host animal as they pass down the gastrointestinal tracts from the rumen.
The ability of the grazing animal to use mature forages is a key factor that allows them not only to graze high-quality plants during the growing season but also to graze dormant plants. Fall, winter and early spring grazing is comprised mostly of dormant forages.
Third: Animal type
The nutrient requirements of animals vary greatly based on age, physiological stage, and to a lesser degree, breed of animal. Several other factors affect animal nutrient requirements.
These include sex of the animal, season and temperature effects, previous nutrition reflected as compensatory gain and level of activity.
Simply put, growing animals (such as yearling steers grazing summer pastures) have different nutrient requirements than mature, non-lactating cows during early winter.
Matching the type of animal, its production stage and the quality and quantity of available forage is a key component of nutritional management of grazing livestock.
Fourth: Supplementation
Determining when the available forage does not meet the nutrient requirements of the animal is the basis for determining when and how to provide supplemental nutrition.
Experience, along with research findings from universities and other parties, provides a starting point for developing this matching process.
In most, but not all cases, protein is typically the first limiting nutrient for grazing livestock, since the energy needs are generally met by the products of fermentation of the rumen.
This is especially true when grazing brown, mature, dormant forage. When protein is deficient, fermentation in the rumen by the microbes is slowed due to insufficient substrates for adequate microbial growth and function.
Supplementing relatively small quantities of protein will frequently enable the rumen to reach a level of fermentation necessary to break down plant carbohydrates for use by the animal.
There are many options for providing supplemental protein to grazing livestock. The key factor in this process is to balance cost and benefit. There are many tools available for establishing this balance. Experience and a sharp pencil are two of these tools.
Mineral supplementation is another important part for achieving optimum production from grazing
livestock. Mineral nutrition is a complex subject and may well provide the basis for future articles.
Summary
The lead sentence in this article referred to all the “built-in” components a cow has. The fact that she is equipped to travel for grazing, harvest and ferment low-quality forage into products she can use to incubate a calf ought not be overlooked by those involved in providing high-quality products from cattle and other grazing livestock for use by mankind. 
Extension Beef Specialist
Colorado State University
