Feed Management to Improve Nitrogen and Phosphorus Efficiency Charles C. Stallings Professor and Extension Dairy Scientist Virginia Tech The two nutrients that have the greatest potential for negative environmental impact when present in waterways are nitrogen and phosphorus. These nutrients can come from many sources including excessive lawn fertilization. Concentrated animal units can also contribute with poultry, swine, and dairy production being the most intensive in their management. Beef cattle are typically maintained on pasture and dispersed over a greater area although there are more beef than dairy cattle in Virginia. Management of excreted nutrients has become a significant issue facing animal agriculture and programs have been designed to address these issues. Nitrogen Excretion Nitrogen in the ruminant animal can be converted into microbial protein which in turn can be used as a source of amino acids for milk protein production or other processes. In reverse the protein breakdown process results in nitrogen that is excreted by the animal. Below is the distribution of excreted nitrogen in lactating dairy cows fed three concentrations of crude protein. The units are expressed as grams (g) of nitrogen (N) per cow per day. Intake N, Urine N, Fecal N, Total Excreted Excreted/Intake, g/day g/day g/day g/day % 12 % protein 359 99 158 257 72 15 % protein 449 138 179 317 71 18% protein 582 228 199 427 73 Source: Tomlinson et al. 1996. American Society of Agricultural Engineers. As nitrogen intake increases so does excretion in both the urine and feces but the increase is greatest in the urine. At 12% protein in the ration the percent of excreted nitrogen in the urine is 39% compared to 44% at 15% protein and 53% at 18% protein. The bottom line is the majority of the excess fed nitrogen is excreted via the kidney in the urine. In addition much of the nitrogen in urine is in the form of urea which can be broken down to ammonia and be partially volatilized into the atmosphere. It is interesting to note that the efficiency of nitrogen utilization (excreted N divided by N intake) was the same across all three protein levels. In this study the milk production was 45 lbs per cow per day and the nitrogen excreted in the milk was approximately 100 grams per cow per day and was not affected by the amount of protein in the ration. The amount retained by the cow however increased with increased protein in the ration going from 16 to 43 to 55 grams at 12%, 15%, and 18% protein. 14
A recent study from the University of Wisconsin looked at the effect of protein source on nitrogen excretion. Alfalfa and corn silages were fed in equal proportions at 50% of ration dry matter. Solvent (high protein degradability, HD) and expeller (low protein degradability, LD) soybean meals were varied to alter the ratio of rumen degradable to undegradable protein. Both soybean meals were fed in all four diets, only the proportion changed with the HD diets having the majority solvent soybean meal and the LD diets having more of an equal mixture of solvent and expeller. DM intake, Milk, Milk N, Milk N, Milk urea nitrogen lbs./day lbs./day g/day % intake mg/dl 17 % protein (LD) 50 85 177 29 11.9 18 % protein (LD) 54 89 189 27 12.5 17.6 % protein (HD) 46 82 169 28 12.6 18.7 % protein (HD) 52 85 181 25 13.4 Source: Flis and Wattiaux. 2005. Journal of Dairy Science. Dry matter intake and milk were increased with addition of expeller soybean meal on both the LD and HD diets. Milk nitrogen in grams per day was also increased by addition of expeller soybean meal indicating more amino acids for milk protein synthesis. Milk urea nitrogen was not different but did tend to be higher on higher protein diets. The next table contains information on nitrogen excretion from this experiment. Intake N, Urine N, Fecal N, Total Excreted, Excreted/Intake, g/day g/day g/day g/day % 17 % protein (LD) 618 194 209 403 65 18 % protein (LD) 707 213 232 445 63 17.6 % protein (HD) 596 202 190 392 66 18.7 % protein (HD) 712 223 208 431 61 Source: Flis and Wattiaux. 2005. Journal of Dairy Science. Clearly more nitrogen consumed results in more nitrogen excreted. This study also demonstrated that as more expeller soybean meal was fed that was lower in rumen degradability than solvent soybean meal, dry matter intake and milk production increased. The opposite was true when solvent soybean meal was fed in excess. Therefore, source of protein is important as is level of ration protein. In addition there were differences in how cows excreted nitrogen based on age or lactation number. First lactation cows excreted additional nitrogen while multiple lactation cows retained more of the nitrogen. The authors concluded that first lactation cows should have rations balanced that are not excessive in protein content especially when containing highly degradable protein sources. A separate first calf heifer group would be needed. Urea is a small molecule that travels in the aqueous (water) phases of the cow and appears in the blood, urine, and milk. Because milk urea nitrogen (MUN) is a breakdown product of protein, it can be used to monitor protein status of cows. A 15
recent report by Wattiaux (Hoard s Dairyman, 2005) indicates that an MUN of 11.5 to 12 mg/dl would be associated with a ration protein level of 16.5% and this can be an optimal situation that does not reduce milk production while avoiding excess urinary nitrogen. Wattiaux estimates that there is 2 mg/dl change for each one percentage unit change in protein when rations contain 15 to 18.5% protein. Herds with an MUN above 14 would have increased urinary excretion of nitrogen. A project through the University of Maryland (Dr. Rick Kohn) is working with milk cooperatives to report MUN levels in bulk tank milk that is collected. Reports are going back to the farm. An incentive program is in place giving herds that test below 11 mg/dl for 3 consecutive months a payment of $150 and those between 11 and 12 for 3 consecutive months will get $100. Some herds in Virginia are receiving these reports. Virginia Tech has support from this project to problem solve in Virginia herds needing assistance. Rotz presented some estimates for nitrogen volatilization, leaching, and denitrification for manure from free stall barns with 6 months manure storage. Comparisons between surface applied, injected, and irrigated manure disposal was estimated. Loss of N was extensive due to volatilization especially when manure is surface applied and irrigated and greater than the total loss due to leaching and denitrification (elemental N formation). With manure injection the N volatilized is about the same as N lost to leaching and denitrification. Surface applied Injected Irrigated N lost by volatilization, lbs./acre 67 41 79 N lost by leaching, lbs./acre 22 25 19 N lost by denitrification, lbs./acre 14 15 12 Source: Rotz. 2005. Dairy Manure Management. Air emissions are becoming an issue with animal production units and ammonia is one of the concerns. The industrial standard is for a maximum of 100 pounds of ammonia per day to be released into the atmosphere. Rotz (Hoard s Dairyman, 2005) estimates emission rates of ammonia on dairy farms ranging from.18 to 1 lb./cow/day depending on the type of housing and manure handling system. The.18 emission rate would mean a 550 cow herd would be needed to produce 100 pounds of ammonia but at the 1 pound rate only 100 cows would be needed. Factors such as temperature (high temperature results in more emissions), manure ph (high ph results in more volatilization), and manure handling all can impact ammonia volatilization in addition to feeding excess protein. Because the majority of excess N is excreted in the urine which is most prone to volatilization a management strategy to reduce ammonia emissions would be to fine tune the feeding of protein so that excess is avoided. Covered manure storage and manure injection into the soil are recommended ways of reducing N emissions. How much protein is needed for lactating dairy cows? Certainly the 12% protein in the experiment above would not be adequate for most modern dairy cows. Could we 16
use less than 18% and lose no milk production? In some cases it seems that we can if we have the proper balance of rumen degradable and undegradable protein in combination with adequate rumen available energy. Also it appears age or lactation number should be considered when managing the lactating herd. Keeping first lactation cows apart from the rest of the herd has merit from both a social as well as a nutritional standpoint. A lower protein level is warranted when feeding first lactation cows relative to older cows because they are usually lower producing and do not utilize the nitrogen as efficiently. Overall herd MUN should be less than 12 mg/dl. Phosphorus Excretion Phosphorus has become an issue as dairy farms have increased in size which increases the need for imported feeds from other areas of the country. The result can be a net increase of phosphorus onto the farm resulting in buildup in the soil and possible water contamination as soil erosion occurs on fields that have had manure applications. Phosphorus, unlike nitrogen, does not volatilize into the atmosphere but stays with the manure and soil particles. Below are results of an experiment looking at phosphorus (P) partitioning in the lactating dairy cow fed different concentrations of phosphorus. Intake P, Urine P, Fecal P, Total Excreted, Excreted/Intake, g/day g/day g/day g/day %.34 % P 91.3 46 47 52.51 % P 138 1.6 86 87 63.67 % P 180 2.3 119 121 67 Source: Knowlton and Herbein. 2002. Journal of Dairy Science. This study demonstrates that very little of the dietary phosphorus appears in the urine. The added phosphorus appears mostly in the feces. Notice that unlike nitrogen, the excreted as a percent of the consumed phosphorus increases as higher levels are supplemented. Because of this it is extremely effective to reduce excretion by limiting intake to the requirement of phosphorus. Milk phosphorus excretion did not vary by diet and was 39 to 41 grams per day. The requirement of phosphorus varies with body weight, milk production, rate of weight gain, and pregnancy. Below are the calculated phosphorus requirements according to the 2001 National Research Council at varying milk productions keeping body weight constant at 1400 pounds and assuming no weight gain or fetus development. These are the numbers that would be generated when calculating phosphorus status in our P Report that we provide to dairy herds participating in our Phosphorus Feeding Incentive Program. Milk, lbs./day 30 40 50 60 70 80 90 17
Dry matter intake, lbs./day 36.2 39.7 43.2 46.7 50.3 53.8 57.3 Required P, grams/day 41.9 50.0 58.1 66.2 74.4 82.5 90.6 Required P, % dry matter.26.28.30.31.33.34.35 Source: National Research Council. 2001. At higher herd production levels (80 to 90 lbs./cow/day) the approximate relationship of milk production is one pound of milk to one gram of phosphorus. For individual cows there is extra needed for growth and pregnancy but these needs are very low relative to the needs for milk production. The P Report for our incentive project is calculated by knowing the dry matter intake (DMI) and concentration (%) of P in the ration which is determined by lab analysis. An example for a herd averaging 80 lbs. of milk of 4.0% fat milk is given below. Group 1 % P in P lbs. P consumed, Cow numbers Total ration consumed grams/cow grams P 53.8 lbs. DMI 0.3684 0.198199 89.98 150 13497 Total P fed, Required P, 89.98 82.5 P fed, % of required 109 Since milk production is an important contributor to the P required as well as dry matter intake I have taken the same results for P in the ration and calculated the P Report for a herd producing 60 lbs. of 4.0% fat milk. Intake of P is lower but so is the required P resulting in P fed as a % of the required being higher than when producing 80 lbs. of milk above. Milk production is a major component when calculating the P needed as a % of the requirement. Group 1 % P in P lbs. P consumed, Cow numbers Total ration consumed grams/cow grams P 46.7 lbs. DMI 0.3684 0.1720 78.09 150 11731 Total P fed, Required P, 78.098 66.2 P fed, % of required 118 18
Nitrogen and Phosphorus Balance Ten Virginia dairy farms are cooperating with us on precision feeding as well as the P incentive program. Ms. Beverly Cox, a Dairy Science graduate student, has been working with these herds and has monitored flow of nitrogen and phosphorus through the farms. She calculates the amount of N and P in the feed and fertilizer and subtracts the amount taken off the farms as meat, milk, or crops. Below I include one year s calculation for two farms. Feed Fertilizer Total In Meat & Milk Crops Total Out Farm A-N lbs. 70,763 12,008 82,572 21,315 26,679 47,994 Farm A-P lbs. 5,640 1,138 6,778 4,827 5,266 10,093 Net Balance N +34,578 lbs. and P -3,315 lbs. Efficiency (In Out) = N 58% and P 149% Farm A is bringing in 47,176 more lbs. of nitrogen than is going out, but is actually sending out more phosphorus than is being brought in. Feed is the primary source of both N and P. Some crops are being moved off the farm in this case. Farm B below, however, does not have any crops being sold. Feed Fertilizer Total In Meat & Milk Crops Total Out Farm B-N lbs. 165,000 7,800 172,900 44,264 0 44,264 Farm B-P lbs. 23,919 2,481 26,401 9,533 0 9,533 Net Balance N +128,636 lbs. N and P +16,867 lbs. P Efficiency (In Out) = N 25% and P 36% In Farm B there is more importation of N and P than in Farm A but actually has less N and P out because of greater N removal in meal and milk. Since no crops are removed from the farm all the output is as meat and milk resulting in a net accumulation for both nutrients. Knowing the source of the nutrients coming onto the farm is the first step in knowing how to reduce amounts. General Conclusions on Reducing Nutrient Output Test feeds to know nutrient content Limit protein (N) and phosphorus in lactating cow rations to the requirement with minimal overfeeding Group cows by lactation number as well as milk production if possible and track dry matter intake for each group and lower the safety factor for uncertainty built into many rations Balance the ration for rumen degradable and undegradable protein and have MUN less than 12 mg/dl Provide adequate rumen available energy (starch or sugar) to utilize the degradable nitrogen 19
Use by-product feeds that are high in P only when economical and evaluate amounts fed Do not offer free-choice minerals containing phosphorus Maximize the amount of forage in the ration when possible References Flis, S. A. and M. A. Wattiaux. 2005. Effects of parity and supply of rumendegraded and undegraded protein on production and nitrogen balance in Holsteins. Journal of Dairy Science 88:2096-2106. Knowlton, K. F. and J. H. Herbein. 2002. Phosphorus partitioning during early lactation in dairy cows fed diets varying in phosphorus content. Journal of Dairy Science 85:1227-1236. Kohn, Rick. 2005. A program to improve dairy herd nutrition using milk urea nitrogen. NRCS Conservation Innovation Grant. National Research Council. 2001. Nutrient Requirements for Dairy Cattle. National Academy Press, Washington D. C. Rotz, C. Alan. May 25, 2005. How much ammonia do dairy farms emit? Hoard s Dairyman, page 371. Rotz, C. Alan. 2005. Manure management choices and whole farm impacts. Dairy Manure Management. Natural Resource, Agriculture, and Engineering Service publication 176. Pages 240-248. Tomlinson, A. P., W. J. Powers, H. H. Van Horn, R. A. Nordstedt, and C. J. Wilcox. 1996. Transactions of the American Society of Agricultural Engineers 39(4):1441-1448. Wattiaux, M. A. October 25, 2005. What can MUN s really tell us? Hoard s Dairyman, page 697. 20