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Animal Science 4332

Ruminant Nutrition

Forages for grazing ruminants

Forages represent an important energy and nitrogen source for beef and dairy cattle. In most instances, forages may make up 80 % of the total feed consumed by these animals. Therefore, the quality of forages is a crucial factor in terms of the impact that the forage has on the performance of the animal (performance is in reference to animal growth or milk production). In terms of forage quality, one must strive to optimize ruminant production from forages by optimizing digestibility and intake, maximizing microbial growth efficiency, and adjusting the balance of nutrients from the forage to suit the animal's needs. Regardless of forage quality, forages differ in anatomical and chemical make-up.

Forage types

In considering different forage types, grasses and legumes come to mind as the main forages utilized in beef and dairy cattle diets. The combination of grasses and legumes together in grazing system provides diverse forage to extend the grazing season. The combination of legumes and grasses differ anatomically and chemically. The nitrogen content of legumes is also quite high and if fed as the only forage source may provide 60 % more nitrogen than a growing ruminant needs. Therefore, the combination of legumes and grasses in a pasture situation would balance nutritional needs by the animal. Some legumes, like alfalfa, tend to cause bloat in ruminants because of the soluble and degradable proteins. However, a variety of alfalfa, AC graze land, reduces the incidence of bloat in cattle because it is less digestible and lower protein content than other varieties.

Chemical differences between legumes and grasses

% Cell Wall
CW H C lignin











Legumes Vs grasses:

Have higher lignin / cellulose

Have lower hemicellulose / cellulose

Lower plant cell wall content

Another important difference between legumes and grasses is that legumes typically have greater digestibility than grasses at similar maturity. This also results in ruminants consuming more legume forage than grasses at similar maturity. The chemical structural of the plant contributes to this difference. The leaves of legumes have lower cell wall and higher non-structural carbohydrates, which are digested very fast as compared to the grasses. The increase in digestibility increases passage rate of the material through the digestive tract, which enhances forage intake. The addition of legumes to grass based diet typically increases forage intake because the digestibility of legume.

Warm season vs. cool season grasses:

These grasses differ in the temperature requirements for growth. Warm season grasses grow best at 60 F while cool season grasses grow best 40 to 50 F. The combination of these grasses together in a grazing program extends the grazing season because they have different growth patterns. Warm season grasses start to grow in mid-summer while cool season grasses start to grow mid-April. The chemical make-up of these grasses differ: Warm season grasses have greater structural carbohydrate, greater hemicellulose and cellulose, lower leaf to stem ratio, lower digestibility, and lower intakes (conflicting information) than cool season grasses.

Plant maturity: Refers to the morphological development, which results in appearance of reproductive cycle. As the plant matures to full bloom or development of seeds the chemical composition of the plant changes. A large part of this change is increase in the stem portion of the plant with little change in the leaf portion. The crude protein decreases, structural carbohydrate increases, and lignin content increases in the plant. Along with these changes is a decrease in the leaf / stem ratio component. With maturation of the plant the stems increase in height, and consequently, the stem fraction makes up a greater portion of plant dry matter. These changes suggest the important of harvesting forages at the appropriate time to emphasize quality.

Plant digestibility also decreases with an increase in plant maturity. The digestibility of stem components accounts for the greatest reduction in digestibility of the plant. The leaf component of legumes accounts for very little of the change in digestibility as the plant matures. With grasses the leaf component does decrease in digestibility with maturity. However the stems account for most of the changes in digestibility with maturity.

Maturity and its effect on chemical composition

Percentage of dry matter

Bud to first flower




Mid to full bloom




Postbloom +





Vegetative to boot

>18 <33 <55

Boot to early head

13-18 34-38 55-60

Head to milk

8-12 39-41 51-65

Dough +

<8 >41 >65

Temperature and drought also affect the chemical make-up of the plant. An increase in temperature causes an increase in lignin and structural carbohydrates. These contribute to changes in the nutritional value of the plant. As the temperatures in June and July increases the maturity of the plants escalate and decrease in quality.

Droughts also have an effect on growth of the plant. With dry conditions, the structural carbohydrates decrease and the leaf to stem ratio also decreases. The effect of drought on the nitrogen content is contradictory; some have shown a positive effect while others have shown a negative effect on nitrogen content.

Factors affecting Forage Quality

1. Affected by chemical composition of the forage, high protein low fiber. Legumes tend to be higher in quality than grasses. Therefore adding a legume to a pasture will improve forage quality because legumes are higher in protein lower fiber content.

In Missouri, it is possible to maintain 20 to 50 % legume in a pasture. One key to maintain a stand of legumes is keep in leafy, vegetative stage of growth. To keep them the pasture must be rested for reseeding and restoring carbohydrates. Should we apply high rates of N fertilizer. Not unless plan to cut or mob graze. The grass will form a canopy over legume and blocks it's growth.

2. Quality also affected by maturity. As the plant matures, it goes from leafy vegetative to stemmy, reproductive stage, protein decreases and fiber increases. Under management intensive grazing, maturity is controlled as paddocks are more uniformly grazed, and higher quality forage.

Another major factor affecting forage quality is plant part. Leaves contain more protein and less fiber than stems and are therefore higher quality. Plant parts are important to consider in legume hay production because leaves can be shattered during raking and bailing, thus lowering the quality. In a grazing situation, plant parts are important in that with a cow's first bite of a vegetative alfalfa, their first bite contains a high proportion of leaves and the second bite contains stems, lower in quality. With vegetative tall fescue,the first two bites are high in leaves.

Nutritive Value of Forages

The nutritive value of the forage is quite variable and fluctuates with the growth of the plant. Cool season grasses reach their growth potential in May and June and come on again in September and October. During the remainder of the year the grass goes dormant. During the grazing season, the cool-season grasses provide adequate protein and energy for cattle and are at their highest nutritional value. During the rest of the year the grass goes dormant and does not meet the animal's nutritional requirements. The practice of extending the grazing season for cool season grasses, such as, tall fescue has provided available forage later in the season, i.e., November and December. The nutritional value of the grass will vary throughout the season with the highest nutritional value occurring in the early bloom stage of development and the lowest nutritional value occurring full bloom stage. Cattle will perform better on the forage early in the growing season when lush and approaching early bloom stage. In a continuous grazing system, cattle will show the following gains relative to nutritional value of forages.

Performance of 550 lb steers grazing forages varying in maturity

Neg TDN Lb / day
Mcal/lb %

Alfalfa, early












Timothy, early












The greater gains for those fed alfalfa, early bloom are attributed to energy and protein content of the forage. As the forage matures, the energy and protein content of the forage declines, and thus the anticipated daily gains of the 550 lb calves change as a reflection of the quality of the forage.

One way to improve steer gains throughout the grazing season is provide a balance supply of nutrients. There are several ways to accomplish this. These are Supplementation; rotation grazing; complementary grazing.

An example of complementary grazing, would be to use tall fescue or other cool season grasses in the early season followed by warm season grasses in mid-summer when they grow well and reach full bloom. This could be followed by placing cattle back on tall fescue in the fall and take advantage of the regrowth of the pasture.

An example of such as system included placing 480 lbs calves on tall fescue pastures for 231 days (Dec. 3 to July 21, 1992) or tall fescue followed by Caucasian bluestem (warm season grass; Dec. 3 to June 1 and switched to Caucasian bluestem until July 21).

Performance of steers grazing different forage systems

Initial Wt. Daily gain Days total costs
Lbs lb / day $

Tall fescue





Tall fescue /Caucasian BS





Link to Forage Selection

On may suggest from these data the importance of balancing nutrient supply to the animal throughout the grazing season. The lack of energy and protein late in the grazing season results in poor performance of growing calves.

Protein content of Forages

Cattle usually select higher quality forage than the average of the standing forage. In order to determine a ruminants protein status when consuming forages, one needs to know the protein degradability and potential digestibility of the undegradable protein fraction in forages. The following table gives on an idea of the degradable and undegradable fractions of a legume and grass.

Protein content of forages










E. Bloom




Mid bloom




Full bloom

















The DIP component of the forage represents 70 to 85 % of the forage crude protein

Content, while the undegradable protein fraction represents 10 to 30 % of the total crude protein. It has been estimated that the undegradable fraction of crude protein represents 0 to 3 % of forage DM while the degradable fraction ranges from 2 to 20 % of the DM. In general, the protein in legumes is more extensively degraded in the rumen than in grasses. One may infer that the efficiency of utilization of protein in legumes is compromised more than that in grasses due to greater losses of N that accompany extensive degradation of protein in rumen. When comparing cool season grasses and warm season grasses, cool season grasses have less undegradable protein than warm season grasses, expressed as a percentage of total protein.

Plant Maturity

As the plant matures, dry matter intake decreases, resulting in reduced postruminal supply. There is a decrease in production of microbial protein and decrease in net N flow to the small intestine. There is also less undegradable intake protein available to the animal because it is lower in digestibility.

The amount of total CP, DIP, and UIP also vary throughout the year. During the grazing season (April through July), the crude protein and degradable protein components are the highest as compared to the rest of the year. Throughout the year, the degradable fraction comprises a larger percentage of total plant CP than the undegradable fraction. These changes in forage composition throughout the year make it a challenge to predict protein requirements for grazing cows and growing calves.

In evaluation of forages for their ability to supply adequate rumen degradable protein for the microbial population, one must have an index of the available nitrogen supplied. The following table presents degradable crude protein as ruminal ammonia need (UFP). The following guidelines may be used to determine if ammonia supply is adequate:

1. Crude protein greater than 10 %.

2. Rumen degradable protein greater than 55 % of crude protein

With these guidelines, one can evaluate the adequacy of the forages for rumen degradable protein.

Physical processing of forages

Grinding, chopping, and pelleting of forages have been used to provide cattle a total mixed ration or a supplement to the existing ration already being fed. Alteration in the physical characteristics (grinding and pelleting) reduces the particle size of forage, as well as decreases the residence time of the forage in the rumen. This effect also reduces the extent of digestion of the ground forage. Fine grinding / pelleting of forages will decrease proportion of microbial N in total N reaching lower gut. This increases the amount of N escaping degradation in the rumen. It may also decrease the supply of amino acids for digestion in the small intestine due to heat damage of the protein in the forage (this especially occurs with the pelleting process).

The grinding and pelleting of forages allow the animal to consume more ground or pelleted forage. This effect is more pronounced with forages of low quality or advancing maturity. This generally increases intake, gain, and improves feed conversions as compared to feeding long or chopped forages.

Quality Corn silage

TDN yield per acre is 30 to 50 % greater when corn is harvested as silage compared to harvesting corn as grain and stover. To make good corn silage or haylage, it has to be harvested at the correct stage of maturity and moisture content. Also the stover needs to be cut at the proper length to ensure good packing of the silage upon storage in order to minimize oxygen build up or anaerobiosis.

Maturity Milk line 1/2 to 2/3 down the kernel

Moisture (%) 65 to 70

Length of cut 3/8 to 1/2

One of the most important factors influencing quality of silage made is the degree to which oxygen is excluded from the silo, stack, or bunker. The most serious problem resulting from this is heat damage, which reduces the nutritional value. This occurs through the combustion of carbohydrates and oxygen, which occurs during plant respiration, and the activity of yeast and molds. These results from slow silo filling, air leaks, slow feed-out. Low moisture content, long chop length, or poor packing.

Phases during ensiling

  1. Aerobic respiration (4 to 5 hrs.)- minimize soluble carbohydrate use so it can be used by microbes.

  2. Once oxygen depleted, anaerobic bacteria ferment sol. Carbohydrate and protein. Yields acetic acid (lasts 1 to 3 days).

  3. Anaerobic bacteria produce lactic acid as end product of fermentation, which is the most desirable end product.

  4. Forage stable at pH 4 and remains stable until fed.

Common silage problems include hot silage, caramelized silage, moldy, rancid odor, vinegar odor, alcohol odor and frozen silage.

Balancing corn silage diets:

Roughage low in energy

Low in protein

Low in Vitamin A and minerals

Hay making

One of the most basic components to a beef cattle operation is adequate supply of high quality pasture.The diversity of climatic conditions precludes providing available pasture on a yearly basis.Therefore it is imperative to provide stored forage during the times of year that forage is dormant and low in nutritional quality.

Harvesting, storing and feeding hay are important to a beef cattle operation.It is crucial to minimize losses during forage production and maximize forage utilization. Under the best conditions, 20 to 25% of the dry weight of forage is lost in the process of haymaking.Proper management is the key to minimize these losses.

Harvest at proper stage of maturity - with grasses and legumes, stage of maturity affects yield.As hay crops mature, digestibility decreases primarily because fiber content increases.Maximum yield of TDN per acre occurs 2 weeks prior to maximum dry matter yield.Stage of maturity at harvest dramatically influences cattle performance.This is influenced by the loss of soluble carbohydrates and protein, as well as the reduction in the digestibility of the plant.

Nutrient and dry matter losses are mainly associated with haymaking.The main losses during harvesting are associated with plant respiration, rain and mechanical handling.When the forage is cut and allowed to wilt prior to baling the length of drying time affects the nutritive value of the forage.The longer drying time for forage affects the loss in soluble carbohydrates through cell respiration.By conditioning wilted forage with conditioner, we speed up the curing process and minimize respiration, which reduces nutrient losses and preserve forage quality. After conditioning, the hay is raked into windrows for baling.Raking can cause leaf shattering, if it is done when the forage is too dry. The ideal dry matter is 50% dry matter. The raking of the hay into windrows enhances the rate of drying.Baling of the hay into square bales or round bales also can reduce forage quality through leaf loss.It is critical that the hay be baled at proper moisture content.For small square bales this is < 20% moisture.For large round bales this is < 18%.If stored at higher moisture content, heating from microbial growth leads to formation of mallard products reducing nutrient digestibility.

Hay should be stored in a barn to protect it from weathering, resulting in the loss of dry matter from spoilage.Storage of big round bales of hay on crushed rock reduces DM losses by 50% compared to storage on the ground.

Improving the quality of Hay by Anhydrous Ammonia Treatment

Justification for Using Anhydrous Ammonia Treatment. One of the challenges for a cattle producer is to have high quality forage for the cow herd for Winter feeding. It is a challenge to harvest hay at the ideal time because of rain. Hay of marginal quality is harvested when yield is more important than quality. Most of the hay is harvested at a CP of 7 % and an average total digestible nutrient concentration of 43 %. Yearling heifers require a diet that is 11 to 12 % CP and 60 to 65 % TDN. Low quality hay plus an energy protein supplement such molasses-urea may provide nutrients necessary for heifer development during the first winter.

One treatment used to enhance hay quality when put up too mature because weather conditions or interest in yield rather than quality is Anhydrous Ammonia treatment.

Purpose: Ammoniation improves N content of the forage and improves fiber digestibility. Work has shown with most low quality hays, The following change in content occur.


Crude Protein

NDF, %

In Vitro OM Dig.,%

Nontreated .Bermuda Grass




Treated Bermuda Grass




The NPN from ammonia has a protein value similar to that of urea which is found in many liquid and dry supplements. For young calves utilization of NPN is not as good as protein utilization from natural protein sources. Although N is increased in low quality hays other nutritional effects more important. Ammoniation also increases feeding value of forage by a chemical breakdown of plant fiber. This results in the opportunity for rumen bacteria to attach to fiber and digest the ammoniated hay. NDF is reduced by ammoniation and greater in vitro organic matter digestion. This is a measure of energy content of forage and is usually 10 to 15 % units greater. Current work at MU has shown that anhydrous ammonia treatment of endophyte infected tall fescue chemically destroys ergovaline ( toxin in hay that adversely affects cattle performance).

Procedure of Ammonia treatment of Hay

Arrange hay stack to minimize costs of materials and labor.

Stack the hay in a 3-2-1 pyramid configuration, Allow a small space ( 2 to 3 in.) between cut edges of adjacent bales so ammonia can circulate within stack. Midway down the length of the stack, a two-foot wide space is left on the bottom layer of bales. A PVC pipe which aids in delivery of ammonia from the tank to the stack is placed into this opening. The PVC pipe is used to deliver ammonia from the tank to the stack is 2 inches in diameter, 20 ft long and capped on one end. It distributes the ammonia more uniformly within the stack. A trench is dug around the stack to secure the plastic. Black plastic of at least 6-mil thickness should be used. Clear plastick which does not contain an ultraviolet inhibitor should not be used because it becomes brittle in a short period due to exposure to sunlight. Check plastic for holes caused by hay stems puncturing during covering. Can use duct tape to seal holes.. It usually takes 5 hours to apply the proper amount of ammonia to the stack. Ammonia will absorb into hay in 2 days. Treatment depends on environmental temperature. The warmer the temperature the faster the reaction time. Hay should remain under plastic for about 30 days before feeding. The estimated cost for application of ammonia to 86 big round bales is $570 ( 1991 estimate).

Ammonia toxicity can occur when cattle are fed ammoniated forage sorghum, hybrid sudan, small grain hay (not straw) and high quality brome and fescue hay. Therefore, avoid treating forages with crude protein content over 7%. The symptoms include extreme excitability, circling, running, convulsions and death. If toxicity should occur, avoid working the livestock and remove the ammoniated forage for several days. A mixture of 50% ammoniated hay and 50% untreated hay should prevent future problems.

Winter hay storage

How much hay will be needed for cow herd to make it through winter?

Estimate hay left over. Base estimate weight on several bales. Adjust estimate for storage and feeding loss.

Calculate number of animal units will be feeding over the winter. Base number of units on 1 unit for a mature 1000 lb animal, 1/2 unit for yearling cow and 1/4 unit for calves. Each animal consumes 30-40 lbs of avg quality hay / day. (as fed basis).

Example: Assume 120 days of feeding hay with following herd;

35 cows X 1 = 35

10 backgrounding steers X 1/2=5.0

15 replacements heifers X 1/2=7.5

8 calvesX1/4 =2.0

1 bull1 1/2=1.5


51.0 animal units

51 animal unitsX120 daysX30 lb / day =183,600 lbs or 230 bales of hay needed

(Assume each bale weighs 800 lbs)

Forage Analysis

Forage analysis may be used to determine TDN and energy values of forages. This is important to determine what nutrients are limiting for growth. By comparing the nutrients provided by the forage with the animal's requirments indicates if supplementation is needed. Supplementation is the addition of a nutrient to a forage diet that is lacking in sufficient amounts for optimal growth.

One component analyzed is NDF. A formula has been developed for cool season grasses and alfalfa to predict intake of the forage by cattle.

Intake as % of BW = 120 / % NDF

1.9 % BW as intake = 120 / 63

For a 1200 lb cow X .019 = 22.8 lb DM intake

% TDN = 88.9- ( 0.779 X ADF )

% TDN = 88.9 - (0.779 X 32 )

64 % TDN = 88.9 - ( 0.779 X 32 )

22.8 lb DM itake X .64 % TDN = 14.6 lb TDN intake

22.8 lb DM intake X .10 CP = 2.28 lb CP intake

One can determine how deficient the forage is for TDN and CP, and supplement to meet the animal's requirements for these important nutrients. Typically forages are deficient in both protein and energy if it is poor quality forage.

Low protein:

For cows fed a low quality hay or grazing dormant pasture, protein is usually the first limiting nutrient, energy may be deficient as well. It is not as deficient as protein. If protein is deficient, then there is insufficient protein for the microbial population to synthesize more microbial protein. This will limit the microbial population and compromise rumen function. The addition of energy with forage protein limiting will not improve forage intake or diet digestibility.

This situation occurs when forages provide < 7 % CP - Intake of forage is not adequate to meet the cow's nutrient requirement. This calls for addition of a protein supplement ( 20 - 25 % CP ).

Supplemental protein will stimulate forage intake and may enhance forage digestibility. However, most research has found passage rate increased and consequently rumen microbial activity increases stimulating microbial protein synthesis. When this occurs, there is a need for additional energy.

Form of protein is important. What form of protein would you add?________________________

With low quality forages, one may also need energy. Several choices of energy may be used.

Corn starch- Form is antagonistic to the rumen environment This environment is currently handling forage fiber and the fibrolytic bacteria breaks down fiber. Addition of starch promotes the proliferation of amylolytic bacteria. The addition of excess starch promotes rapid degradation of starch reducing rumen pH, which the fibrolytic population is sensitive to low pH. This may reduce fiber digestion and forage intake. The recommendation is to feed .25 % BW corn starch which has been shown not to adversely affect fiber digestion and forage intake. A small amount of non-structural carbohydrate actually stimulated forage digestion.

Fibrous energy- by-products are an excellent source of fiber and minimal starch. This fiber is acomplementary source of energy for the rumen bugs who utilizing forage fiber. These forms of fiber do not have a negative effect on fiber digestion.

Forage intake- Forage intake will respond to supplementation depending on the quality of the forage. If the protein and energy in the forage were balanced, any type or level of supplementation had a negative effect on forage intake. However, when the forage was low in crude protein relative to energy, the effect of supplementation on forage intake was positive regardless of level and type of supplementation. When the forage was moderately unbalanced in protein relative to energy, the supplement had positve effect on forage intake at low levels of supplementation and a negative effect at high levels of supplementation.

The following figure describes the relationship between supplemental Non-structural carbohydrate intake and change in forage intake. This implies that supplemental Non-structural carbohydrate to forages

containing greater than 8 % CP usually decreased forage intake by cows. Likewise, when forages contained less than 8 % CP, the opposite occurred.

Figure. Relationship between supplemental Non-structural carbohydrate intake and change in forage intake

Associative Effects

When balancing diets for cattle, associative effects are ignored, and it is assumed that combination of two feed ingredients, like forage and grain will contribute nutrients to meet the animal's nutritional requirements without interacting and influencing forage utilization. The addition of supplemental feed ingredient may have a positive or negative associative effect on forage utilization. The addition of supplemental protein to a low quality forage diet results in an increase in forage intake and / or digestibility of the forage. Likewise, a negative associative effect make occur when supplement suppresses intake and digestibility of the forage so that the intake or digestible nutrients is less than would be expected from the forage and supplement separately. For example, the addition of supplemental corn to a mature forage diet results in negative effects on forage intake and digestibility. The amount of negative effect depends on forage quality, amount of supplement fed, and the grazing conditions.

Substitution Rate

Substitution rate may be defined as the change in forage intake in kg DM per kg supplement DM fed. A positive substitution rate indicates that forage intake is reduced by supplement intake substituting for forage. Likewise, a negative substitution rate indicates that forage intake is increased by supplementation.

In the following figure, a strong relationship was found between substitution rate and forage % CP. Substitution rate was zero when forage CP was equal to 7.4 % averaged -.83 for forages below 7.4 % CP and averaged .78 for forages above 7.4 % CP. This means that each unit of supplement resulted in a .78 unit decrease in forage intake when forage CP was above 7.4 %, while .83 unit increase in forage intake when forage was below 7.4 % CP. Figure 3 describes this relationship.

Figure 3. Relationship between substitution rate of supplements and forage CP

Form of Protein on Forage Utilization

When forages are consumed by ruminants, these forages are fermented by microbial population in the rumen. The fermentation of the forages is dependent upon a ready supply of available nitrogen or crude protein. It is critical that the protein is readily degradable in the rumen for optimal use by the rumen microbial population. The intake of low-quality forage by steers was increased dramatically by supplementation of DIP ( digestible intake protein ). In figure 1 supply of supplemental DIP and starch had different effects on forage intake of low-quality forage by steers. The addition of DIP increased forage intake and digestibility.

Supplemental DIP increased forage intake, available protein in the intestines, and VFA production.

Figure 1. Effect of level of Supplemental DIP and Starch on intake of low-quality forage by beef steers

Figure 2. Effect of increasing % of Supplemental DIP from NPN on Forage Intake and Digestion

Protein supplement for moderate energy diets

Feed protein vary in rumen solubility and result in variation in the rate and extent of digestibility in the rumen. Proteins escaping ruminal degradation are classified as ruminally undegradable proteins ( RUP), or bypass protein. Sources of RUP are not equal in quality for ruminants based on essential amino acid content. These protein sources may provide a large amount of protein postruminally but the protein has a poor balance of amino acids. Feathermeal is a good example of this. It is not readily degradable in the small intestine. Microbial protein has a better profile of amino acids compared with certain animal and vegetable proteins commonly used as supplements. Diets need to be balanced to provide nutrients in the correct amounts and forms for maximal microbial protein synthesis. This will improve protein status as well as improve feed intake and energy status of the animal. There is considerable data to reveal that feeding RUP supplements ( corn gluten meal, dehydrated alfalfa meal, brewer's grains, distiller's grain, and blood meal ) in combination with urea improved growth rate or feed efficiency compared with feeding soybean meal or urea alone. When performance improved, it was likely attributed to an increase in flow of essential amino acids that were limiting with either the soybean meal or urea alone. In these situations, one must realize that the supplement was provided with moderate to low energy diets based on forages.

For high concentrate diets based on corn, feeding high levels of RUP may actually reduce performance compared with feeding soybean meal. This may occur if a large proportion of dietary protein passes out of the rumen intact, ammonia nitrogen concentration may be insufficient for optimal synthesis of microbial protein. This is especially true for corn based diets because corn protein is quite resistant to ruminal degradation ( RUP = 50 to 60 % of total corn protein ). In this case it is important that some highly degradable source of nitrogen, such as urea, be fed in combination with slowly degradable protein to meet ammonia requirements of ruminal microbes.

Supplementing diets with RUP does not always improve amino acid flow to the small intestine or growth rate and feed efficiency. This may be due to reduction in microbial proteins synthesis when diets contain high levels of RUP. In some cases protein is not the first limiting nutrient so increasing protein supply has minimal effect on growth rate. Positive responses to RUP occur when rapidly growing, immature ruminants are fed diets containing low to moderate levels of metabolizable energy.

Crude Protein Strategies for Winter Nutrition of Cows

Sources of Supplemental Protein

Oilseeds and oilseed meals: Cottonseed, soybean meal, canola, sunflower.

Animal and grain byproducts: blood meal, fish meal, feather meal, brewers grain, distillers grains

Legume hays: alfalfa hay

Non-protein nitrogen: urea, biuret.

Available in 2 forms: dry feeds: meals, cubes, cakes, pellets, pressed blocks, alfalfa hay.

Liquid feeds: molasses-mixes, hardened molasses blocks or tubs.

When considering these choices, one bases his/her decision on two things:

Supplemental delivery system

Cost / lb of supplemental Crude protein.

Example:alfalfa hay at 19 % CP and cost $100 / ton vs soybean meal at 49 % CP, and costing $215 / ton.Based on cost which one would you choose?

Soybean meal: 2000 lb X .49 CP = 980 lbs of CP;$215 /980 lbs = $.22 / lb

Alfalfa hay: 2000 lb X 19 % CP = 380 lb CP;$100 / 380 lbs = $.26 / lb

Based on these calculations and costs, soybean meal would be the choice of supplemental protein.

Choice of delivery method will determine if a CP supplement will be hand-fed or self-fed.Hand-feeding involves regularly providing a supplement to animal in a manner that allows rapid consumption.Self-feeding involves periodically providing large quantities of supplement with assumption that animals will consume the supplement in consistent controlled amounts over an extended period of time ( salt mixes, molasses mixes, blocks, tubs, as examples).Self fed supplements usually require less labor compared to hand-feeding but they are more expensive per pound. They may also result in more variation in supplemental intake.

Winter supplementation is expensive, consisting of cost of the supplement, labor and equipment associated with supplement delivery.Other than determining type and quantity of a CP supplement to purchase, a beef producer has little control over supplement cost. However, a beef producer has significant control over labor and associated supplement delivery costs. One method would be offer CP supplementstwice or three times per week, rather than feeding supplement every day in order to reduce labor costs.

Recent research indicates that infrequent supplementation of CP to beef cattle fed low-quality forage is economically feasible management practice. Infrequent supplementation of CP to beef cattle improved nitrogen utilization and animal performance compared to no nitrogen supplementation. Animal performance and nitrogen utilization were not greatly different between daily and infrequent supplementation schedules. However, there were suggestions that more frequent CP intake may increase forage intake and improve nitrogen utilization efficiency.Consequently decreasing the frequency of CP supplementation is a management practice that can decrease labor and associated costs of a winter supplementation program while maintaining animal performance. However, infrequent supplementation of urea should be conducted with extreme caution to minimize potential for urea toxicity. In contrast biuret does not pose the toxicity concerns associated withurea and should be safe to supplement infrequently.