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Interpreting Milk Urea Nitrogen (MUN) Values


Introduction

This fact sheet has been developed to support the implementation of the Natural Resources Conservation Service Feed Management 592 Practice Standard. The Feed Management 592 Practice Standard was adopted by NRCS in 2003 as another tool to assist with addressing resource concerns on livestock and poultry operations. Feed management can assist with reducing the import of nutrients to the farm and reduce the excretion of nutrients in manure.

The Natural Resources Conservation Service has adopted a practice standard called Feed Management (592) and is defined as “managing the quantity of available nutrients fed to livestock and poultry for their intended purpose”. The national version of the practice standard can be found in a companion fact sheet entitled “An Introduction to Natural Resources Feed Management Practice Standard 592”. Please check in your own state for a state-specific version of the standard.

Milk processing plants and DHI can provide dairy managers with milk urea nitrogen (MUN) values on bulk milk and individual cow milk samples. Milk Urea Nitrogen is a useful tool that can allow dairy managers to monitor changes in the feeding and management of their herds. The following points can allow you to interpret MUN test results from your herd.

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Milk Urea Nitrogen (MUN)

Milk urea nitrogen is the fraction of milk protein that is derived from blood urea nitrogen (BUN). In Holstein’s, MUN normally represents about 0.19 percentage points of the normal 3.2% total milk protein.

Casein and/or whey proteins that contribute amino acids for human use or cheese production are not included in MUN values. Average MUN values will range from 10 to 14 milligrams per deciliter (usually reported as a whole number such as 12). When cows consume feed containing protein, If bacteria cannot capture the ammonia and convert it to microbial protein, the excess ammonia is absorbed part of the protein is degraded to ammonia by rumen microbes (rumen degraded protein or RDP). across the rumen wall. Because ammonia can shiftblood pH, the liver converts ammonia to urea to be excreted or recycled. Urea diffuses freely across cell membranes, therefore MUN concentrations represent blood urea concentrations. Thus, if BUN values are elevated, MUN will be elevated. If MUN values are high, your herd is possibly wasting feed protein along with excreting excess nitrogen into the environment. If MUN values are too low, the rumen bacteria yield can be reduced thereby limiting milk production and milk protein yield.

Feeding Factors That Impact MUN

The key factor is providing adequate rumen available carbohydrates to provide the energy for the rumen microbes to convert ammonia into microbial protein. The following feeding situations could lead to higher MUN values in your herd.

  1. Feeding too much total crude protein in the ration may result in the excess protein being wasted.
  2. Feeding too much rumen degraded protein (RDP) and/or soluble protein can raise MUN even if ration crude protein was normal.
  3. If rumen acidosis occurs, microbial protein growth will be inhibited and ammonia is not captured.
  4. Rations low in fermentable carbohydrate (such as starch, sugar, and/or digestible fiber) can reduce microbial growth leading to higher MUN values.

Target MUN values

Every herd can have a different optimal MUN depending on the time of feeding relative to milking time, total mixed rations (TMR) compared to component-fed herds, cow eating patterns, and other factors that affect BUN values. The power of a MUN tests is to monitor changes in feeding and management programs within a herd.

  1. Develop a MUN baseline that is “normal” for your herd (values may range from 8 to 16).
  2. When the farm baseline changes by more than 2 to 3 points (normal variation), look for changes in your herd that caused this MUN shift.
  3. Look at weekly averages as large variations occur day to day.
  4. DHI and milk plant MUN values will vary due to machine standards and sampling differences.

Feed and Management Changes Leading To Higher MUN Values

  1. New crop corn silage may not have the same level of fermentable carbohydrate (less starch or starch is not available).
  2. Putting cows on lush pasture can increase total and degradable protein intake.
  3. Shifting to a different crop of hay silage that is wetter or higher in crude protein can elevate MUN.
  4. Grinding your grain coarser may reduce the rate of fermentation in the rumen.
  5. Shifting from processed corn silage to unprocessed or improperly processed corn silage means less fermentable starch is available.
  6. Shifting to a more degradable protein source (shifting from heat-treated soybeans to raw soybeans for example) results in more rumen ammonia.

Feed and Management Changes With Low MUN Values (< 8-9)

If the rumen does not maintain a minimum level of ammonia, milk yield and milk protein yield may drop because of reduced microbial protein synthesis. If your herd MUN is low, consider adding supplemental protein, different protein sources and/or other ration change and then monitor your herd for changes in MUN concentrations.

Herd vs. Individual MUN Values

Herd MUN values are similar to herd somatic cell counts when interpreting results. DHI processing centers may provide MUN group averages summarized by lactation number, days in milk, and milk production. Pennsylvania workers recommend a minimum of 8 to 10 cows per group in order to calculate an unbiased group MUN value. There are a number of factors that can influence your MUN values. These include:

  • Breed – Holsteins usually have a lower MUN value than other dairy breeds. However, this may be due to body weight rather than a breed difference.
  • Season – MUN values tend to be higher in the summer months.
  • Sampling time – MUN values usually peak 3-5 hours after feeding.
  • Milking frequency – Herds milked 3x tend to have higher MUN values than herds milked 2x.

AM-PM samples – The AM MUN value is usually lower than PM samples taken from the same herd. When comparing MUN values in your herd between months, be sure to account for differences in sampling times.

Fine Tuning MUN Values

MUN is one tool to evaluate ration protein and energy status. Remember that MUN’s can be impacted by heat stress (MUN values are higher in the summer). Evaluate the following management factors along with herd or group MUN values.

  1. Check rations to determine if the crude protein is too low (less than 15 percent for example) or too high (over 18 percent crude protein). Review the level of RDP (60-65% of the total crude protein), RUP (35-40 percent of total crude protein), and SP (50% of RDP).
  2. Check ration starch levels (24 to 28 percent of the ration dry matter) and ration sugar levels (4 to 6 percent of total ration dry matter).
  3. Evaluate the ratio of true milk protein to milk fat. For Holsteins, the ratio of milk true protein to milk fat is 82 percent (for example 3.0 percent true milk protein and 3.7 percent milk fat). A low MUN could result in a value of less than 75 percent.
  4. Evaluate manure consistency. Cows with low MUN could have firm manure compared to cows with looser manure and higher MUN’s. However, there are a number of others factors that can contribute to manure consistency differences in a herd.

Applying MUN Values to Calculate Nitrogen Losses

Wisconsin workers have developed an equation to predict the loss of nitrogen based on body weight and MUN values. Other equations are also available and could be used.

Urinary excretion of nitrogen = Body weight x 0.0129 x MUN (mg/dl)

Two examples are calculated below using a low (10 mg/dl) and average (14 mg/dl) MUN’s.

1500 lb Holstein cow x 14 MUN x 0.0129 = 271 grams of urinary nitrogen 1500 lb Holstein cow x 10 MUN x 0.0129 = 194 grams of urinary nitrogen

The difference of 77 grams represents a loss of one pound of dietary protein or 2.2 lb of soybean meal plus the added environmental risks of disposing of the urinary nitrogen. This is equal to about 52 lbs. of N excreted per cow during a 305-day lactation.

Take Home Message

  • MUN values can be used to the efficiency of microbial protein synthesis there by reducing nitrogen excretion into the environmental
  • MUN values will vary from herd to herd, so the key benefit is to make comparisons within a herd or groups of cows in a herd
  • If MUN levels (10-14 mg/dl) are outside normal ranges, look at ration balancing results, milk components, feeding management and nutrient balance.

References

Jonker, J.S., R.A. Kohn and J. High. 2002. Use of milk urea nitrogen to improve cow diets. J. Dairy Sci. 85:939-946.

Kauffman, A.J. and N.R. St-Pierre. 2001. The relationship of milk urea nitrogen to urine nitrogen excretion in Holstein and Jersey cows. J. Dairy Sci. 84:2284-2294.

Nousiainen, J., K.J. Shingfield and P. Huhtanen. 2004. Evaluation of milk urea nitrogen as a diagnostic of protein feeding. J. Dairy Sci. 87:386-398.

Wattiaux, M.A., E.V. Nordheim and P. Crump. 2005. Statistical evaluation of factors and interactions affecting Dairy Herd Improvement milk urea nitrogen values in commercial Midwest dairy herds. J. Dairy Sci. 88:3020-3035.

“Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, sex, religion, age, color, creed, national or ethnic origin; physical, mental or sensory disability; marital status, sexual orientation, or status as a Vietnam-era or disabled veteran. Evidence of noncompliance may be reported through your local Extension office.”

 

Disclaimer

This fact sheet reflects the best available information on the topic as of the publication date. Date 6-20-2007

This Feed Management Education Project was funded by the USDA NRCS CIG program. Additional information can be found at Feed Management Publications.

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This project is affiliated with the LPELC.

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Project Information

Detailed information about training and certification in Feed Management can be obtained from Joe Harrison, Project Leader, jhharrison@wsu.edu, or Becca White, Project Manager, rawhite@wsu.edu.

Author Information

Mike Hutjens
Extension Dairy Specialist
University of Illinois, Urbana

Larry E. Chase
Extension Dairy Nutritionist
Cornell University, Ithaca, NY

Reviewer Information

Dave Casper – Agri-King, Inc.

Jim Drackley – University of Illinois

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Feeding Dairy Cows: In Vitro NDF Digestibility


Introduction

This fact sheet has been developed to support the implementation of the Natural Resources Conservation Service Feed Management 592 Practice Standard. The Feed Management 592 Practice Standard was adopted by NRCS in 2003 as another tool to assist with addressing resource concerns on livestock and poultry operations. Feed management can assist with reducing the import of nutrients to the farm and reduce the excretion of nutrients in manure.

The Natural Resources Conservation Service has adopted a practice standard called Feed Management (592) and is defined as “managing the quantity of available nutrients fed to livestock and poultry for their intended purpose”. The national version of the practice standard can be found in a companion fact sheet entitled “An Introduction to Natural Resources Feed Management Practice Standard 592”. Please check in your own state for a state-specific version of the standard.

Conclusions reached regarding in vitro neutral fiber digestibility (IVNDFD) and its impact on lactation performance in a literature review for a symposium presentation at the 2006 ADSA/ASAS Annual Meeting (Shaver, 2006) were as follows:

  • IVNDFD has been related to > milk production across an array of different forages.
  • Milk production response to IVNDFD is thru DMI, and not energy density.
  • DMI and milk production responses to IVNDFD > in higher producing cows.
  • Benefits of brown midrib corn & sorghum silages for IVNDFD, DMI, and milk production have been observed consistently.
  • More IVNDFD/in vivo research is needed with legumes & other grasses.
  • Increased IVNDFD has not been fully exploited by researchers in trials attempting to maximize dietary forage or optimize forage mixtures, or by field nutritionists feeding higher forage diets with the aim of improving cow health.

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IVNDFD Analysis

Several commercial testing laboratories offer wet chemistry IVNDFD measurements. Ranges for IVNDFD of forages are presented in Table 1. The IVNDFD values are highly variable among and within forage types. Introduction of low-lignin, brown midrib hybrids for production of corn and sorghum silages has widened the variation in IVNDFD for these forage types (Oba and Allen, 1999b). NIRS calibrations for predicting IVNDFD on hay-crop forage and corn silage samples are available at some commercial forage testing laboratories. However, Lundberg et al. (2004) found poor prediction by NIRS of legume-grass silage and corn silage IVNDFD. It is hoped that NIRS calibration equations can be improved upon in the future.

The NRC (2001) recommended a 48-h IVNDFD for use in the NRC (2001) model, and for that reason we used 48-h IVNDFD measurements in MILK2000 (Schwab et al., 2003). However, debate continues within the industry about the appropriateness of 48-h vs. 30-h IVNDFD measurements. Some argue that the 30-h incubation better reflects ruminal retention time in dairy cows (Oba and Allen, 1999a) and that most of the in vivo trials that have evaluated effects of varying IVNDFD on animal performance also performed 30-h IVNDFD measurements (Oba and Allen, 2005). Labs and their customers also like the faster sample turn around that is afforded by the 30-h incubation time point. For that reason, and also for improved lab operation efficiency, a 24-h incubation time point is being employed by some labs. However, some argue that the 48-h incubation time-point is less influenced by lag time and rate of digestion, and thus is more repeatable in the laboratory (Hoffman et al., 2003). Hoffman et al. (2003) provided data on the relationship between 30- and 48-h IVNDFD measurements that showed a strong positive relationship (r-square = 0.84). But, the lab average at a specific incubation time point and the relationship between incubation time points within a lab can be highly variable among labs making the development of a universal incubation time point adjustment equation difficult. The average lignin-calculated corn silage NDF digestibility in the NRC (2001) is 59%. This reference point is important for adjustment of IVNDFD values from different labs and varying incubation time points so that the resultant TDN and NEL values are comparable to NRC (2001) values.

Average IVNDFD values for selected high-fiber by-product feeds (Peter Robinson, CA-Davis, personal communication) are presented in Table 2. The IVNDFD values are highly variable among these high-fiber by-product feeds. The IVNDFD values for these high-fiber by-product feeds were poorly related to lignin-calculated (NRC, 2001) NDF digestibility. High digestible NDF (dNDF; % of DM) for soy hulls and beet pulp relative to other high-fiber by-products suggest a high potential for using these ingredients at reasonable inclusion rates to partially replace forage with low fiber digestibility to increase diet dNDF. Monitoring and maintaining effective NDF in the diet is critical when employing this feeding strategy.

The distribution of 48-h IVNDFD for high-group TMR samples from commercial dairies analyzed at the University of Wisconsin Forage Testing Laboratory (Marshfield, WI; Hoffman, 2003) is presented in Figure 1 with an average IVNDFD of 57.2% of NDF. The IVNDFD range for these high-group TMR samples is wide and raises concern over intake limitations on the low end and lack of effective fiber on the high end. Analyzing for IVNDFD offers another tool for troubleshooting fiber status of dairy cattle diets.

Table 1. Variation within forages for neutral detergent fiber digestibility measured in situ or in vitro.
Forage IVNDFD (% of NDF)
Nocek and Russell, 1998 Legumes 31-63
Grasses 41-77
Corn Silage 32-68
Allan and Oba, 1996 Alfalfa 25-60
Whole-Plant Corn 30-60
Hoffman, 2003 (UWFTL) Legumes 35-65
Grasses 25-75
Corn Silage 40-75
Chase, 2003 (Dairy One) Legumes 34-57
Grasses 25-75
Corn Silage 45-64

 

Table 2. Content and digestibility of NDF for selected high-fiber by-product feeds.
Ingredient NDF, % DM1 IVNDFD, % NDF2 dNDF, % DM
Forages 40-60 30-60 10-35
Corn gluten feed 36 80(1)3 29
Distillers grains 39 75 (14) 29
Brewers grains 47 50(2) 24
Wheat midds 37 50(3) 19
Beet pulp 46 85(10) 39
Citrus pulp 24 85(2) 20
Soy hulls 60 90(2) 54
Whole cottonseed 50 50(36) 25
Cottonseed hulls 85 20(4) 17
Almond hulls 37 40(5) 15
1NRC, 2001.
230-h IVNDFD (% NDF) adapted from Dr. Peter Robinson, CA-Davis.
3(n).

 

Figure 1. Distribution of 48-h IVNDFD (% of NDF) in data set of 377 high-group TMR samples from commercial dairies analyzed at UW Soil & Forage Analysis Lab, Marshfield, WI (Hoffman, 2003).

 

References

  • Allen, M., and M. Oba. 1996. Fiber digestibility of forages. Pages 151-171 in Proc. MN Nutr. Conf. Bloomington, MN.
  • Arieli, A., and G. Adin. 1994. Effect of wheat silage maturity on digestion and milk yield in dairy cows. J. Dairy Sci. 77: 237-243.
  • Aydin, G., R. J. Grant, and J. O’Rear. 1999. Brown midrib sorghum in diets for lactating dairy cows. J. Dairy Sci. 82: 2127-2135.
  • Bal, M. A., R. D. Shaver, H. Al-Jobeile, J. G. Coors, and J. G. Lauer. 2000. Corn silage hybrid effects on intake, digestion, and milk production by dairy cows. J. Dairy Sci. 83: 2849-2858.
  • Chase, L. E. 2003. Update on forage digestibility. Page 25 in Proc. 2003 Dealer Seminars. Cornell Univ. Coop. Ext. Anim. Sci. Mimeo Series. No. 223.
  • Chow, L., M. Oba, V. Baron, and R. Corbett. 2006. Effects of advanced in vitro fiber digestibility of barley silage on dry matter intake and milk yield of dairy cows. J. Dairy Sci. 89(Suppl.1):263(Abstr.).
  • Dado, R. G., and M. S. Allen. 1996. Enhanced intake and production of cows offered ensiled alfalfa with higher neutral detergent fiber digestibility. J. Dairy Sci. 79: 418-428.
  • Dhiman, T. R., and L. D. Satter. 1997. Yield response of dairy cows fed different proportions of alfalfa silage and corn silage. J. Dairy Sci. 80: 2069-2082.
  • Grant, R. J., S. G. Haddad, K. J. Moore, and J. F. Pedersen. 1995. Brown midrib sorghum silage for midlactation dairy cows. J. Dairy Sci. 78: 1970-1980.
  • Hoffman, P. C. 2003. New developments in analytical evaluation of forages and total mixed rations. Proc. Symposium & Joint Mtg. Of WI Prof. Nutrient Applicators, WI Custom Operators, and WI Forage Council. WI Dells, WI.
  • Hoffman, P. C., Lundberg, K. L., L. M. Bauman, and R. Shaver. 2003. In vitro NDF digestibility of forages: The 30 vs. 48 hour debate. Univ. of WI Extension Focus on Forage Series. Vol. 5, No. 16. http://www.uwex.edu/ces/crops/uwforage/30vs48-FOF.htm.
  • Hoffman, P. C., S. J. Sievert, R. D. Shaver, D. A. Welch, and D. K. Combs. 1993. In situ dry matter, protein, and fiber degradation of perennial forages. J. Dairy Sci. 76: 2632-2643.
  • Kendall, C., and D. K. Combs. 2004. Intake and milk production of cows fed diets that differed in dietary NDF and NDF digestibility. J. Dairy Sci. 87(Suppl.1):340(Abstr.).
  • Ivan, S. K., R. J. Grant, D. Weakley, and J. Beck. 2005. Comparison of a Corn Silage Hybrid with High Cell-Wall Content and Digestibility with a Hybrid of Lower Cell-Wall Content on Performance of Holstein Cows. J. Dairy Sci. 2005 88:244-254.
  • Llamas-Lamas, G., and D. K. Combs. 1990. Effect of Alfalfa Maturity on Fiber Utilization by High Producing Dairy Cows. J. Dairy Sci. 73: 1069-1080.
  • Lundberg, K. L., P. C. Hoffman, L. M. Bauman, and P. Berzaghi. 2004. Prediction of forage energy content by near infrared reflectance spectroscopy and summative equations. Prof. Anim. Sci. 20:262-269.
  • Mertens, D. R., H. G. Jung, M. L. Raeth-Knight, and J. G. Linn. 2005. Impact of alfalfa hay neutral detergent fiber concentration and digestibility on Holstein dairy cow performance: I. Hay analyses and lactation performance – USDFRC. J. Dairy Sci. 88(Suppl.1):250(Abstr.).
  • National Research Council. 2001. Nutrient Requirements of Dairy Cattle. 7th rev. ed. Natl. Acad. Sci., Washington, DC.
  • Oba, M. and M. S. Allen. 2000. Effects of brown midrib 3 mutation in corn silage on productivity of dairy cows fed two concentrations of dietary neutral detergent fiber: 1. Feeding behavior and nutrient utilization. J. Dairy Sci. 83:1333-1341.
  • Oba, M. and M. S. Allen. 1999a. Effects of brown midrib 3 mutation in corn silage on dry matter intake and productivity of high yielding dairy cows. J. Dairy Sci. 82:135-142.
  • Oba, M. and M. S. Allen. 1999b. Evaluation of the importance of the digestibility of neutral detergent fiber from forage: effects on dry matter intake and milk yield of dairy cows. J. Dairy Sci. 82:589-596.
  • Oliver, A. L., R. J. Grant, J. F. Pedersen, and J. O’Rear. 2004. Comparison of brown midrib-6 and -18 forage sorghum with conventional sorghum and corn silage in diets of lactating dairy cows. J. Dairy Sci. 87: 637-644.
  • Raeth-Knight, M. L., J. G. Linn, H. G. Jung, D. R., Mertens, and P. R. Peterson. 2005. Impact of alfalfa hay neutral detergent fiber concentration and digestibility on Holstein dairy cow performance: II. Lactation performance – St. Paul. J. Dairy Sci. 88(Suppl.1):250(Abstr.).
  • Ruiz, T. M., E. Bernal, C. R. Staples, L. E. Sollenberger, and R. N. Gallaher. 1995. Effect of dietary neutral detergent fiber concentration and forage source on performance of lactating cows. J. Dairy Sci. 78: 305-319.
  • Schwab, E. C., R. D. Shaver. J. G. Lauer, and J. G. Coors. 2003. Estimating silage energy value and milk yield to rank corn hybrids. J. Anim. Feed Sci. Technol. 109:1-18.
  • Shaver, R. D. 2006. Forage intake, digestion and milk production by dairy cows. J. Dairy Sci. 89(Suppl.1):298(Abstr.).
  • Tessmann, N. J., H. D. Radloff, J. Kleinmans, T. R. Dhiman, and L. D. Satter. 1991. Milk production response to dietary forage:grain ratio. J. Dairy Sci. 74: 2696-2707.
  • Tine, M. A., K. R. Mcleod, R. A. Erdman, and R. L. Baldwin, VI. 2001. Effects of brown midrib corn silage on the energy balance of dairy cattle. J. Dairy Sci. 84: 885-895.

“Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, sex, religion, age, color, creed, national or ethnic origin; physical, mental or sensory disability; marital status, sexual orientation, or status as a Vietnam-era or disabled veteran. Evidence of noncompliance may be reported through your local Extension office.”

Disclaimer

This fact sheet reflects the best available information on the topic as of the publication date. Date 5-25-2007

This Feed Management Education Project was funded by the USDA NRCS CIG program. Additional information can be found at Feed Management Publications.

Image:Feed mgt logo4.JPG This project is affiliated with the Livestock and Poultry Environmental Learning Center.

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Project Information

Detailed information about training and certification in Feed Management can be obtained from Joe Harrison, Project Leader, jhharrison@wsu.edu, or Becca White, Project Manager, rawhite@wsu.edu.

Author Information

Randy Shaver
Professor and Extension Dairy Nutritionist
Department of Dairy Science
College of Agricultural and Life Sciences
University of Wisconsin – Madison
University of Wisconsin – Extension

Reviewer Information

Jim Barmore – Nutrition Consultant
Pat Hoffman – University of Wisconsin

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Silage and Dry Hay Management

Introduction

This fact sheet has been developed to support the implementation of the Natural Resources Conservation Service Feed Management 592 Practice Standard. The Feed Management 592 Practice Standard was adopted by NRCS in 2003 as another tool to assist with addressing resource concerns on livestock and poultry operations. Feed management can assist with reducing the import of nutrients to the farm and reduce the excretion of nutrients in manure.

Forages stored as silage and dry hay crops can be in excess of 50% of the total dry-matter in a ration, and must have maximum nutrient availability in the tightly balanced rations fed to high-production dairy cows. Feeding high quality forages minimizes nutrient loss of the livestock enterprise through nutrient conservation either in the hay bale or in the silo, and by how well the animal utilizes nutrients from preserved forages during digestion.

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Ensiled Forages

Oxygen consuming aerobic and non-oxygen consuming anaerobic bacteria are involved in silage fermentation. Aerobic activity occurs while the silo is being filled and at feedout and are the primary times when dry matter losses occur in the silo. Good silo management minimizes aerobic activity, thus reducing dry-matter losses and maximizes the anaerobic conversion of water-soluble carbohydrate to silage acids, reducing pH to a range that is inhospitable to spoilage organisms. Silage fermentation can be divided into six phases.

Forage Quality Fig 1.jpg

Phase 1

The first phase begins when the plant is harvested. During this phase, aerobic micro-organisms coming in with the crop cause nutrient loss by converting water-soluble carbohydrates to carbon dioxide, water, and heat. In addition, other gases are produced, due to the enzymatic proteolysis of protein, that have environmental concerns such as ammonia nitrogen and various forms of nitrogen oxides. Phase 1 fermentation continues until either oxygen is depleted or water-soluble carbohydrate is exhausted. Aerobic activity should last only a few hours if ideal ensiling moisture, chop length, compaction, and covering management guidelines are practiced by the silage management team.

Phase 2

Depletion of trapped oxygen during the initial aerobic phase triggers the second anaerobic phase of fermentation with the production of several fermentation end-products. These bacteria produce short-chain volatile fatty acids (acetate, lactate, propionate, and butyrate), ethanol, and carbon dioxide. In addition, more nitrogenous end-products can be generated due to continued enzymatic proteolysis of protein as described during the aerobic phase. The phase 2 bacteria tend to be inefficient fermenters, contributing to dry matter losses of forages stored in silos. The proportions of fermentation end-products produced depend on crop maturity, moisture, and epiphytic bacterial populations of the harvested forage.

The 2nd phase bacteria create an environment for another more efficient class of anaerobic bacteria, commonly known as lactic acid bacteria (LAB). An initial drop in pH signals the end of the early anaerobic phase, which generally lasts no longer than 24 to 72 hours.

Phases 3-4

Phases three and four of fermentation occur when LAB convert water-soluble carbohydrate to lactic acid. As the heat generated in phase 1 starts to dissipate and pH becomes lower, other LAB will become active in the production of lactic acid. This acid is the strongest and most efficient volatile fatty acid for rapidly reducing pH. Lactic acid generally predominates in the best-quality silage (more than 60% of the total volatile fatty acids) and can be present at levels as high as 3% to 6% of dry matter. A predominance of LAB relative to other silage acid producing bacteria creates a faster fermentation, conserving more nutrients in the form of water-soluble carbohydrate, peptides, and amino acids. Anaerobic phases continue until forage pH is sufficiently low to inhibit, but not destroy, the growth potential of all organisms. Natural fermentation accomplished solely by epiphytic organisms and unassisted by any type of silage additive takes 3-7 days in corn silage and 7-14 days with legume silages. This time span depends on the buffering capacity, moisture, and maturity of the crop to be ensiled. The rate and end-point of the final pH drop in the ensiled crop depends largely on the type and moisture of forage being ensiled. Corn silage pH terminates at or below pH 4. Legumes, which have less water-soluble carbohydrate content and a higher buffering capacity, generally reach a terminal pH of approximately 4.5. When terminal pH is reached, the forage is in a preserved state.

Phase 5

The fifth stage of fermentation, the stable phase, lasts throughout storage. This phase is not static because other anaerobic bacteria become active during this period producing end-products that: 1) are anti-mycotic and will be conducive to bunklife stability and 2) will enhance starch and fiber digestibility of silages during feedout. Silage management practices resulting in high silage densities, maintaining silo structure integrity, and face management dictates the efficiency of phase 5 fermentation.

Phase 6

The final fermentation phase, occurs when silage is fed from the storage structure. This phase is as important as the others but is often neglected. Up to 50% of dry-matter losses result from secondary aerobic spoilage on the surface of the silage in storage and in the feedbunk. Aerobic microbial activity is stimulated because oxygen is introduced into the silo. The aerobic activity produces heat and reduces the palatability and nutrient availability of silage. Bunklife challenges increase with high application of manure that may have inoculated the crop with mold and yeast spores. Forages wilting in swaths and windrows can become contaminated with soil-borne organisms through raking or by rain splashing soil onto the swaths. Aerobic stability of silage is more of a problem if the crops had been exposed to environmental stresses.

Harvesting, Ensiling, and Feedout

There are nine rules for successful fermentation. First, the storage structure must be of the correct size so that silage removal rates can be maintained at least 6 inches per day for bunker and piled silos. The feedout rate for upright silos is 4 to 6 inches/day in summer and 2 to 4 inches/day in the winter.

The second and third management rules are to harvest at proper maturity and moisture level. Table one lists recommendations based on crop and type of storage structure. Proper maturity and moisture provides optimal nutrition for dairy production, more water-soluble carbohydrates, and promotes the elimination of oxygen to maximize anaerobic fermentation.

Forages ensiled at moisture levels greater than 70% may undergo undesirable secondary clostridial fermentation. Wet forages have a low concentration of water-soluble carbohydrate. Thus, anaerobic bacteria may not have enough sugar to produce sufficient volatile fatty acids to achieve a pH below 5. A pH over 5 creates an environment suitable for secondary fermentation by clostridia. These anaerobes degrade lactate and amino acids and produce butyric acid (which is a weaker acid). They also consume lactate (thus causing terminal pH to rise) and yield an unpalatable, rancid silage.

Clostrial fermentation can produce silage with extended aerobic stability, it is nevertheless undesirable because it can break down amino acids and reduce palatability, which can reduce intake. Unwilted silages may require a pH near 4.0 to completely inhibit clostridial fermentation. Alfalfa and grasses are swathed at 80% to 85% moisture and require wilting to less than 70% before ensiling. Corn forage, in contrast, is chopped at the proper moisture and requires no wilt time.

Legumes and grasses must have proper wilt time between cutting and harvesting to permit evaporation of moisture. Wilting concentrates plant sugars essential for fermentation. Creating wide swaths during harvest permit a faster wilt time and conserves valuable plant sugars and protein nutrients. The following table from University of Wisconsin research shows that wide swaths conserves non-fiber carbohydrates (NFC) and results in higher relative forage quality (RFQ). Merging equipment is then used to build windrows just before chopping the crop.

Difference in composition of alfalfa haylage made from narrow and wide swaths, UW Arlington, 2005
Undersander, University of Wisconsin
Factor Wide Narrow Difference
NDF, % 37.8 40.1 -2.3
NFC, % 38.4 36.5 1.8
Ash, % 9.3 9.9 -0.6
TDN, 1x 63.5 62.6 0.9
Lactic acid, % 5.6 4.6 1.0
Acetic acid, % 2.4 1.9 0.5
RFQ 166 151 15

The fourth rule is that chopper knives must be sharp and the shearbar properly adjusted for desired theoretical length of cut (TLC). Sharp knives ensure a clean chop and prevent shredding, decreasing chances of effluent production. Table one suggests lengths of cut for various forages. This adjustment is critical to maximize forage compaction for efficient fermentation while providing sufficient particle length for proper rumination. If corn forage crops are kernel processed, the roller mill settings must be adjusted so that all corn kernels are fractured during harvest. The shearbar TLC is usually longer with processed corn silage compared to non-processed for providing better source of effective fiber to the dairy or beef animal.

The fifth rule is that the structure must be filled as rapidly as possible to diminish phase 1 losses. Bunker silos and piles should not have more than a 3:1 slope on sides and ends so that proper packing is achieved during silo filling.

The sixth rule is to pack silage bunkers and piles so that at least 800 lbs of tractor weight is used per hour per ton. For example, if 100 tons of forage is delivered to the silo/hr, then a tractor with 80,000 lbs (100 X 800) of pack weight or 40 tons needed to meet packing needs. No more than 6 inches of forage should ever be packed at any given time. Tractor weight delivered to forage is drastically lessened when silage depths are in excess of 6 inches. The person on the pack tractor has the most important role in filling bunker silos. Packing ensures the elimination of oxygen, thus minimizing aerobic activity and maximizing anaerobic fermentation.

The seventh rule is to seal the silo properly. A plastic tarp is usually used to cover the entire bunker surface, and a net of tires holds the tarp in place. Double sheets of tarp and lining the bunker walls with tarp will further reduce dry matter losses during fermentation. A new “saran wrap” cling type plastic known as Silo-Stop® is available for placement between the silage surface and tarp, which will further mitigate surface spoilage losses. Kansas State research shows that dry matter losses in the top four feet of uncovered horizontal silos can be in excess of 33% while normal dry matter losses are about 15%. A favorable return on investment for covering exists by minimizing the amount of surface dry-matter losses.

The eighth rule is to maintain a proper feedout rate. Losses resulting from slow removal account for up to 50% of dry-matter losses, which will be in the form of plant sugars, not fiber. Aerobic organisms consume water-soluble carbohydrate, thus reducing the energy content of the forage.

Silages should be removed from bunker and pile faces by shaving the silage face from top to bottom with the loader bucket rather than by lifting the bucket from the bottom to the top. Lifting creates fracture lines in the stored mass, thus allowing oxygen to enter and sustain aerobic activity. Even if farmers remove a desirable 6 inches from the silo face every day, oxygen may penetrate several feet into the stored mass allowing aerobes to generate heat. Consequently, farmers cannot get ahead of aerobic instability.

Silage additives should only be used as the ninth rule if the other conditions for producing high-quality silage can be met. The most common additives are bacterial inoculants, enzymes, acids, and nutrients. Good silage can be made better through the proper use of effective silage additives.

Dry Haycrop Forages

Haycrop preservation is a result of moisture reduction, which creates an environment unsuitable to spoilage from microbial activity. Haycrops dry in 3 phases. Phase 1 is very rapid loss of moisture down to 60-65% moisture. Phase 2 is a slower process down to about 40% moisture. Phase 3 is the longest phase reaching moistures levels that can be safe for storing dry hay. Hay does not become static until it reaches about 12% moisture and the equilibrium humidity is below 65% at which time most fungi will not grow.

If hay is baled at higher moistures and not protected by a preservative or inoculant, several peaks of heating may occur. The first temperature peak will generally occur within a few days and can be the result of aerobic bacterial growth, fungal growth and/or plant respiration. If oxygen and a favorable moisture level are available, microorganisms begin to multiply, generating heat up to 130 to 140 F. The rise in temperature tends to kill most microorganisms resulting in the gradual decline in internal bale temperatures. The initial heating in hay baled at lower moistures typically drives off moisture. However, in higher moisture bales, the hay moisture combines with water generated in the respiration process, allowing for unusually prolonged conditions that prove optimum for bacterial and fungal growth. The magnitude of peak temperatures will usually be lower each time. Eventually the temperature will stabilize near ambient temperature. These secondary temperature peaks are generally the result of fungal growth. Aerobic fungi are the primary microbes responsible for the breakdown of complex carbohydrates and subsequent generation of heat.

Harvest and Storage Losses

Storage losses are directly related to microbial growth and to subsequent heating. The extent of heating depends largely on: (1) the moisture of the hay, (2) the density and size of the bale, (3) the rate of bale dry-down and (4) the microbial populations that came in with the crop. Microbial activity in hay does not terminate at baling, especially when baling at higher moistures (20-30%) to reduce leaf shatter losses.

Respiration Losses

Cells of cut forages are alive and functioning until the moisture content reaches about 47-48%, below which the cells die. If drying conditions are poor and the cells live a relatively long time, carbohydrates will be depleted and forage quality is diminished. Under good drying conditions, respiration accounts for 2-8% loss in dry matter with losses up to 16% under slow drying conditions.

Management practices that shorten drying time resulting in reduced respiration and harvest losses include: 1) cutting early in the day to maximize solar drying (although plant sugars are lower during morning hours), 2) cutting when anticipated weather will allow for relative humidity of the air to be below the equilibrium humidity of the forage, 3) mechanical or chemical conditioning to crush stems for water escape and 4) maximizing hay exposure to wind and sunlight by creating wide and thin windrows.

Weather Losses

Rain lowers the quality of hay through leaching of water-soluble carbohydrates and prolonging respiration losses. The extent of leaching loss is influenced by several factors including type of forage, stage of maturity, moisture content at the time of rainfall, amount of rainfall, frequency of rain and mowing/conditioning treatments. Alfalfa harvested in the bud stage undergoes more extensive leaching loss than hay harvested in full bloom presumably because the amount of soluble nutrient decrease as the alfalfa plant matures.

Mechanical Losses

Mechanical losses can range from 8-45% and is due to “leaf shatter”. Alfalfa leaves dry down 2-1/2 to 5 times faster than stems and as plant moisture decreases to below 30%, leaves become extremely brittle. Leaf loss is nutritionally important because alfalfa leaves comprise approximately 50% of the crop dry matter and contain over 70% of the plant protein, and 65% of the digestible energy. The extent of mechanical leaf loss is dependent upon crop maturity, moisture content, and rake or baler design. Nearly 50% of the total mechanical losses occur during mowing-conditioning and raking. The final field operation also causes reduction in dry matter yields. Losses from conventional, small rectangular balers range from 3-8% while large baler losses may be as high as 15 percent.

Storage Losses

Hay stored at less than 15% moisture and stored under cover will have up to 10% dry matter losses. When baling moisture exceeds 20%, excessive heating due to spoilage microorganisms result in a browning reaction which reduces the nutritive value of the hay. Excessive heat damage can reduce protein and energy digestibility of the hay.

Mold growth in improperly cured hay can adversely affect palatability and feed intake, although less than 5% of the molds commonly found in hay produce any mycotoxin. Feeding moldy alfalfa hay results in significantly lower dry matter intake, reduced weight gains and poorer feed conversion compared to feeding mold-free hay.

Table 1. Harvest and Moisture Recommendation Table
—–Silo Type—–
Crop Maturity Horizontal Stave Sealed Length of Cut
Silage Crops ……..% moisture…….. inches
Corn Silage Milk line 1/2 – 3/4 Down the kernel (Verify with whole plant DM determination)
Non-Processed 65-72 63-68 50-60 3/8 – 1/2
Processed 62-70 62-68 50-60 3/8 – 3/4
Alfalfa Mid-bud 1/10 bloom and wilt to –> 60-70 63-68 40-60 1/4 – 1/2
Cereal silage Milk or soft dough and wilt to –> 67-72 63-68 55-60 1/4 – 1/2
Grasses When first stems head out 60-70 63-68 40-60 1/4 – 1/2
Forage sorghum Grain medium to hard dough or as leaves begin to lose color 62-72 63-70 60-70 3/8 – 1/2
Dry Hay Crop Bale Type Moisture Range
Alfalfa Mid-bud 1/10 bloom Large Round/
Mid-Square		 Preser-
ative		 No Treatment
Grass When first stems head out Large, Square Bales 13 – 20% <13%
Cereal Milk or soft dough Small, Square Bales 13 – 18% <13%

Selected References

Mahanna, W.C. 1994. Hay Additive Review: Where We’ve Been, Where We’re Going. 24th National Alfalfa Symposium. Feb. 24-25, 1994, Springfield , IL.

Seglar, W.J. 1997. Dairy Production Management-Silage Management. Veterinary Compendium-Food Animal Medicine and Management, Feb. 97.

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Disclaimer

This fact sheet reflects the best available information on the topic as of the publication date. Date 2-2007

This Feed Management Education Project was funded by the USDA NRCS CIG program. Additional information can be found at Feed Management Publications.

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This project is affiliated with the LPELC.

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Project Information

Detailed information about training and certification in Feed Management can be obtained from Joe Harrison, Project Leader, jhharrison@wsu.edu, or Becca White, Project Manager, rawhite@wsu.edu.

Author Information

William J. Seglar DVM, PAS
Nutritional Sciences
Pioneer Hi-Bred International, Inc.
7100 NW 62nd Ave, PO Box 1100
Johnston, Iowa 50131-1100
Bill.Seglar@pioneer.com

Reviewer Information

Brad Harman – Pioneer Hi-Bred International

Greg Zuver – Nutrition Consultant

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