Many farmers still view livestock manure as a waste product that has to be disposed of as efficiently as possible. In this case, “efficiently” means with the least possible investment in time and equipment. In the past, it wasn’t uncommon for farmers to spread an entire winter’s production of dairy manure on the one or two fields closest to the barn, even though those were the same fields receiving the previous winter’s entire manure production. (Hint: When you have to measure application rates in inches of depth rather than tons or gallons per acre, that’s too much manure.)
Fertility on these fields soon reached astronomical levels, “off the chart” on the soil nutrient scale—especially in the case of phosphorus—it can literally take generations to farm excessively high fertility back to normal levels. Ironically, soil fertility would have been even higher had the farmers used good manure management, including incorporation of manure, timing application rates to when the crop needs the nutrients, and avoiding spreading near ditches and streams where runoff could quickly move off-site. I think we’ve come a long way since then, aided by a higher percentage of manure that’s stored (versus daily spread), and by more efficient manure handling equipment, as well as by increased regulations such as CAFO (Concentrated Animal Feeding Operations), and more responsible manure management by farmers and farm employees.
We can no longer treat manure as a waste product—it’s a highly valuable resource. On moderate and large livestock farms, increasing government regulations are dictating when, where and how much manure can be applied to cropland. Also, the non-farming public is increasingly bothered by the smell of manure and by the sight of spilled manure in roadways. Nuisance lawsuits have been on the increase, and farmers have lost their share of them. This will only increase as livestock farming rubs shoulders with non-farm residents. And, small farms aren’t immune to these problems—if dairy manure contaminates a neighbor’s well it doesn’t make any difference if the manure was from a 50-cow farm or a 500-cow one.
Dollars and Sense
Using a recent analysis of the slurry dairy manure at Miner Institute, just the nitrogen, phosphorus and potassium in one, 5,000-gallon spreader load is worth over $100. That doesn’t include the value of the many other nutrients in the manure, plus the soil-building properties of the organic matter. Following are a few ideas to help maximize the value of livestock manure: dollars and sense
The nutrients in manure are highly available to plants. While a portion of the nitrogen in manure is in the fibrous fraction and not available the year of application, the phosphorus and potassium are just about as plant-available as the P and K in commercial fertilizers. And, much of the N that isn’t available the year of application is slowly released over the next couple of years. Therefore, the corn crop in a field that receives manure every year gets a lot of N the year of application, but also a little bit from that applied one and two years ago—just a little bit, but enough to make a difference, especially if manure application rates were high.
Manure is a multivitamin. Micronutrient deficiencies are less likely to occur on fields that receive livestock manure. On average, only about one-third of the nutrients that ruminants consume are used to produce milk and meat; the remaining two-thirds are excreted. These include not only the major nutrients nitrogen, phosphorus and potassium, but a host of other nutrients, many of which we scarcely think about in planning fertilizer programs. Zinc is one of the very few micronutrients commonly deficient in the Northeast, primarily in corn production. We did a comparison of the soil test zinc level in fields at Miner Institute that had been receiving regular applications of manure versus those that hadn’t been manured in recent years. Not surprisingly, manured fields had significantly more plant-available zinc than non-manured fields. Regular manure applications—not every field every year, but every field at least once every few years—will prevent most micronutrient deficiencies because most of the grain we feed our cows was produced in other areas, and with two-thirds of the nutrients passing through the cow into her manure, we’re essentially translocating the minerals in the grain from where they were produced onto our cropland.
Not testing manure for nutrient content is like buying fertilizer without asking for the analysis. If we buy fertilizer we’re sure to know the analysis, but many farmers have never had an analysis on their livestock manure and have no idea of how many pounds of nutrients are in a ton (or 1,000 gallons). Manure nutrient values vary a lot depending on the amount of bedding used, the solids content of the manure, and even the diet fed the herd. (A high-producing dairy herd will usually have manure that’s higher in nutrient content. Therefore, better cows equals better manure.) Since the nutrients in manure are highly available, manure can be used to replace commercial fertilizer, in whole or in part. Farm regulatory officials realize how important it is to know manure nutrient content, which is why CAFOs are required to test every storage or source of manure every year. Therefore, if a CAFO farm has a slurry manure pit and a stack of composted manure, separate analyses are required each year on both pit and stack. All livestock farms should do regular manure testing, not because of government requirements, but to save money on fertilizer. Plus, it’s just the responsible thing to do.
Slurry and liquid manure pits should be tested several times during unloading. If you buy 10 tons of 15-15-15 fertilizer you can be confident that the analysis of the last ton of fertilizer you apply will be almost exactly the same as the first ton. This is not the case with the manure in liquid and slurry pits. Over time there’s separation between the solid and liquid portions of the manure, and the nutrient content of the solid versus liquid fractions are much different. Almost all the phosphorus in manure is in the solids (feces), while almost all the potassium is in the liquid (urine). Agitating a large manure pit seems to stir it up quite a bit as the top crust gets broken up, but as soon as agitation is done some resettling and separation occurs. The result is that as the manure is pumped out, even if periodic re-agitation is done, the analysis of the first third of the manure removed will be a lot different than the last third.
We found this happening as we unloaded the slurry manure pit at Miner Institute. The first third of the manure we spread was very low in solids and since the phosphorus is in the solids and the potassium in the liquids, it had a wide ratio of K to P. But, as we pumped the pit down, the solids content increased and the ratio of K to P narrowed considerably. The last third of the slurry had by far the most phosphorus and the narrowest ratio of K to P. Pennsylvania State University sells a manure sampling kit containing multiple sample bottles and recommends sequential testing of slurry pits at least once. That recommendation was made before the price of fertilizer increased by multiples; with current fertilizer prices I’d recommend sequential sampling—three samples should be enough—every year because differences in precipitation and rations can combine to change nutrient content.
We used to say that one of the drawbacks in fertilizing with manure was that we couldn’t change the nutrient content as we can with commercial fertilizer. Well, we still can’t change the nutrient content, but we can use these significant and predictable differences in K to P ratio in deciding what fields to manure first and which to manure last. Fields testing high in soil test P (usually those close to the barn) get the first manure out of the pit since each tanker load contains a lot less P in relationship to K. Fields testing low in P get the last manure out of the pit, which is higher in solids content, and each tanker load contains a lot more P. This should be an easy sell to farmers: if a farmer had one load of manure that was only 2 percent solids and another load that was 6 percent solids, which do you think would be more economical to haul a few miles from the home farm to apply on what is often the lowest fertility fields? Nobody likes to haul manure that’s only 2 percent solids, but since it has to be hauled, close is better than far.
Even moderate-sized livestock farms have many thousands of dollars of highly plant-available manure nutrients. A small investment in time, manure analysis and planning manure application schedules (rather than taking the simplest route) can go a long way towards reducing commercial fertilizer needs.
Editor’s note: This article was originally published in the March 2009 issue and has been updated for accuracy.