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Spray or Fertilize?

Posted May 1, 2008

With higher (much, much higher) fertilizer prices and higher herbicide prices, a common question we get involves whether you get a "bigger bang for the buck" from fertilizer or herbicides on introduced pastures. Ideally you would use both on introduced pastures that have weed problems, but input prices have made this a less than ideal world in pasture management.

Let's start with herbicides. Probably the most consistent work I have seen in agricultural research is that which shows that a pound of weeds killed will result in a pound of grass gained. A moderate to heavy infestation of weeds weighs in the range of 500-1,500 pounds of dry matter per acre. While these weeds may be grazed somewhat while they are small, they are obviously not grazed much after that or they wouldn't be thought of as weeds.

If you use a herbicide that costs $10 per acre (including application) to control the weeds, you have gained 500-1,500 pounds of grass for your money. This equates to a grass value of $13-$40 per ton of dry matter forage gained for your herbicide cost. Unless you have high wildlife goals, weed control should be a priority in fields that have moderate to heavy weed pressures. If wildlife goals are important, they should be weighed against the grazing value obtained through the herbicide use. Also, if you have desirable legumes, you must decide whether the legume is giving you more than the weeds are taking away before you determine whether or not to spray.

To determine if you have enough weeds to spray, scout the fields at the time the weeds are emerging and beginning to grow to determine the species and quantity of weeds in your fields. If you don't have enough weeds to justify spraying, you can save money. It is difficult to describe a moderate to heavy weed infestation. If you're pretty sure weeds are causing you to lose significant grazing, it is probably enough weeds to justify spraying.

Does fertilizer pay? Most work in the southern Oklahoma/north central Texas area shows that nitrogen fertilizer will give a yield increase of about 30 pounds of dry matter forage per pound of nitrogen above the amount produced with no nitrogen. If urea is $500 per ton, this means that the cost of nitrogen fertilizer to produce 1 ton of dry matter forage is $37. While this is much higher than in the past, it is still cheaper than buying feed and bringing it in.

The above calculation only considers nitrogen (N). Phosphorus (P) and potassium (K) prices have gone up as much or more than nitrogen. If you need to use P and/or K with nitrogen, fertilizer use on pastures becomes unprofitable at current prices. If your soil is low in P and/or K and you do not apply them as fertilizer, nitrogen use efficiency will not be 30 pounds of dry matter forage per pound of nitrogen applied. It will be much less. Therefore, it is not advisable to apply only nitrogen fertilizer to soils that are low in P and K because the yield increase you receive will be unprofitable.

One possible way to maximize fertilizer inputs on pastures is to examine your soil tests and use nitrogen fertilizer on the fields that have adequate P and K and only need nitrogen. Do not use nitrogen on the fields that need P and K. If you can get animal manures, apply them only on the fields that need P and K since they have about as much P2O5 and K2O as they do N and will build up the P and K levels in the soil.

If you fertilize fields with moderate to heavy weed pressures, you definitely need to use herbicides. Fertilizing without controlling weeds will result in very abundant, large and healthy weeds.

In summary, if you have a lot of weeds, it is definitely economically feasible to control them unless you have very strong wildlife goals. It is still economically feasible to fertilize introduced pastures if you only need nitrogen (soil test to find out if fields are sufficient in P and K) or if you can get animal manures delivered and spread at a reasonable price.