Manure is an inevitable byproduct of the production of meat and milk destined for human consumption. Excessive excretion of manure and manure nutrients represents inefficiencies that increase feed costs, increase the environmental impact of dairy farming and increase costs associated with moving and storing manure. Profitability often can be enhanced when feeding and management practices are used that reduce manure production per unit of milk produced. Furthermore, good environmental stewardship will maintain the generally positive public image of dairy farming.
Society is increasingly concerned about the environmental impact of manure and manure nutrients. In response, the federal government and many states have developed environmental rules regulating certain dairy farms. Although regulations vary, the amount of phosphorus (P) and nitrogen (N) excreted via manure are of major regulatory importance. The selection of proper feed ingredients and diet formulation can affect the mass of manure produced and the amount of specific nutrients (e.g., N and P) excreted.
Average manure production
Over the past several years, our laboratory has conducted numerous experiments measuring fecal and urinary output by lactating dairy cows fed a variety of diets. The current database contains data from 14 different experiments with cows (232 observations) fed 55 different diets. Not all samples were assayed for N and P. Therefore, the number of observations is 202 for N data and 161 for P data.
In all experiments, cows were housed in metabolism stalls for four to six days, fed for ad libitum consumption and all feces and urine produced was collected, weighed and sampled. All animals were Holstein cows in their second or greater lactation and varied greatly in milk production, bodyweight and days in milk (DIM). The average milk production in this data set is very close to the current U.S. average.
The objectives of the individual experiments included the evaluation of different types of forages, fat supplements, byproduct feeds and mineral supplementation. A diverse array of feedstuffs was used, but corn silage and alfalfa silage were the predominant forages fed (alfalfa hay and orchardgrass silage were also included in some experiments). Dry ground corn was the predominant starch source and soybean meal was the predominant source of supplemental crude protein. Several byproducts including soy hulls, dried distillers grains, wheat middlings and animal protein meals were fed. The concentration of crude protein and P varied among diets, but in most cases, P was fed to only slightly exceed the NRC recommendations in place when the experiment was conducted.
The average amount of manure (feces plus urine) produced daily by cows in this data set was about 140 pounds and ranged from 59 to 224 pounds per day. The vast majority (87.5 percent) of manure was water. The excretion of urine averaged 45.1 pounds per day (5.4 gallons) and ranged from 18.5 to 101 pounds per day (2.2 to 12.2 gallons per day). Excretion of wet feces averaged 94.8 pounds per day. The excretion of urine was more variable than excretion of feces. On average, slightly less than one-third of manure was urine, but this proportion was highly variable ranging from 16.5 to 62 percent urine.
Excretion of N and P via manure averaged 385 and 47 grams per day, respectively. On average, 50 percent of the N excreted was via urine, but almost all the P excreted was via feces.
Over the past two years we have been conducting a very large experiment to evaluate the relationships we observed with our original data set and to expand our information regarding factors that influence manure excretion and excretion of N (108 observations). Those data have not been fully analyzed yet, but some preliminary findings are included in this [article]. For the new data set, dry matter intake (DMI) averaged 53 pounds per day and milk production averaged 84 pounds per day.
Factors affecting manure excretion
The original database was used to evaluate major dietary (concentrations of forage, corn silage, NDF, CP and P) and cow factors (DMI and milk production) that influenced excretion of manure and manure nutrients. The only two independent variables that were significantly related with wet manure production were DMI and milk production. The relationship between DMI and manure production was much stronger than the relationship between milk yield and manure production.
Although manure excretion increased, on average, with increasing milk production, substantial variation is evident meaning increased milk production does not necessarily increase manure production. Not unexpectedly, DMI was clearly the most important single factor affecting manure production, but actual manure excretion varied about ±22 pounds from predicted excretion at a specific DMI. With our new data set that included cows with higher average milk yield and higher average DMI than the original data set, the relationship between DMI and manure production was very similar.
Assuming no effect on DMI, cows fed a diet with no corn silage (all hay crop) would produce, on average, 25 pounds per day more manure than would cows fed a diet with 100 percent of the forage as corn silage. At the average DMI of this database (46 pounds), this is equivalent to a 16 percent reduction in manure excretion.
This relationship was confirmed with our new data set. Cows fed diets with 75 percent of the forage as corn silage (the remaining forage was alfalfa silage) produced 14 percent less manure than cows fed with 25 percent of the forage as corn silage (165 versus 191 pounds per day).
Excretion of fecal DM
To better understand the relationships between manure production and dietary and cow factors, manure was partitioned into feces and urine. Excretion of fecal water was not related to any variable except DMI. Therefore, only excretion of fecal DM will be discussed.
As expected, DMI accounted for most of the explainable variation in excretion of fecal DM. In all equations the coefficient associated with DMI was 0.35, meaning the average DM digestibility of these diets was 65 percent (i.e., 100 - 35). The concentration of dietary NDF and the percent of forage as corn silage also were significantly related with excretion of fecal DM, but the effects were quantitatively small.
A one percentage unit increase in the concentration of dietary NDF was associated with a 0.07-pound-per-day increase in excretion of fecal DM. A one percentage unit increase in the concentration of corn silage (as a percentage of forage DM) was associated with a 0.015-pound-per-day decrease in excretion of fecal DM. The concentration of dietary NDF and the proportion of corn silage were negatively correlated. Therefore, the effects of one cannot be statistically separated from the other. Most likely, NDF concentration was primarily responsible because average NDF digestibility is less than average DM digestibility.
Excretion of urine
As with excretion of fecal DM and manure, DMI (pounds per day) was strongly related to urine excretion but the percentage of forage as corn silage also was strongly related with urine excretion. The effect corn silage has on total manure excretion is caused almost entirely by its effect on urine excretion. Replacing hay crop forage with corn silage reduced excretion of urine. On average, cows fed diets in which all the forage was corn silage would be expected to produce about 24 pounds per day less urine than cows fed diets with hay crops providing all the forage.
In our new data set, cows fed diets with 25 percent of the forage as corn silage produced, on average, 22 pounds more urine per day than cows fed diets with 75 percent of the forage as corn silage. The reason decreasing corn silage (and increasing hay crop) increased urine output was most likely the increased concentration of potassium in hay crops compared with corn silage.
Excretion of N and P via manure
Because of the importance of P in nutrient management plans and environmental regulations, P excretion was extensively examined using the database described above. The only variables related to excretion of P via manure were P intake and milk production, but the equation based solely on P intake was almost as accurate as the equations using P intake and milk production.
Based on this equation, if increased N intake results in increased milk production, the marginal efficiency of N utilization can increase. For example, if a cow consumes 46 pounds of DM of a diet with 17 percent crude protein (CP) (2.72 percent N) and produces 66 pounds of milk, N intake equals 571 grams and expected excretion of N equals 382 grams per day (67 percent of N intake is excreted in manure). If intake increases to 48 pounds (17 percent CP) and production increases to 73 pounds, N intake is 598 grams and expected N excretion is 397 grams (66 percent of N intake is excreted in manure).
The only other factor tested that affected excretion of N in manure was the concentration of corn silage (percentage of forage DM). Including that term reduced residual variation only slightly, and the resulting model was not appreciably more precise or accurate. The coefficient was 0.28, which means changing from a diet with all the forage as hay crop to one with all corn silage would be expected to reduce manure N excretion by 28 grams per day (7 percent of average N excretion in this data set). Corn silage reduced fecal N excretion but slightly increased urinary excretion of N.
Brown midrib corn silage and manure excretion
Corn silage made from brown midrib (BMR) hybrids usually has higher in vitro NDF digestibility and lower lignin concentrations than silage made from conventional hybrids. In vivo digestibility of nutrients has not been affected consistently by hybrid. The effect of BMR silage on excretion of nutrients and manure has not been examined until very recently. Prior to our experiment, data from two studies suggested manure N may be decreased when cows were fed BMR silage. Excretion of manure N was not statistically analyzed in either study, but numerically, manure N as a percent of N intake was lower for cows fed BMR silage than for cows fed the control silage.
To determine effects of BMR silage on manure excretion and N metabolism we conducted a digestion study comparing BMR silage with a conventional hybrid. Diets were 55 percent corn silage (either BMR or conventional) and 45 percent concentrate and contained either 14 or 17.5 percent CP. Cows fed the BMR silage tended to excrete less manure (154 versus 160 pounds per day) than cows fed the conventional hybrid.
As expected, increasing dietary CP concentration greatly increased excretion of N via manure (374 vs. 465 grams per day for the 14 and 17.5 percent CP diets, respectively). At a specific N intake, cows fed BMR silage excreted 14 grams per day less N than cows fed the conventional hybrid. Hybrid affected manure N excretion, but the response was quite modest (reduced manure N by 3.6 percent).
(In)efficiency of the dairy industry
With all the recent interest and increasing regulations, one would think excretion of manure and manure nutrients by dairy cows is worse now than in the past. Based on USDA statistics, 20 years ago we had 11 million dairy cows in the U.S. producing 143 billion pounds of milk per year. Based on our equation, in 1985 the U.S. dairy herd produced about 690,000 tons of manure each day.
In 2005, we had nine million dairy cows producing 177 billion pounds of milk per year. Again based on our equation, manure production was about 621,000 tons per day. In the last 20 years, the U.S. dairy industry has reduced manure production by about 10 percent and increased milk production about 23 percent. Clearly, we should continue to develop and use new methods that reduce the environmental impact of the dairy industry, but we also should be proud of the progress already made.
From our data set, “manure inefficiency” averaged 2.2 pounds of manure per pound of milk produced. But because cows produce manure even when they produce no milk (averaged 100 pounds per day from our data), manure inefficiency will decrease as milk production increases. Cows producing 40 pounds of milk will average 3.1 pounds of manure per pound of milk, but for cows producing 80 pounds of milk, the ratio drops to 1.9 pounds of manure per pound of milk.
On average, only 33 percent of the N and 40 percent of the P consumed by these cows was secreted in milk or retained in the body. Although these efficiencies appear low and can be increased, they are similar to the average N use efficiency (33 percent) of cereal grain production. Efficiency of P use (grams of P in milk per gram of P consumed) is increased by:
1. not overfeeding (feed to just meet NRC requirements for absorbed P)
2. using highly available sources of P
3. obtaining high production (high-yielding cows have higher P efficiency than low-yielding cows)
New technologies are being evaluated and developed that may improve P efficiency further.
Efficiency of N use is increased by:
1. not overfeeding protein
2. using feeds with high protein digestibility (e.g., not using heat-damaged forages)
3. providing the proper balance of rumen degradable and undegradable protein
4. having high-producing cows
We must continue to strive to improve efficiency of N use (i.e., milk N output divided by N intake), but obtaining maximum efficiency may not be desirable or economically sustainable. In the U.S., cows are most commonly fed and managed as groups. Because of variation in nutrient requirements among cows and variation in nutrient supply (i.e., variation in feed composition), a diet balanced to meet the protein requirements of an average cow in a group will, on average, result in reduced milk production for that group, although it will usually increase N efficiency.
St-Pierre and Thraen concluded that if all cows in the U.S. were fed for maximum N efficiency, rather than maximum income-over-feed costs (and concomitantly lower N efficiency), it would cost the industry approximately $4 per pound of reduced N excretion. Perhaps it would make more economic sense to put greater emphasis on proper crop fertilization practices and crop selection than on maximizing N efficiency of the cow.
Conclusions
The average lactating dairy cow produces about 140 pounds of manure per day of which approximately 85 percent is water. Replacing hay crop forage with corn silage significantly reduced the volume of urine excreted resulting in less total manure production. Cows fed diets with all the forage provided by corn silage would be expected to produce about 25 pounds per day less urine than cows fed diets with hay crops providing all the forage.
Excretion of P and N via manure was mostly affected by intake of P and N respectively. Excretion of P via manure was not affected by forage type, but diets with high concentrations of corn silage relative to hay crop forages excreted less N in manure. Cows fed BMR silage also tended to excrete less N in manure than did cows fed conventional corn silages. PD
References omitted but are available upon request at editor@progressivedairy.com
—Excerpts from 2006 Penn State Dairy Cattle Nutrition Workshop Proceedings
