Energy results from the digestion of carbohydrates, protein, and fat.
Net Energy Lactation (NEl) is the feed energy available for maintenance and milk production after digestive and metabolic losses.
Various methods exist for predicting the amount of energy used from a feed.
Cows require energy for maintenance, growth, milk production, reproduction, and body reserves. A cow’s total energy requirement
After water, energy is the largest nutrient requirement of the dairy cow. Energy cannot be measured with chemicals in a laboratory. Since energy comes from the digestion of carbohydrates, protein, and fat, researchers attempt to predict the energy value of feeds based on the amount of each and their assumed (or measured) digestibility and availability to the cow.
Energy is usually expressed in terms of calories. A calorie (cal) is the amount of energy needed to raise the temperature of 1 gram of water for 16.5 to 17.5oC. In Europe, they use the term joule. Once calorie equals 4.184 joules (J). A kilocalorie (kcal) is 1000 calories. A megacalorie (Mcal) is 1000 kcal or 1 therm.
Energy Utilization Scheme
NEm – Net Energy used for Maintenance – Energy for Breathing, Walking, Etc.
NEg – Net Energy used for Growth – Energy for Muscle and Bone Production
NEl – Net Energy Lactation – This includes energy used for maintenance and milk production because they are used with the same efficiency so it is easier to leave them together.
Gross Energy (GE) is the energy generated when a feed is burned. Heat production is measured using a bomb calorimeter. This is easy to measure in a lab but it is relatively useless for nutritionists. Cornmeal and straw have similar amounts of Gross Energy but everyone knows that the two won’t yield the same amount of milk when they are fed to a cow. For this reason, researchers have tried to develop a system to quantify the amount of energy in a feed that is available to the cow and can be used for productive purposes.
Digestible Energy (DE) is the energy derived from the digestion of a feed. It is fairly simple to measure the amount of DE in a feed. Simply feed it to the cow (as the sole ration) and collect all of her manure. The difference between the total heat given off when the feed is burned versus the total heat given off when the manure is burned is the Digestible Energy.
Metabolizable Energy (ME) is the energy available for metabolism. Net Energy (NE) is the energy available for production and maintenance. It gets a bit more difficult to measure Metabolizable Energy and Net Energy. The only true way to do it is to put a cow into a chamber and measure all inputs and outputs (including heat and gasses) from her body. Of course, this is very expensive and impractical to do on a routine basis. So, researchers have come up with some conversion factors. For example, ME is about 82% of DE. NEl is about 64% of ME. In Europe, ME usually is used to express energy requirements and energy content of feeds. In the U.S., the Net Energy system is most often used.
For NEl, it is assumed that the feed is being fed to a cow with an intake level that is three times what her intake would be if she were only maintaining herself. When a cow eats more feed, it passes through her digestive tract more quickly. Thus, the NEl estimates for a feed fed to a cow at three times maintenance are lower than they would be for a cow that was just maintaining herself. The 1989 NRC discounts the energy value of a feed 4% for each unit of maintenance. All feeds, regardless of their rate of digestion, fiber content, and particle size, are adjusted to the same extent to account for the higher dry matter intake of a lactating cow. The 1989 NRC has tabular values for the NEl content of various feeds. Most of these were derived from estimates of TDN (developed by Morrison at Cornell (to be discussed)) or DE.
NEl includes energy used for maintenance and milk production because energy is used with the same efficiency whether for milk production or for maintenance. For growing heifers, both NEm and NEg should be used to be technically correct. The two are separate requirements. Energy is used with a different efficiency by the heifer for maintenance than for gain.
Many laboratories calculate the amount of NEl in a forage based on its ADF content. Using databases containing the ADF content of feeds and the NEl content of those feeds, regression equations have been developed to predict NEl from the ADF content of a feed. As ADF increases, NEl decreases. Unfortunately, different laboratories often use different equations. This makes for confusion. For example, here are three different equations for corn silage that are commonly used:
Corn Silage NEl Estimates Using Three Different Equations
|NEl = 94 – (0.80 x %ADF)
|NEl = 104.4 – (1.24 x %ADF)
|NEl = 97.6 – (1.26 x % ADF)
Two Different Variations Used to Predict NEl Content of Feeds and Forages:
1. Van Soest Discount System
As previously noted, the NRC system discounts the energy of feeds by 4% per unit of maintenance intake regardless of the type of feed it is. Peter Van Soest of Cornell University recognized that this is not biologically correct. The decline in digestibility of feeds that occurs as cows consume more depends not only on rate of passage but also on its rate of digestion. Feeds that are more slowly digested by the rumen bacteria will be affected more by an increase in rate of passage.
Van Soest calculated NEl values for feeds but then he assigned different discount values relative to level of intake above maintenance for each feed. Feeds that have a high cell wall content and a low degree of lignification have the largest discounts. For example, grasses would have larger discounts than legumes, such as alfalfa. Molasses, a highly digestible sugar, would have a discount of 0%.
2. Ohio State Equations (Weiss)
Bill Weiss at The Ohio State University recognized that while ADF is negatively correlated with the digestibility and energy content of a feed, there is more to accurately predicting the energy content of feeds sent in for laboratory analysis. Two feeds may have the same amount of ADF yet different amounts of NDF, ash (or mineral), lignin, and fat. The digestibility of the NDF may also differ. He developed equations for estimating the NEl content of feeds and forages based on the amount of digestible protein, digestible NFC, digestible fat, and digestible NDF in a feed. These equations are in use by some commercial laboratories. Some also evaluate digestible NDF using in vitro or in situ techniques for greater accuracy.
Cows require energy for the following functions: maintenance, growth, milk production, reproduction, and body reserves (listed in order of priority). A cow’s total energy requirement will be the sum of what she needs for each function. For example, a cow weighing 1300 pounds (590 kg) making 100 pounds (45.5 kg) of milk containing 3.5% milkfat will require 9.57 Mcal/day for maintenance and 31 Mcal/day for milk production. That makes her total NEl requirement equal to 40.57 Mcal/d or 0.78 Mcal/pound of dry matter if she consumed 52 pounds of dry matter per day.
Maintenance – Animals need energy for basic maintenance functions, such as, breathing, eating, digestion, walking, and keeping warm or cool. The cow’s size and physiological state are two of the biggest factors affecting the maintenance energy requirement. Other factors include, cold, heat, wind speed, humidity, hair length, hair color, mud coating, bunk space availability, floor surface, stall comfort, and activity level. The 1989 NRC and most computer ration balancing programs assume that the cow is in a thermoneutral environment and is not affected by her environment. So, when using those programs, it is up to the on-farm observer to assess body condition score and environmental conditions to determine if the extra maintenance needs of the cows are being met or if the ration energy content should be adjusted. The 1989 NRC requires 9.57 Mcal of NEl per day for maintaining a cow weighing 1300 pounds (590 kg).
The Cornell Net Carbohydrate / Protein System does attempt to take many of the above environmental factors into account to determine the cow’s maintenance energy requirement. Maintenance requirements can increase by as much as 30% when a cow is coated with mud and is out in a cold wind with high humidity.
Growth – The 1989 NRC estimated energy needed for growth of muscle and bone based on work done with beef breeds. More research needs to be done to understand the composition of body weight gain in dairy breeds, the efficiency with which energy is used at various ages, and genetic merit differences. Unfortunately, many dairy producers freshen heifers still having a significant amount of growing to do during their first lactation. Energy is diverted away from milk production in these animals and usually this is seen by a relatively flat milk production curve (poor peak but good persistency). To account for this growth the 1989 NRC recommends that maintenance energy requirements be increased by 20% during the first lactation and 10% during the second lactation.
Milk Production – Energy requirements for milk are generally expressed as a constant function of fat-corrected milk production. For example, the 1989 NRC requires 0.31 Mcal of NEl to produce one pound (0.45 kg) of milk containing 3.5% fat. It requires 0.40 Mcal of NEl to produce one pound (0.45 kg) of milk containing 5.5% fat.
Reproduction – The1989 NRC simply increases the maintenance energy requirement by 30% for a cow during her last two months of pregnancy. This is to allow for more energy to be provided to the developing calf. This is a fairly simplified approach that needs improvement. New research suggests amino acids may provide a great deal of energy to the fetus at this time. In addition, the extra needs of growing twins within the cow need to be addressed.
Body Reserves – The 1989 NRC requires 2.32 Mcal of NEl for every one pound of body condition needed to be gained. Many researchers have worked to fine-tune a 5-point body condition scoring system for dairy cows. Cornell researchers found that one point of body condition represents 121 pounds (55 kg) of body weight in mature Holstein cows having a body condition score between 2 and 4. Using the body condition scoring system, rations should be adjusted to maintain body condition between 3 and 3.75.
Old Energy Systems
1. Total Digestible Nutrients (TDN) is the sum of digestible crude fiber, digestible crude protein, digestible fat multiplied by the factor 2.25, and digestible nitrogen-free-extract. Feeds need to be analyzed via a complete digestion trial to accurately determine TDN. Many of the book values used for TDN were assigned to feeds by Frank Morrison at Cornell University and first published in the 1936 edition of his book “Feeds and Feeding”. TDN is commonly reported on forage analysis reports today but is usually not used for the purpose of ration balancing. TDN is expressed as a percentage of feed dry matter.
2. Estimated Net Energy (ENE) was also developed by Frank Morrison. He realized the productive value of TDN in forages was less than that for concentrates. TDN values were adjusted downward to try to subtract off the digestible energy that was used for non-productive purposes, such as for heat.
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