the rations we feed dairy cattle impact manure ch emistry ... · land-spreading in comprehensiv e...

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In the USA, to protect lakes and streams against runoff from agricultural land, rules within the original Clean Water Act were updated to include guidelines for the land application of animal manure. The focus of these rules was on manure P management because the control of P (pollutant source) is considered essential to the protection of surface water quality.

The Clean Air Act was updated later to include the emission of NH3

and other hazardous gasses from livestock farms. Atmospheric NH3

combines with other chemicals to form particulates which are harmful to human health, and the deposition of NH3-base compounds acidifies and fertilizes natural ecosystems which lead to their degradation. The control of urea contained in dairy cattle urine is a critical first step in mitigating NH3 emissions from dairy farms.

The rations we feed dairy cattle impact manure chemistry and nutrient dynamics in soil, water, airJ. Mark Powell

Midwest Manure SummitUniversity of Wisconsin Extension

Green Bay WIFebruary 25, 2015

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The research presented today is trans-disciplinary in nature. The new water quality regulations based partially on manure P management, and the new air quality standards based partially on NH3 emission, created immediate needs for information on relationships between concentrations of P and crude protein (CP) in dairy rations and how these components may be modified to satisfy the nutritional demands of healthy, high producing dairy cows, while at the same time reduce total concentrations and labile forms of P and N in manure.

Determinations of how ration components impact milk production, profits, manure chemistry, water and air quality required trans-disciplinary dairy nutrition-soil science research. Neither dairy nutrition nor environmental sciences alone could have provided this information to the dairy industry.




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The links between manure chemistry and environmental is based on research using diets commonly fed to Wisconsin dairy cows. Very few of the diets impacted milk production.

For example, in a recent review of five nutrition trials with lactating cows (18 dietary treatments comprised mostly of alfalfa silage, corn silage, corn grain, protein supplements and other minor ingredients fed as total mixed rations to 203 mid-lactation cows) seventeen of the diets resulted in significant reductions in urinary urea excretion and only two resulted in significant reductions in milk production: (1) a reduction in dietary CP from 18.4% to 15.1% (achieved by reducing rolled high-moisture shelled corn and solvent-extracted soybean meal in the diet) decreased milk production by 1.1 kg/cow per day, and (2) the reduction in dietary CP from 17.3% or 16.1% to 14.8% (due to reductions of solvent-extracted soybean meal in the diet) decreased milk production by 1.9 kg/cow per day.




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The phosphorus, nitrogen and carbon contained in dairy manure have potential environmental implications. The control of these nutrient excretions in manure through diet manipulation is a key pathway to mitigating potential negative impacts of dairy production systems on water, air and global climate change.




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Regulations on manure land spreading practices were enacted during the late 1990’s to protect lakes, streams and other surface water bodies from runoff and environmental contamination. For example, the phosphorus risk index was developed which includes manure application guidelines based on a combination of soil test P, concentrations of P in manure and other factors. This is why we embarked on research related to relationships between dietary P, manure P and runoff P from manure-amended soils.

During the early to mid 2000’s environmental policy related to agriculture shifted somewhat from concerns related to water quality to concerns related to the emission of ammonia (NH3) and other hazardous gasses from livestock operations. For the dairy industry, the main concern was the emission of NH3, which is generated from the large amounts of urea contained in the urine excreted by dairy cows. This is why we embarked on research related to relationships between dietary crude protein (CP), manure chemistry and environmental outcomes, including gaseous emissions.




Most confinement dairy farms in the Midwest and Northeast regions of the U.S follow a fairly generalized formula of how to produce milk. Cows and replacement heifers are fed primarily homegrown feed and protein and mineral supplements are purchased to compliment dairy diets. The principal diet ingredients are forages from silages of alfalfa and corn, and corn grain. Soybean meal is the most important protein supplement.

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Each diet component has differential impacts on manure chemistry and the environment. For example, mineral P in diets increases the excretion of total P and soluble P in feces (very little P excreted in urine). Runoff of soluble P from cropland after manure application can be related back to the P excreted in manure, which is linked to the amount of mineral P in cow rations. Likewise, the type and amount of crude protein (CP) and forage fed to dairy cows impact the forms of N excreted in manure and therefore manure N cycling in soil, including plant N uptake.

Ammonia emissions from soil after manure application can be related back to the urea N excreted by dairy cows in urine, which is linked to the concentrations of CP in cow rations and the type of forage fed. The type of forage fed impacts enteric methane from dairy cows, the carbon chemistry of feces and carbon sequestration in soils.




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As dietary P concentrations increase, the excretion of total P and soluble P in feces increases. Increases in soils test P and runoff of P from cropland after manure application can be related back to the P excreted in manure, which is linked to the amount of mineral P in cow rations. Manure from cows fed high P diets requires more land for land-spreading in Comprehensive Nutrient Management Plans (CNMP) than manure from cows fed a P adequate diet.




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On-farm P cycling and P balancing are illustrated with this graphic.

Phosphorus inputs to the farming system include: feed, fertilizers, mineral and protein supplements, etc.

Outputs from the farming system include: exported milk, meat, and crop products.

A consequence of P inputs exceeding P outputs is the net importation (or positive balance) and storage of P on the farm. The result will be a build-up of soil test P values over time. The latter, in turn, increases the potential for P loss to lakes and streams if runoff and erosion occur on the farm.

One strategy for balancing P is to decrease dietary P inputs to recommended levels.




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Consumption of dietary P in excess of requirements is excretedentirely as soluble P in dairy feces.




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This study illustrates the effects of manure from diets that contained excessive dietary P on runoff losses of P.

Results indicate that when manures from dairy cows fed different concentrations of P are applied at the same rates, the high P diet manure will release more P in runoff than the low P diet manure. In June, dissolved P (DP) losses from the high P diet plots were almost 10 times higher (2.84 vs. 0.30 ppm P) than runoff from the low P diet plots. P losses in October runoff were lower, but trends were the same with DP losses from the high P diet fields almost four times higher(0.89 vs. 0.21 ppm P) than the low P diet treatments.

Excess P in dairy diets increases the potential for P loss in runoff from land-applied manure.




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As dietary P exceeds animal nutritional requirements (3.5 g of P per kg of dry matter intake) the P concentration in manure increases. This decreases the N:P ratio in the feces (urine contains very little P) relative to the N:P ratio requirements of field crops. This means that if a manure N management strategy is adopted (manure land applications are based on crop N requirements) more P is land applied with manure than the crop needs. This increases P in runoff and soil test P.




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Unnecessary mineral P supplements in dairy rations was found to be excreted entirely as water soluble P in manure and, after manure land application, increased soil test P levels, P loss in runoff, and the cropland area requirements (this slide) in order for producers to comply to comprehensive nutrient management plans (CNMPs).




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As dietary P exceeds animal nutritional requirements (3.5 g of P per kg of dry matter intake) the P concentration in manure increases. This decreases the N:P ratio in the feces (urine contains very little P) relative to the N:P ratio requirements of field crops. This means that if a manure N management strategy is adopted (manure land applications are based on crop N requirements) more P is land applied with manure than the crop needs. This increases P in runoff and soil test P (this slide).




Our research on commercial dairy farms confirmed that Pconcentrations in dairy feces can be used to determine the amount of P consumed.

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Two recent global and national reports highlight the environmental implications of too much agricultural nitrogen. Along with global climate change, it can be said that this is the next ‘big environmental issue’ that agriculture will be asked to address.




Most confinement dairy farms in the Midwest and Northeast regions of the U.S follow a fairly generalized formula of how to produce milk. Cows and replacement heifers are fed primarily homegrown feed, and protein and mineral supplements are purchased to compliment dairy diets. The principal diet ingredients are forages from silages of alfalfa and corn and corn grain. Soybean meal is the most important protein supplement on many dairy farms.

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Management can have a large impact on the relative amount of feed protein that is transformed into milk protein. For example, in Wisconsin, dairy producers who balance rations, feed TMR, use Posilac and milk 3x per day put 22 to 33% more feed protein into milk protein (rather than manure) than farmers who do not follow these practices.

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Similar to what we found with dietary phosphorus, the type and amount of protein supplement fed to lactating dairy cows impact manure chemistry, especially N excretion in urine. These impacts then n to influence various environmental outcomes, such as N gas emissions and manure N availability to crops after manure land application.




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As nitrogen (protein) intake by dairy cows exceeds requirements, feed N use efficiency (relative amount of consumed N that is secreted as milk N) decreases (the blue milk line, bottom graph). This is accompanied by an increase in urinary N (the orange line, middle graph).




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As nitrogen (protein) intake by dairy cows exceeds requirements, total N excretion in manure increases and there is a shift away from fecal N, which is stable) to urinary N, which is highly reactive.




For dairy barns and soils after manure application, manure from cows fed a low protein diet emits less manure than manure from cows fed dietary CP excessively.

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Of the total feed N consumed, a general average of 24% is secreted in milk and 38% is excreted in feces and 38% in urine. Urinary urea N (UUN) is the most reactive N source on dairy farm. UUN is principal N source for plants after manure land application, and the N source controlling emissions of ammonia (NH3) and nitrous oxide (N2O) from dairy manure.

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There is ample evidence that milk urea nitrogen (MUN) can be used to predict feed N intake and feed N use efficiency (the proportion of consumed dietary N that is secreted as milk N).

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General interactions between dietary N, MUN, UUN and atmospheric N emissions from dairy farms are depicted in this slide. Urea in ruminant livestock is released to the general circulation as blood urea N (BUN). The urea in blood may be either recycled back to the digestive system or excreted by the kidney as UUN. Urea diffuses readily in most body tissues including the udder and eventually the milk. Nutritionists have found highly significant relationships between BUN and MUN and between MUN and daily excretion of UUN. After excretion, urease enzymes, which are present in dairy feces and soil, hydrolyze UUN to ammonium (NH4

+). A portion of this NH4+ is

converted to NH3 gas in barns, manure storage areas, and fields following land application of manure. Some emitted NH3 combines with other constituents in the atmosphere to form NH4

+ and NO3-

containing compounds. A portion re-enters the terrestrial N cycle (depicted with vertical, dashed-lined arrows) to become indirect sources of N2O emission from soil. Reductions in UUN through improved feeding practices can reduce the pool of N for nitrification and denitrification thereby reducing direct N2O emission from manure-amended soils.

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For most dairy cows, reducing dietary CP to recommended levels would enhance feed N use efficiency and reduce MUN and UUN without sacrificing milk production. This slide is a summary of 5 dairy nutrition trial that included eighteen dietary CP treatments (range of 14.8% to 19.4% DM), Sixteen reductions in dietary CP lead to significant reductions in MUN, seventeen resulted in significant reductions in UUN and only two resulted in significant reductions in milk production: (1) a reduction in dietary CP from 18.4% to 15.1% (achieved by reducing rolled high-moisture shelled corn and solvent-extracted soybean meal in the diet) decreased milk production by 1.1 kg/cow per day (Broderick, 2003), and (2) the reduction in dietary CP from 17.3% or 16.1% to 14.8% (due to reductions of solvent-extracted soybean meal in the diet) decreased milk production by 1.9 kg/cow per day.

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Dietary CP can be used to accurately predict both MUN and UUN, and MUN can be used to accurately predict UUN.

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MUN records of 37,889 cows in 197 herds in Wisconsin (2010-2011) revealed that approximately from one-half to three-forth of tested cows were likely consuming dietary CP in excess of requirement. Farm simulations were used to quantify the effect of dietary CP (as indicated by this MUN distribution) on whole-farm N emissions (slide 31).

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Each 1.0 mg/dL reduction in MUN would reduce state-wide (Wisconsin) NH3 emissions by about 12 g N/cow per day, and N2O emissions by about 0.6 g N/ cow per day. Therefore, each MUN decrease of 1 mg/dL (within the range of 16 to 10 mg/dL) can be associated with decreases in NH3 emission of 7.2 to 11.3% and N2O emissions of 6.8 to 12.2%. On a state-wide basis, reductions in MUN to 12 to 10 mg/dL (which indicates adequate dietary N intake) would reduce NH3 emissions by approximately 29% to 43% and N2O emissions by 15% to 22%.

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Fact Sheet




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One approach to enhance feed protein utilization and reduce N excretion by dairy cows is to increase the concentrations of tannin in their diets. Modest amounts of condensed tannins (2 to 4% of DM) in forages, as is found in birdsfoot trefoil, reduces protein breakdown during ensiling and rumen fermentation by up to 50%. In a New Zealand study and a Wisconsin study, tannins in birdsfoot trefoil were found to increase milk production and milk protein concentrations.

Consumption of tannin containing forages by lactating cows has been found to also impact urinary N excretion and fecal N chemistry, which impacts NH3 loss from barn floors and from soil after manure application, N transformations in soil and N availability to plants.




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Compared to feeding alfalfa (Alf) to lactating dairy cows, birdsfoottrefoil having low (BF-T-Low) and high (BF-T-High) tannins increased manure N excretion, yet these forages shifted excretion from urine (reactive N) to feces (stable N). After application to soils, manure for the cows fed the birdsfoot trefoils emitted less ammonia that manure from cows fed alfalfa.




The effectiveness of tannins in reducing NH3 emissions from dairy manure can be attributed to two factors: (1) reductions in urinary N excretion by dairy cows and therefore the pool of N available for transformation to NH3; and reductions in urease activity in feces.

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The relative effectiveness of feeding tannin in reducing NH3 emissions depends on the concentration of CP in the diet. Relative reductions in NH3 emission were greater at low CP diet than the high CP diet. This difference may be attributed to (1) lower amounts of urine N excreted and therefore applied to barn floors when cows are fed lower CP diets, and (2) lower pH of excreta derived from low CP diets. Reductions in NH3 emission due to decreased urease activity are associated with substrate-tannin complexes, which prevented urea attack by urease enzymes, and/or to an inhibitory effect of the tannin extract itself.

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The dynamic nature of N transformations in agricultural systems necessitates a broad understanding of possible tradeoffs between N use, N incorporation into products, N conservation and N loss. Tradeoffs can occur between feed N use, manure N excretion, crop N use and environmental impacts. The conservation of one manure N form may result in the loss of other manure N forms. For example, excessive feeding of CP to dairy cows reduces feed N use efficiency and increases urinary N excretion and NH3 loss from dairy farms, but feeding lesser amounts of CP may lead to reductions in the fertilizer N value of manure. Likewise, although corn silage yields more DM and feeds more cows than alfalfa silage, this shift in feeding practices and associated land management practice impacts manure chemistry and overall N dynamics on a dairy farm.

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After application to soil, slurry from cows fed a higher CP emitted much more NH3 than slurry from cows fed a lower CP (slide 24) diet. After NH3 subsided (48 h after slurry application), inorganic N levels were 30% greater in soils amended with the higher CP slurry than in soil amended with the lower CP slurry. Also, feces from cows fed a high CP diet mineralize more N in the soil than feces from cows fed a low CP diet.




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The greater mineralization of N in soil from feces of cows fed the high CP diet resulted in greater plant yield than feces from cows fed a low CP diet.




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Many dairy farmers are growing and feeding more corn silage. This may have implications for various components of the nitrogen cycle on dairy farms.

For example, after application to soil, feces from cows fed a greater proportion of corn silage than alfalfa silage significantly reduced concentrations of inorganic N (immobilization of N) in soil compared to soils amended with feces from diets that contained lower amounts of corn silage. In a companion greenhouse trial, plant yield and N uptake were also significantly lower in pots amended with feces from corn silage than in pots amended with feces from alfalfa silage.




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The N contained in dairy feces can be divided into two general pools: (1) endogenous N consisting of microbial products from the rumen, intestine, hind gut, and digestive tract; and (2) undigested feed N (NDIN). Digested and undigested N in feces can have variable effects on the soil N cycle. For example, endogenous N may be rapidly available to crops but fecal fiber N may take a much longer time to mineralize. The amount of total N, fiber N, and the C/N ratio of dairy feces and slurry is highly influenced by the type and amount of forage fiber and CP consumed by a dairy cow (CS= corn silage; AS = alfalfa silage; LP = low crude protein; HP = high crude protein).




The feces from cows fed different diets contain a narrow range (11 to 18) of C/N ratios. Within this range, one would expect very little difference in fecal N mineralization after application to soil. Livestock manures that contain C/N ratios of ≥19 are expected to initially immobilize soil inorganic N, while manure having C/N ratios of ≤16 readily mineralize N. The slight differences in C/N ratio of dairy feces, however, impact soil N mineralization, plant N uptake and yield. More detailed analyses of the C compounds (e.g., non-structural carbohydrates, hemicellulose, cellulose, lignin), and secondary compounds (e.g. tannins, polyphenolics) in dairy feces may more fully describe the impacts of ruminant diets on the chemical composition and decomposition of manure C and N in soil.

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SOME READINGS

Ebeling, A.M., L.G. Bundy, J.M. Powell, and T.W. Andraski. 2002. Dairy diet phosphorus effects on phosphorus losses in runoff from land-applied manure. Soil Sci. Soc. Am. J. 66:284-291.

Misselbrook, T.H., J.M. Powell, G.A. Broderick, and J.H. Grabber. 2005. Dietary manipulation in dairy cattle: Laboratory experiments to assess the influence on ammonia emissions. J. Dairy Sci. 88: 1765-1777.

Powell, J.M., Z. Wu, and L.D. Satter. 2001. Dairy diet effects on phosphorus cycles of cropland. J. Soil and Water Conserv. 56 (1) 22-26.

Powell, J. M., D.B. Jackson-Smith, and L.D. Satter. 2002. Phosphorus feeding and manure nutrient recycling on Wisconsin dairy farms. Nutr. Cycl. Agroecosyt. 62:277-286.




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Powell, J.M., M.A. Wattiaux, G.A. Broderick, V. Moreria, and M.D. Casler. 2006. Dairy diet impacts on fecal chemical properties and nitrogen cycling in soils. Soil Sci. Soc. Am. J. 70:786-794.

Powell, J.M., G.A. Broderick, J.H. Grabber, and U.C. Hymes-Fecht. 2009. Effects of forage protein-binding polyphenols on chemistry of dairy excreta. J. Dairy Sci. 92: 1765-1769.

Powell, J.M., C.J.P. Gourley, C.A. Rotz, and D.M. Weaver. 2010. Nitrogen use efficiency: A potential performance indicator and policy tool for dairy farms. Environ. Sci. & Policy.13: 217-228.

Powell, J.M., M.A. Wattiaux, and G.A. Broderick. 2011. Evaluation of milk urea nitrogen as a management tool to reduce ammonia emissions from dairy farms. J. Dairy Sci. 94 :4690–4694. doi: 10.3168/jds.2011-4476.

Powell, J.M., M.J. Aguerre, and M.A. Wattiaux. 2011. Tannin extracts abate ammonia emissions from dairy barns. J. Environ. Qual. 40: 3: 907-914. 2011.




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Powell, J.M. and Broderick, G.A. Trans-disciplinary soil science research: Impacts of dairy nutrition on manure chemistry and the environment. Soil. Sci. Soc. Am. J. 75:2071–2078.

Powell, J.M. Alteration of Dairy Cattle Diets for Beneficial On-Farm Recycling of Manure Nutrients. pp 21-42 In: Applied Research in Animal Manure Management. Zhongqi H. (Ed.) Nova Science Publ. Inc.

Powell, J.M., C. A. Rotz and M. A. Wattiaux. 2014. Potential use of milk urea N to abate atmospheric nitrogen emissions from Wisconsin dairy farms. J. Environ. Qual. (in press)

Satter, L.D., T.J. Klopfenstein, G.E. Erickson, and J.M Powell. 2005. Phosphorus and dairy-beef nutrition. In A.N. Sharply & J. T. Sims, eds. Phosphorus Agriculture and the Environment. pp. 587-606. ASA-CSSA-SSSA Monograph N. 46. ASA-CSSA-SSSA, Madison, Wisconsin.




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