A575
Nutrition For Beef Cattle - Introduction, Digestion, Nutrients

type of program and calving season | factors affecting nutrient requirements | the digestion process | nutrients for beef cattle

Introduction: In many ways, cattle producers have expertise in nutrition. Each day when cattle are fed or when grazing, they are taking in nutrients (protein, energy, minerals, etc.) that are vital for their life and production. Cattlemen are responsible for providing the feeds that supply the nutrients for the beef animal. If provided in the proper balance and portions at an economical level, the desired results should be obtained. If shortages occur with any required nutrient or if the nutrients are fed in excess or the sources are extremely costly, then economic losses will be experienced. Because the largest cost in maintaining a beef cow and producing a calf is feed expenses, it is logical to concentrate on this high ticket item. Cattlemen often want direct answers concerning their feeding program such as "how much and what type of supplement should I feed to grazing cows in early winter." Unfortunately, there is not an accurate answer to this question until more is known about the type of cattle and the base forage the cattle are consuming. Perhaps if ranchers would couple their excellent experience in feeding cattle with some basic facts on the nutrition of the cow, then the feeding program could be fine tuned and more profit gained.

This information will discuss both simple and basic nutritional concepts. Sometimes a basic concept appears to be anything but simple to a person who is attempting to use the concept and apply it in a practical feeding situation. The challenge and objective of this discussion is to try to keep the basic concepts simple to understand and yet show why it is important to further understand cow nutrition. Many textbooks and other publications go into great detail and allow further pursuit of a more thorough understanding of basic nutrition.

Many factors influence profitability of a cow/calf operation. Four major factors are listed below:

1. Yearly costs of keeping a cow.
2. Number of cows exposed to the bull that wean a calf.
3. Weaning or yearling weight of calves.
4. Price received for calves and cull cows.

The first three factors can be affected by good management practices. Cow/calf managers should continue to explore avenues to reduce yearly cow costs, increase the number of cows weaning a calf, and wean the heaviest calf possible for a given set of feed resources.

As stated previously, feed costs are the greatest expense in keeping a cowherd and the nutrition program dictates reproductive performance. The ultimate goal for a cow/calf manager is to keep feed costs low but still meet the nutrient requirements of the cowherd so reproductive performance is not impaired. Once these two factors are balanced, producers through new genetics of added growth or milk production can match increased weaning weight with the most economical feed resources available.

The nutritional program should be simple and should supply the needed nutrients for the cow to give birth to a strong, healthy calf, milk reasonably well and rebreed by 80 to 85 days after calving. Managing feed resources to attain a consistently high reproductive rate at a low cost is important in maintaining profitability for the cow/calf enterprise.

Individual producers have little control over calf prices even though the breeding program yields calves with high market appeal. Calf supply and demand has the biggest influence on the price received.

 

Spring Calving: Spring calving cows usually graze summer pastures for most of their lactation. In late fall and early winter, a time that coincides with the middle one-third of pregnancy, cattle graze winter range, crop residues, or are fed low quality harvested forages.

Good quality forage is needed to feed cows 30 to 60 days before and after calving so weight and condition losses are minimal before going to spring and summer pastures. Spring calving programs should be synchronized with the forages available so the greatest nutrient requirements of the cow are when the nutrient quality of the forage is greatest.

Fall Calving: In most parts of the U.S., cows will be lactating and breeding during a time of the year when non-harvested forage quality usually is low. Therefore, fall calving programs require good quality harvested feeds for adequate milk production and early rebreeding.

Cows must be fed adequately after calving to the end of the breeding season to ensure a high conception rate. Crop residues such as cornstalks, milo stubble, or meadow re-growth are essential feed resources to make a fall calving program economical. Protein for lactating cows not provided by crop residues must be met using an economical protein source.

A major advantage of fall born calves is that they are old enough to make excellent gains grazing spring and summer pasture.

Winter Calving: Winter calving has the advantage that calves are old enough to use the extra milk from cows grazing high quality grass, and older calves are better able to utilize high quality summer pastures.

A major disadvantage is that cows will have to be fed more liberally during early lactation because breeding likely will occur in dry-lot and not on high quality spring pasture. In addition, more facilities for calving and protection from winter storms are required to minimize calf death losses.

1. Stage of Production: The beef cow’s nutritional requirements are influenced by stage of production. The production cycle of cowherds can be divided into four stages: (1) calving to breeding – 70 to 85 days; (2) breeding to weaning – 120 days; (3) mid-gestation – 100 days; and (4) late gestation – 60 to 70 days. Important nutritional considerations in each of the four stages of production are as follows:
   
   Calving to Breeding: Cows are lactating during this stage of production, causing nutrient requirements to be greater than at any other stage. Cows in moderate body condition need to be fed to meet their nutrient requirements and to maintain body condition during the winter to have a short interval from calving to breeding.
   
   Cows in good body condition can lose some condition after calving and still attain a high rebreeding percentage. If cows in good condition are fed to lose weight and body condition after calving, it is essential that spring pastures are early so cattle are maintaining or gaining weight prior to the beginning of the breeding season.
   
   Rebreeding performance for cows calving in thin body condition can be highly variable. If thin cows experience little or no stress from calving to breeding, re-breeding can be high. If thin cows experience stresses related to nutrition, weather and calving, breeding performance probably will be low. Severely restricting feed to cows in thin and moderate condition after calving will reduce reproductive performance of cows and growth rate of calves.
   
   Breeding to Weaning: Milk production for most beef breeds will be declining during this stage of production and as a result nutritional requirements also are declining. Spring calving cows of average or low milking ability usually will gain weight during this period if on good summer pasture. Limiting nutrition during this time period will result in lighter calves at weaning. Cows bred for high milk production may lose weight and enter mid-gestation in thin condition. Restricting nutrition to cows at this time has little effect on the developing fetus.
   
   Mid-Gestation: Nutrient requirements for the beef cow are lowest during this stage of production because calves are weaned and the nutrients required by the developing fetus are minimal. Cows in good body condition can lose some weight condition during this period without severely reducing productivity. Cows in thin or moderate body condition must gain or maintain weight and body condition, or performance will be reduced.
   
   Late-Gestation: The fetus is growing rapidly during this stage of production causing the nutrient requirements of the cow to increase. The gain in weight of the fetus, fetal fluids and membranes is about one pound daily for the last 70 days before calving.
   
   Cows in good condition can lose some weight during this period and yet give birth to a strong, healthy calf. Cows in thin condition should be fed to maintain or gain weight and body condition. Cows experiencing excessive weight losses during this period will be slow to cycle and rebreed after calving.
    
2. Age of Cow: After calving, first calf heifers need to be fed separately from mature cows until spring pasture is available. Rations fed to lactating two-year-olds need to contain a higher percentage of energy, protein, calcium and phosphorus than those fed to mature cows before and after calving. Handling first-calf heifers separately from mature cows is also important because younger cattle are low in the "pecking order" and tend to get less than their share of the ration.
   
   Bred replacement heifers and two-year-olds just weaning their first calf may be managed together, especially if the two-year-olds are thin at the tine their first calf is weaned. Older bred cows that are thin also could be fed with this group.
    
3. Cow Size and Condition: For satisfactory performance, large frame cows need more feed than cows of smaller frame size. Weight variation due to differences in condition does not appreciably affect the amount of feed needed for satisfactory production as long as the weather is not severely cold.
   
   For example, a thin 1,000 pound cow and a fleshy 1,200 pound cow both need about the same feed as a 1,100 pound cow of the same frame size during the dry period. However, thin cows need extra energy during cold stress to maintain normal body temperature.
   
   Healthy, mature cows of different frame size and condition can be fed together before and after calving if rations are fed to appetite. A more detailed discussion on body condition scoring can be found on page A85.
    
4. Milking Ability: Superior milking beef cows require rations containing more energy, protein, calcium and phosphorus than average milking beef cows if they are to rebreed and produce a calf every year. First-calf heifers, regardless of milking ability, must be fed to gain weight the first three months of lactation to rebreed. This may require feeding high energy feeds such as grain or corn silage after calving until pasture is available.
   
   In addition, mature, superior milking cows need top quality forages or feeds high in energy in their rations after calving, or rebreeding performance will be low.
    
5. Weather: On most winter days cows fed recommended amounts of feed will produce enough heat to maintain body temperature. In the northern plains, cold weather stress probably does not justify feeding high energy feeds if cattle are fed a full feed of forage properly supplemented with protein.
   
   When weather conditions make it impossible to get adequate feed and water to cattle for long periods of time, cow performance can be reduced. Cows in moderate to good condition can withstand stress situations better than thin cows.
   
   
   

Digestion simply refers to the breakdown of feedstuffs. In order for it to be utilized by the cow, a feed stuff must be broken down to a level so it is absorbed through the gut lining.

The stomach of the cow has four compartments: the rumen, reticulum, omasum and abomasum. The rumen is the largest and plays the greatest role in forage utilization. The rumen contains billions of microorganisms, both protozoa and bacteria, which are involved in forage breakdown. This breakdown is accomplished by secreting enzymes that aid in the fermentation of the feeds in the rumen. Much of the fermentation products are volatile fatty acids (VFA), which are absorbed through the rumen wall and then travel via the portal blood vein to the liver. They are then converted to glycogen or blood sugar and ultimately energy. The reticulum is used to form boluses and serves as a "pump" to push the cow’s "cud" up the esophagus so it can be chewed and mechanically broken down further. While the cow is chewing her cud, she is adding a large quantity of salvia (approximately 5 gallons/day), which is rich in buffering minerals that aid in controlling the proper level of acidity in the rumen. Saliva also contains considerable urea which is being recycled.

Because of the cow’s ability to recycle urea, many feel this is the reason that a protein supplement can be fed successfully every 3 days. She can maintain the nitrogen (from protein) in the system for a long period of time by recycling. Perhaps the cow should be given credit for being one of the first recyclers. The function of the omasum is not well known. It appears to be partially responsible for absorbing moisture from the feed in the stomach. The abomasum is often referred to as the true stomach that is very similar to the stomach in the monogastric or simple stomach animals such as the pig or human. Because the abomasum is relatively high in acid content, very few live microbes are present and digestion is via breakdown by enzymes.

After the feed and microbial particles pass through the abomasum, it travels to the small intestine where many of the small nutrients are absorbed. The unutilized or unabsorbed feed particles pass on to the large intestine where some water is absorbed before passing on out of the cow.

On the average, feed particles are in the digestion tract (front to end) 24-36 hours; however, this can vary considerably. Finely ground and high quality feeds will pass more quickly. Finely ground forage will pass faster than long unground forage, and consequently, the digestibility of the ground forage will be lower. Poorly digestible forages will pass at a much slower rate. Reports would indicate that some particles stay in the digestive tract for over 7 days.

It is important that we understand that we are actually feeding two different parts of the cow’s digestive tract. The rumen microorganisms have their own ecosystem and requirements, and are extremely important in energy and protein utilization. Digestion and absorption in the small intestine are considered as a second part of the cow’s digestive tract. It is important that the rumen microorganisms’ requirements are met so the level of microbial activity remains high. this helps ensure that forage digestibility, which basically occurs in the rumen, will be at a high level. Most of the microorganisms are specialists. A group of the organisms are forage digesters or specifically breakdown cellulose and some other fiber particles in the forage. Others "specialize" in starch utilization which is found in grain. When a combination of forages and grains are fed there are some antagonisms or negative associative effects. Small amounts of grain (2 lb or less) or starch does not appear to create negative effects on the forage digesters. When the level of grain in the forage diets is increased, then a shift will occur away from forage digestion in the rumen and forage digestibility will decrease. Consequently, less energy will be gained from the forage. This is especially true when the rumen microorganisms are short of protein. Protein is used as a "body building" nutrient for the organisms, thus protein must always be adequate so the level of the rumen microbial activity and digestion can remain high.

Some protein in feeds, especially the protein in supplements that have been heated in processing, will not be broken down in the rumen. This protein passes on through the rumen and is digested and absorbed in the small intestine. This is a very efficient process and once the rumen microorganisms’ needs are met, it would be desirable to meet the remainder of the cow’s protein needs by feeding high by pass protein sources such as blood meal, meat meal, corn gluten meal, feather meal, plus others. As more is learned of the cow’s actual protein requirement and the actual by pass value of protein sources are determined, the more the by pass protein concept will be used in balancing rations.

The life of the rumen microorganisms is variable, but in all cases is short (measured in hours). Thus, the population is in constant turn over. As the rumen microorganisms, which are single cell "animals," build their bodies or cell walls they synthesize protein from nutrients available in the rumen. As they die of old age (a few hours after they are formed) they pass on down to the small intestine where they are digested and absorbed. This is referred to as microbial protein. At this point the microbial protein is utilized in the same manner as the protein that by passed the rumen, which origin may have been from soybean or cottonseed meal. Again, it is important to understand how starch sources, such as grains, are utilized by the rumen organisms and when and why they can be beneficial or detrimental. The same is true with protein and understanding the protein and energy relationship and interdependence.

Neither protein nor energy is the most important. They are both important and both must be supplied in adequate quantities to acquire satisfactory performance. Some have felt that energy is the nutrient needed for reproduction and protein is needed for growth. The fact remains that they are both needed for maintenance, growth and reproduction.

Nutrients are those compounds or components of feeds that provide the total nutrition (life and production) for the cow. Energy is the nutrient that comprises the largest and often most expensive portion of the cow’s diet. It may not be the largest out-of-pocket expense because many ranchers raise the sources of energy in their own forage based operations.

The 1996 National Research Council – Requirements for Beef Cattle has detailed discussions with numerous literature citations for the various nutrients required by beef cattle. In addition, they have chapters on estimating intake, requirements for stressed cattle, and software on ration balancing.

In order to gain some consensus of opinion on exact cattle requirements, the National Research Council appointed a subcommittee on beef cattle nutrition to evaluate all published data and publish exact requirements for major and minor nutrients for all classes of beef cattle. New requirements are published about every 10 years. The committee has historically met for at least a two year period and has in-depth review and discussion of new nutritional concepts, as well as new research that deals with animal requirements. The subcommittee is made up of highly recognized and respected nutritionists from across the U.S., each having specialties in the various areas of cattle nutrition. They also seek the expertise of others in forming final requirements. Finally, after days and months of review and discussion by the leading experts in the field, the new requirements are published. This is often referred to as the nutritionists’ bible. In the past 10 years, nutritionists have used the 1984 Nutrient Requirements of Beef Cattle. In May of 1996, the 1996 Nutrient Requirements of Beef Cattle was released. As with any new publication, the 1996 requirements suggested several changes, however, not without controversy.

How accurate are the 1996 Nutrient Requirements of Beef Cattle? It is not uncommon to hear comments such as "Those published requirements don’t apply to your situation because your cattle and conditions are different, and as a consequence, my recommendations are different and better." The producer should question what research the feed company bases their recommendations on, and is it thorough and unbiased. The 1996 published requirements are the "state of the art" from the top nutritionists in the United States. The challenge is to apply the requirements in a practical way. The following will be a summary of the important points found in each chapter of the 1996 NRC.

There are many different ways of expressing energy. From a chemical basis, the gross energy is simply burning or ashing a sample and determining how much heat or energy is given off. Although this gives an indication of how much energy is available, it has limited value for estimating the energy utilized by livestock because digestibility is not considered. Most values for energy that are published in feed tables of composition are based on feeding trials. These feeding trials actually measure the amount of energy that is utilized by the animal. Digestible energy (DE) is simply the energy fed minus the energy in the feces. Metabolizable energy (ME) takes it one step further by accounting for energy lost in the feces, urine and gases. Net energy (NE) accounts for the losses mentioned in ME plus heat loss of the body.

In the past, the beef cow industry has used Total Digestible Nutrients (TDN) in balancing the cow’s energy requirements. When most of the commonly used feeds are utilized, TDN still serves very well in determining how to meet the cow’s energy requirements. The new publication justifiably uses the net energy system - net energy for maintenance (NEm) and net energy for gain (NEg). The net energy system has been used in balancing energy needs in the feedlot industry for the past 20 years. Most cow diets can still adequately use the TDN when considering a dry pregnant cow for maintaining weight, and the average lactating cow when no weight gain (or loss) is desired. The major advantage of the net energy system is it allows for precise estimates of the amount of weight gain (or loss) that is desired or that could be expected. For example, if a group of cows is in condition score 4 in November and it is desired to raise their condition to a condition score of 5 by March, the net energy system provides the tools to very accurately estimate the amount of feed needed to raise the weight (or condition score) to the desired level.

The 1996 net energy system breaks out the energy requirements for maintenance of the cow, development of the fetus, lactation and body weight gain (or energy available if weight loss occurs). Maintenance is simply the feed energy required that will result in no weight gain or loss of the cow. This includes the energy for body functions such as the digestion process, temperature regulation, physical activity and other metabolic activity. The maintenance requirements are adjusted for several breeds of cattle. In general, the breeds of cows that have higher milk production have higher maintenance requirements. This is due to the fact that higher milking cows tend to have larger internal organ weights, which have very high energy needs.

In the case of pregnant cows, the energy requirement for pregnancy is also calculated. This varies with the birth weight of the calf, which the user inputs into the program. Lactation is also a large energy requirement that the new system calculates based on the amount of milk given at peak milk production. Even though it is obvious that high milk producing cows have higher energy requirements than low milking cows, peak milk production obviously is not practical to determine in range cattle. Consequently, the new publication gives guidelines for peak milk production quantities based on weaning weight.

After all of the energy requirements for maintenance, pregnancy and/or lactation are met, the remaining energy can be utilized for body weight gain. If the energy demands for maintenance and production are not met, weight loss of the cow will result, with the possibility of lowered milk production. The new system predicts the number of days that it will take to gain or lose one condition score (approximately 75-80 lbs. of body weight).

Feedstuffs analysis reports include both NEm and NEg. In beef cow rations the new system only utilizes the NEm of the feeds for the various energy components. The analysis for net energy levels of feeds is not actually determined directly; however, it is estimated based on the fiber (acid detergent fiber - ADF) level of the feed. Total digestible nutrients are also estimated in feed analysis. Formulas have been published to determine (estimate) NEm and NEg, if TDN content is known (or estimated).

Perhaps the biggest change and most difficult to apply is the new protein requirements. In the past, crude protein has been utilized. It has long been recognized that the crude protein system has many flaws, and in fact, is a fairly crude measurement of protein availability in feeds and the animal requirements. For example, with the crude protein system, it is assumed that protein from non-protein nitrogen, such as urea, is equal to protein from all natural sources, such as soybean meal or cottonseed meal. We have known for years that this is simply not true. We also know that the rumen microorganisms in beef animals have nitrogen or protein requirements, yet the animal has protein requirements for maintenance of the digestive tract, nervous system, muscle structure, etc., plus muscle growth. In the past, the crude protein only assumed one requirement for the entire animal. The crude protein system worked relatively well as long as the limitations were recognized and appropriate adjustments were made.

The 1996 NRC requirements utilize metabolizable protein which establishes separate requirements for the rumen microorganisms and the animal. Metabolizable protein is the protein that reaches the small intestine and is made up of microbial protein (protein that is made by rumen microorganisms) and undegradable intake protein (UIP). In the past, UIP has been referred to as by-pass protein or the protein that escapes or bypasses the rumen microorganisms without breakdown. In the small intestine, protein is digested efficiently, similar to digestion in monogastric animals. The requirement for the rumen microorganisms is referred to as degradable intake protein (DIP) and is derived from protein that is digested or broken down in the rumen. It is important that the DIP requirement be met to provide a high level of microbial activity and to assure optimum levels of fiber digestion.

Some degradable intake protein (DIP) can come from non-protein nitrogen (NPN) sources such as urea. The amount of NPN that is utilized in high roughage diets for bacteria protein synthesis continues to be debated. It is known that there are limitations on how much is incorporated into microbial protein; however, in the ration formulation model that is supplied with the 1996 manual, it assumes that NPN from urea is utilized with the same efficiency as degraded natural protein from sources such as alfalfa hay. The protein in alfalfa is approximately 85% degraded in the rumen. It appears that the rumen bacteria can utilize the nitrogen from NPN just as effectively as from degraded protein from alfalfa or soybean meal; however, when natural proteins are degraded they supply branch chained fatty acids, which appears to be very important in bacterial protein synthesis. It is also known that urea breaks down at a much faster rate in the rumen than carbohydrates are broken down in forages. Because of the lack of synchrony, one would not expect the nitrogen from NPN to be utilized as effectively as nitrogen from natural proteins, which break down slower. Fortunately, the ruminant animal has the ability to recycle nitrogen through the saliva; otherwise, very little of the nitrogen from NPN would be utilized. The debate will continue on how efficiently NPN will be utilized under the various conditions encountered with the range beef cow.  Even though many trials have been conducted to determine NPN utilization, the committee that published the 1996 requirements simply stated, "Until more information is available, it is advisable to use caution when using urea in low-protein, high-forage diets."

In order to determine the level of metabolizable protein that reaches the small intestine, the level of microbial protein must be estimated as well as knowing the level of UIP in the feed or the amount of the fed or grazed protein that escapes the rumen undigested. Tables are available to give estimates of UTP for many feeds and are used in ration formulation. Currently, commercial laboratories do not analyze for UIP and DIP which is a limitation when utilizing the metabolism protein system. Feeds can still be analyzed for crude protein and then UIP and DIP estimated.

In addition to knowing how much protein bypasses the rumen (UIP) and reaches the small intestine, the portion of microbial protein that is produced needs to be estimated. The new NRC model assumes that the amount of microbial protein produced is in relation to the amount of energy in the ration. They use TDN as the indicator of energy. In general, they assume that microbial protein synthesis is 13% of the TDN in the ration; however, this efficiency factor can be altered in the model. In low-energy, high-roughage rations, a factor of 8 to 9% appears to be more accurate in predicting microbial protein yield. If the 1996 NRC computer model is used, there is an opportunity to change the microbial yield factor from 13 to a different number. Until more definitive research is conducted, it is recommended that 9 be used as an appropriate factor.

One of the major problems of any accurate ration formulation is determining intake accurately. This is especially a major problem in grazing animals. The formulation is only as accurate as the accuracy of determining the level of nutrients that the animal is consuming. This depends on the amount and the nutrient content of feeds consumed. Because this is often not known, it requires rough guidelines and commonsense. Fortunately, the new NRC requirements provide estimates of intake. This is based on the type and size of animal, stage of production, and the type of ration. These estimates of intakes are based on research trials where actual intake was determined under various conditions. Unless intake is known or research under similar conditions in your area is utilized, it is recommended that the intake calculated by the 1996 model be used as intake estimates.

The new 1996 model allows for input in accounting for the size, breed, stage of production, level of milk production expected, and body condition of the cow. Adjustments in maintenance are made based on breed. Body condition is used and the output estimates the number of days to gain or lose one condition score rather than predict daily gain or loss.

Environmental factors include temperature, wind speed, coat of the animal and the effects of grazing or dry lot conditions. These factors certainly do effect nutrient requirements in "real world" situations; however, they are very difficult to utilize with accuracy in ration formulations. Milk production can be roughly estimated from weaning weight and research data; however, any estimate under practical conditions would be a very rough estimate. The model makes very precise calculations based on often rough estimates.  Environmental factors, such as wind and temperature, are important; however, the user is usually making estimates in the future which is always subject to large differences to what actually occurs. Also temperatures and wind speeds from the weather bureau should be modified to predict actual temperatures for the cattle. The amount of natural protection cattle utilize is important. Cattle will use one another for protection and aline their bodies to minimize weather effects.

The model assumes approximately 40% higher maintenance requirements for grazing animals versus those in dry lot. Even though there is a cost of maintenance when animals are traveling to graze and the model’s cost of maintenance is based on research data, personal observations and experience would indicate the model’s estimates of increased maintenance while grazing may be too high for the intermountain high plains area. After working with the model, it appears that several animal and environmental factors have relatively large effects on the requirements. As a consequence, a high level of commonsense and practical experience are needed for the best diet recommendations.

Frequently, the question arises regarding whether cattle should be fed and supplemented differently utilizing the new 1996 NRC requirements than the way cattle were fed prior to 1996. Under many practical feeding or grazing conditions, it does not appear major changes will be needed to meet the animals’ requirements as published in the new publication. The system does allow for fine tuning of diets in both protein and energy supplementation. For example, good performance is achieved in many lactating cows when fed prairie or meadow hay and supplemented with 2.5-3.0 lb. of a 30% soybean meal-wheat mids blended supplement. However, when these feeds are calculated in the model, it shows that the ration is deficient in metabolizable protein although DIP is in excess. The model would predict that the digestibility of the forage would be at the maximum level and the microbes would produce maximum level of microbial protein, but cow performance would still be reduced because inadequate protein would be present in the small intestine. Higher levels of the 30% supplement would partially offset this problem, however the most economical and practical way would be to incorporate a low level of a source of protein that is high in UIP (by-pass) protein, such as blood meal. This deficiency can be corrected with 0.4 lb. of blood meal. The need for UIP appears to be more critical in younger 2-year-old first calf heifers.

Another area where fine tuning of rations can be achieved is designing feeding programs to achieve a specific condition score at a given point in time. For example, let’s assume that a group of cows is in body condition 4, approximately 90 days before calving, and it is desired to increase their condition one score before calving. By utilizing the new model, rations can be designed to theoretically meet this goal. The model can also be utilized to balance rations for yearling replacement heifers to meet gain expectations.

There are many situations that will cause some changes in what would be recommended utilizing the new model compared to what was used in the past. It is not possible to address all of the minor and major differences that the new model presents in this paper. The 1996 NRC publication addresses all of the various changes, and if in-depth knowledge is desired, then reading the publication and utilizing the new software is recommended.

The 1996 NRC requirements have two significant changes that will change the way nutritionists balance rations. Rather than utilizing TDN as the energy term, net energy for maintenance and gain is utilized. Crude protein was replaced with two protein fractions - Undegradable Intake Protein (UIP) formally called by-pass protein and Degradable Intake Protein (DIP).

The new metabolizable protein (UIP + DIP) system will correct some of past erroneous assumption that all crude protein is equal, which has long been known to be inaccurate. The new protein system establishes requirements for the rumen microorganisms and for the animal. Even though the new system will be more difficult to apply under practical conditions, the concepts are more precise, and accurate predictors of performance will provide more economical cost of production. Factors that involve the animal characteristics and environmental factors can cause major differences in requirements. As with any good feeding program, commonsense is needed to carry out a practical and economical feeding system.

Ivan G. Rush, Beef Specialist
University of Nebraska
Panhandle Research and Extension Center