Credit Supplement Feed on Pasture, Save Money on Fertilizer

By Jane Fyksen, Crops Editor
Friday, October 13, 2006 12:03 PM CDT

Most graziers supplement their cattle on pasture.

Those additional nutrients passing through the animals and deposited with manure tend to be an overlooked aspect of supplementing grazing cattle.

With rising fertilizer prices and the need to protect water quality by using fertilizer nutrients efficiently, the fertilizer value of supplemental feed should be taken into account. What's more, taking these "credits" can also offset some of the cost of buying supplemental feed.

The University of Missouri helps producers calculate the value of supplemental feeds as fertilizer for their pastures.

Here are daily intakes and fertilizer values of the nutrients fed a beef cow for 100 days (for nitrogen, 35 percent of what's excreted by the livestock is assumed to be "available"):

Wet brewers grain (30 percent DM) - 9 percent protein, 0.2 percent P, 0.03 percent K.

Unfortunately for producers' pocketbooks, cattle don't use the nutrients in their feed with 100 percent efficiency. Most consumed nutrients are redeposited on pasture in manure and urine. In fact, they usually excrete about as much protein and minerals as they consume. Growing cattle will excrete 50 to 70 percent of consumed N, 65 to 75 percent of the P and 65 to 85 percent of K. For mature cattle, figure 100 percent excretion for calculations.

There are three steps in figuring fertilize nutrient value of supplemental feeds on pasture or winter feeding grounds. First, determine how much feed animals are eating. Second, determine the amount of nutrients in the feed available for use as fertilizer. Third, determine the effective nutrient application rate onto a paddock.

The first step is fairly straightforward. Here are typical daily intakes "as fed" (or "wet" basis): Fescue hay - 30 pounds per head; alfalfa hay - 30, DDGS - 6 to 10, condensed distillers solubles - 3 to 9, wet corn gluten - 13 to 23, dry corn gluten - 6 to 10, soy hulls - 6 to 10, wet brewers grain - 17 to 30. It's best to use actual numbers for your farm, though.

For Step 2 (determining nutrients available for use as fertilizer), the Missouri experts review that nutrient availability is affected by the concentration of nutrients in the feed, percentage of those nutrients excreted by the animals and the availability of the manure nutrients as a fertilizer source.

For the amount of supplemental feed (SF) from step one, use values above to calculate the amount of N (in pounds) available as fertilizer:

N equals (SF multiplied by protein concentration (percent) divided by 100) multiplied by excreted N (percent) divided by 100 multiplied by 35 percent fertilizer availability divided by 100 multiplied by 1 pound N fertilizer divided by 6.25 pounds protein.

Note that phosphate and potash in manure are 100 percent equal to fertilizer nutrients. However, N in manure is substantially less available than fertilizer N (35 percent of the N excrete in urine and manure is available as fertilizer; the rest is lost as ammonia or tied up in unavailable forms in the manure).

Step 3, as noted, is to determine the effective nutrient application rate for a pasture. The rate (R) at which nutrients are "applied" by grazing cattle is affected by the availability of nutrients in the manure (calculated in Step 2), the proportion of supplement feed fed in a particular paddock and paddock size. The formula is: R equals available nutrients (pounds) multiplied by percent of feed fed on that pasture divided by 100, and that number divided by pasture size (acres).

Unrolling hay is one way to increase distribution of nutrients in a pasture. Say you feed 100 cows 30 pounds of grass (fescue) hay a day from mid-December to early April (110 days). The winter pasture is 100 acres. What's the N value of the imported hay to this pasture?

Step 1: Amount of supplemental feed (SF) equals 100 cows multiplied by 30 pounds of hay per cow per day multiplied by 110 days equals 330,000 pounds of feed.

Step 2: Available N equals (330,000 pounds feed multiplied by 12.5 percent N divided by 100) multiplied by 100 percent excreted divided by 100 multiplied by 35 percent available divided by 100 multiplied by 1 pound N fertilizer divided by 6.25 pounds protein equals 2,310 pounds N.

Step 3: Nitrogen application rate equals (2,310 pounds N multiplied by 100 percent on the field divided by 100) divided by 100 acres equals 23 pounds N per acre.

Producers should note that the phosphate and potash application rate is more than double the annual removal rate for most pastures, so soil test levels for those nutrients will increase over time if the pasture is used year after year for winter feeding.

The available nutrients in manure are worth $2,967 or $29.50 an acre or $18 per ton of hay if you assume fertilizer value of 40 cents per pound of N, 30 cents per pound of phosphate and 20 cents per pound of potash.

Here's one more example, of a farmer feeding 40 cows 8 pounds of DDGS a day from January to early March (60 days) in a 25-acre pasture.

Step 1: SF (amount of supplemental feed) equals 40 cows multiplied by 8 pounds DDGS per cow per day multiplied by 60 days equals 19,200 pounds of feed

Step 2: Available N equals (19,200 pounds feed multiplied by 32 percent protein divided by 100) multiplied by 100 percent excreted divided by 100 multiplied by 35 percent available divided by 100 multiplied by 1 pounds N fertilizer divided by 6.25 pounds protein equals 344 pounds N.

Step 3: N application rate equals (344 pounds N multiplied by 100 percent on field divided by 100) divided by 25 acres equals 14 pounds N per acre.

The N application rate is close to the spring N needs of many pastures and the phosphate application rate is nearly the annual removal rate for most pastures. The amount of available nutrients in the manure are worth $194, or $7.75 per acre or $20 a ton of DDGS if you assume the same fertilizer values as above.

Barriers to feed as fertilizer

Cattle are, of course, your "fertilizer spreader." Urine typically contains more than 50 percent of the total N and potassium excreted by cattle. Conversely, manure contains 80 percent of the total P excreted. The nutrients in supplemental feed are only valuable as fertilizer if you can spread them evenly around the pasture - instead of around feeders and stock tanks and shady areas.

Regularly move feeders and feeding areas around the pasture. Increase stocking density by rotating livestock frequently. Don't use the same area for supplemental feeding every year. Maintain a setback of at least 100 feed from where you're feeding and a nearby stream. Avoid placing feeding areas where runoff will easily move into surface water, particularly in the winter when ground is frozen.

Missouri research reveals that moving the hay feeding area every two days results in higher pasture yield and higher P and K soil test levels in fields. Feeding hay in one spot resulted in similar yields in most of the pasture as if no hay had been fed at all. Thus, to capture fertilizer benefits of supplemental feed, feeding areas must be moved regularly. You can't fertilize a field with a manure spreader by just running it in one corner of the pasture. Similarly, you can't benefit from fertilizer nutrients in supplemental feeds if the feeder is always in the same spot.

 

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