This is an excerpt from information provided by the Beef Cattle Research Council of Canada, so naturally it’s focused on cattle. But the descriptions of energy and protein and the definitions of measurements of nutritional value cover all forages and ruminants. You’ll find this helpful no matter what livestock you raise.
Cattle require five key nutrients: energy, protein, water, minerals and vitamins
Energy is necessary for maintenance (feed digestion, core body functions, and activity requirements) and to support growth, lactation, and reproduction1. It accounts for the largest proportion of feed costs and is the nutrient required by cattle in the largest amount.
The components of feed that determine its energy content include carbohydrates, fats and proteins. On a feed test, energy content is usually expressed as total digestible nutrients (TDN); however, more precise terms such as metabolizable energy (ME) or net energy (NE) for maintenance (NEm) or production (NEg) may be preferred by nutritionists. These terms better reflect the amount of energy from feed that contributes to animal productivity. Energy deficiency caused by low intake or poor feed quality will limit growth, decrease milk production, reduce body condition, and (depending on timing and duration) may have negative consequences for reproduction.
Gross energy (GE) is the total amount of energy in the feed. But not all this energy is available to the animal. Feed energy is lost as it passes through the animal and is excreted as feces, urine, various gases, and heat. These losses are a normal consequence of feed digestion and the amount of energy lost at each step differs based on the quality of the feed. Digestible energy (DE) provides an indication of the portion of energy that the animal can digest, with the help of the rumen microbes. Metabolizable energy (ME) is the amount of energy available to the animal for metabolism and body functions after losses in energy from rumen fermentation (carbon dioxide, methane) and urine have been accounted for. Net energy (NE) is the amount that is available to the animal to maintain itself, grow, produce milk and reproduce.
Neutral detergent fibre (NDF) and acid detergent fibre (ADF) are indicators of the amount of fiber in a forage. Higher values indicate poorer digestibility and voluntary intake may be reduced.
NDF is a measure of the “bulkiness” of the diet and is mainly hemicellulose, cellulose, and lignin which make up cell walls and provide structure. Due to limitations in the analysis it also includes a portion of the protein and insoluble ash in the plant. When NDF increases, animals consume less.
ADF measures cellulose and lignin and is an indication of digestibility and energy intake. When plants mature, lignin content increases, resulting in higher ADF and reduced digestibility. Feeds high in ADF are less digestible than those high in starches and sugars. The starches and sugars in feed are classified as non-structural carbohydrates (NSC). Even in forages, NSC are an important source of energy.
Protein is required for maintenance, growth, lactation and reproduction. It is a component of muscles, the nervous system and connective tissue1.
Protein requirements depend on cattle/animal age, growth rate, pregnancy and lactation status. Young, growing cattle (and other animals), as well as those in late pregnancy or lactation, have increased protein requirements.
Most protein that ruminants ingest is broken down by the rumen microorganisms and resynthesized as microbial protein. Forages contain crude protein (CP) in two forms. The greatest portion of protein in forage is referred to as true protein, but forages also contain low amounts of non-protein nitrogen (NPN), which rumen microbes can use to synthesize microbial protein.
True protein in forages can be further classified as rumen undegradable protein (RUP or rumen bypass protein) and rumen degradable protein (RDP). RUP are peptides and amino acids that are digested in the abomasum and absorbed in the small intestine, while RDP is degraded or broken down by the microbial population in the rumen into ammonia and volatile fatty acids. Microorganisms in the rumen combine the ammonia supplied by RDP or other non-protein nitrogen sources (e.g. urea) with rumen digestible carbohydrates to synthesize microbial crude protein (MCP). MCP is digested in the abomasum, with the resultant amino acids absorbed in the small intestine. The amount of protein that reaches the small intestine depends upon the availability of RDP and the rumen digestible carbohydrate. If energy is deficient in the diet, surplus ammonia is converted to urea in the liver and then lost through urine. If protein is deficient in the diet, digestibility of fiber decreases due to diminished microbial activity and muscle will be degraded to meet the animal’s requirements for amino acids for core body functions. (Ed. Note: This is why we sometimes feed protein tubs to animals eating mature grasses. The protein helps microbes turn all that fiber into useable protein for the cow.)
Microbial protein makes up close to 70% of all protein absorbed from the small intestine and the protein contributions from microbes may be close to 100% for cattle fed low-quality forage.
In most cow-calf diets, forages with adequate digestibility will provide enough MCP to meet the cows’ requirements. But, for animals with higher protein demands, like growing calves or lactating cows, it can be beneficial to feed proteins that bypass the rumen and are absorbed in the small intestine, improving protein bioavailability. Extra protein can be provided by feeds that are high in RUP, like alfalfa dehydrated pellets, distillers’ grains, or alternative feeds like canola meal. Most forages have higher levels of RDP, particularly legumes.
During summer months, while forages and legumes are actively growing, they may supply up to 20% crude protein (CP) with a high level of RDP, but during the winter, protein levels drop off dramatically. Native pasture, for example, may test as low as 3-7% CP. Putting up good quality feed is key to supplying the beef herd with adequate quality forage sources through the winter4.
Water is an essential nutrient for cattle, accounting for between 50 and 80 percent of an animal’s live weight. Insufficient water intake reduces animal performance faster and more dramatically than any other nutrient deficiency.
For livestock to maximize feed intake and production, they require daily access to palatable water of adequate quality and quantity. Factors that determine water consumption include air and water temperature, humidity, moisture content of the feed/forage, cattle type (calf, yearling, bull, cow), the physiological state of the animal (gestation, maintenance, growing, lactating) and water quality.
Total dissolved solids (TDS) is the main indicator of water quality and is a measure of dissolved inorganic salts in water. TDS is impacted by high or low pH levels, sulphates, nitrates, salinity, excessive mineral levels, algae and bacteria. Testing water sources to ensure that cattle have access to adequate amounts of quality water is important.
The four main functions of water in the body are:
• to help eliminate waste products of digestion and metabolism
• a major component of secretions (milk, saliva) as well as individual and fetal growth
• as an aid in body’s thermoregulation processes through evaporation of water/sweat from the skin’s surface and respiratory tract
• to regulate blood pressure5
Read more about water requirements for beef cattle here.
Next week we’ll take a look at mineral and vitamin requirements. Stay tuned!