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Grazing Management Q&A

University of Missouri forage and grazing experts Rob Kallenbach and John Lory take on common questions from farmers and ranchers about pasture fertility and grazing management.

Q: When is the best time to fertilize pastures?

Kallenbach: The first question to answer is whether you even need the extra forage from fertilizer. Research says you can get about an extra 20 pounds of forage production per acre for every pound of nitrogen (N) you put out there, up to about 100 pounds. If you get the full effect of 100 pounds, it would be worth an extra ton of forage per acre.

I’d be careful about overapplying N in the spring. For many farms, grass grows lush then anyway, and fertilizer gives you more growth than you can manage. When you stimulate cool-season grass growth in the spring, its competitive and shading effects can crowd out legumes like clover. If you fertilize in August, you might get more fall growth when you can use it.

I recommend that you put about one fourth of your fertilizer on in the spring for early grazing and maybe another fourth in May or June to extend good forage into summer. Then apply the rest in late summer to help extend the grazing season as far into fall as possible. That’s better use of your fertilizer dollars.

Q: What’s the right mix of legumes in a pasture?

Lory: Legumes are good for their grazing quality, and they fix N on their roots, which helps the surrounding grass plants, too. Up to 50% of the N in grass from a legume-grass mix is fixed by the legume and transferred.

We like to see 30% of the plants in a pasture as legumes such as red clover, lespedeza, or bird’s-foot trefoil. It’s a good thing to have two or more legumes in the pasture mix to reach the 30% goal. A complex mix of species seems to give more stable yields. 

Your eyeball is not necessarily the most accurate way to estimate clover percentages; you will have a tendency to overestimate the legume. 

Q: Last summer and fall it was very dry. Did I waste my pasture fertilizer?

Kallenbach: In the tests we’ve done in dry weather, we don’t get as much effect from fertilizer. Remember that forages grow longer than annual crops. In Missouri, we can have forage growth all the way into December if the weather cooperates. Fertilizer that doesn’t get utilized in the fall tends to carry over into the spring in pastures.

Lory: Fertilizer carries over in forages better than grains because of the lower potential for saturated soils. Forages have extensive root systems that build organic matter in the soil and promote aeration. 

Q: Is red clover the best pasture legume?

Kallenbach: Yes, I’d say, in general, but clover doesn’t do much when it is really hot; it goes dormant. Lespedeza is a better legume at that point, but it will only fix about as much N as it uses itself, and not much is left for the surrounding plants. As with most things, it’s a trade-off. A mix of pasture forages is best for a variety of reasons.

Q: How much fertilizer value do I get from the cows’ manure droppings in the pasture?

Lory: There are two answers to this. With stable nutrients like phosphorus and potassium, you probably can meet the full requirements through urine and manure deposits. Every manure pile provides three or more years of phosphorus where it lands. Intensive grazing promotes good manure distribution over time, so manure can fertilize the whole field. You can even increase soil test levels on a pasture if you also winter-feed on it. 

As for N, you always lose some of it through ammonia volatilization and poor availability. A cow excretes about 50% of her N in manure and 50% in urine. Urine patches will have high losses of N as ammonia. Organic N in the manure is not all available to grasses. In general, we assume that a little over a third of N excreted by a cow is recovered by the forages.

The challenge is the patchwork pattern that N is distributed in manure. Some areas get all they need; others get none. That means that pastures need some form of supplemental N, either from legumes or applied fertilizer.

Q: It often appears that cattle won’t eat the grass that grows lush around a manure pile. Do they?

Kallenbach: They have an initial aversion to it, but as new forage grows in that spot, they come back and eat it. Sometimes, I think the reason it looks like they don’t eat there is because the grass near a manure pile is the fastest growing and it goes mature. Then it is less palatable to them. 

Q: Does it pay to spread manure piles by harrowing?

Lory: We get that question so much that we’ve thought about doing a research trial. Of course, harrowing doesn’t have any effect on the urine patches, only on the manure piles. Manure piles have all the phosphorus, maybe half the N, and even less of the potassium. The potential benefit will be to spread out the phosphorus and perhaps speed the breakdown of manure piles. 

When I managed pastures years ago, I harrowed. Now I wouldn’t recommend it, because I think your time is better spent improving other aspects of pasture management like grazing more intensively.


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