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Just for kicks: What does my hay really cost?

Woody Lane for Progressive Forage Grower Published on 30 March 2016
Stacked big bales in the field

Each summer, nearly every farmer spends weeks making hay or silage and then during the rest of the year either feeds it to animals or sells it to willing buyers.

On the other hand, we all know of farms that have worn-out hay fields of low fertility, fields that need renovation with lots of fertilizer and seed to return to good production. This is a relationship that needs adjusting. Let’s talk.

To make hay, we cut the forage, dry it enough to prevent mold growth, package it into convenient bales and then haul it to a barn or market. From our fertility perspective, silage is just like hay, only wetter, and since the fertility principles are exactly the same, we’ll keep things simple by only focusing on hay.

So how much fertility is removed from the field when we make hay? Yes, yes, we’ve all been exposed to those popular “rules of thumb” – helpful adages like “one ton of hay equals 4.2 light-years of proteinaceous megajoules” – but I can never remember these rules either, so let’s compute the numbers ourselves. Then you can do it for your own place.

Let’s assume that we make a grass-legume hay of average quality. Let’s say that this theoretical hay contains (on a dry matter basis) 14 percent crude protein, 0.30 percent phosphorus, 2.3 percent potassium, 0.22 percent sulfur, 0.45 percent calcium and 0.21 percent magnesium.

The standard abbreviations are P, K, S, Ca, and Mg. And because we are interested in soil fertility, we must convert the crude protein value to nitrogen (N). Since crude protein is defined as N x 6.25, the N content of our hay is 2.24 percent (= 14 ÷ 6.25). We also must recognize that our hay is not completely dry, in spite of our best efforts.

Let’s say that our hay contains 10 percent moisture, which means 90 percent dry matter. Therefore, one ton of our hay (2,000 pounds) contains 1,800 pounds of dry matter, and this dry matter contains (rounding off for ease of reading) 40 pounds of N, 5 pounds P, 41 pounds K, 4 pounds S, 8 pounds Ca, and 4 pounds Mg. (Remember that we add two decimal places when converting a percentage to a multiplier. For example, 0.30 percent P means that we must multiply by 0.0030).

Our field, of course, will produce more than 1 ton of hay per acre. I know that university agronomy trials often publish optimistic results with hay yields of 8 tons or more, but let’s be more conservative with our field. Let’s estimate that with three cuttings in a season, we will harvest a total of 4 tons of hay per acre (= 160 bales at 50 pounds per bale).

Multiplying these 4 tons by the amounts of each nutrient per ton, we calculate that our total harvest contains 160 pounds N (= 4 x 40), 20 pounds P, 164 pounds K, 16 pounds S, 32 pounds Ca, and 16 pounds Mg. This is the fertility removed from each acre of our field when we haul the hay back to the barn.

160 pounds of N. Wow! Where is all this N going to come from? Well, legumes can “fix” N from the atmosphere, and if the paddock contains a high percentage of well-nodulated legumes, those plants may provide 50 to 100 pounds of N. Also, under proper conditions, the soil’s organic matter will release some N into the soil.

But the rest? The rest must come from an external source like fertilizer or manure. If all 160 pounds are derived from a fertilizer like urea, we would need to apply 348 pounds of urea to each acre (urea is labeled as “46-0-0-0” which means that it contains 46 percent N).

If we used a blended fertilizer like 16-16-16-6 (commonly called “triple-16”), which contains only 16 percent N, we would need to use 1,000 pounds of this fertilizer to deliver those 160 pounds of N.

But it gets worse. Let’s focus on P, K, and S, because these are the major mineral constituents of soil fertility. No atmospheric “fixing” here – these nutrients must all come from the soil or from fertilizer. Some soil particles do indeed break down and release minerals, but not very quickly.

But supplying these nutrients with triple-16 (16-16-16-6) may be a bit daunting. Realize that the nutrient levels in fertilizers aren’t always what they seem.

Although the tag values for N and S do represent their percentages in a fertilizer, the tag values for P and K do not represent the percentages of P and K. Why? Because, by law, these tag values actually represent the percentages of P2O5 (“phosphorus pentoxide”) and K20 (“potash”), which contain 44 percent P and 83 percent K respectively.

Therefore, in terms of real nutrients, 16-16-16-6 should properly be labeled at 16-7-13-6, a revision that would not be overwhelmingly popular with fertilizer companies.

If we want to use triple-16, we would need to apply 286 pounds per acre to supply 20 pounds P – and hang on to your hats – 1,262 pounds to supply 164 pounds K. (The details for K: We need 164 pounds of K using a fertilizer containing only 13 percent K. Therefore, 164 ÷ 0.13 = 1,262). And supplying 16 pounds of sulfur would require 267 pounds of triple-16. Maybe we should all buy stock in fertilizer companies.

Alternatively, we could select fertilizers with higher levels of K, such as muriate of potash at 0-0-60-0, which could supply our needed K with only 329 pounds of fertilizer.

But remember that these fertilizer amounts do not improve the fertility of the soil, they just maintain it. They only replace the nutrients removed by our hay crop. They don’t correct any pre-existing nutrient deficiencies. If we actually wanted to improve soil fertility over previous levels, we would have to add even more fertilizer.

Fertilizer is not free. You can apply your own prices to these fertilizers, but for grins, let’s say that triple-16 costs $17 for a 50-pound bag (a current local price), which means that 300 pounds of this fertilizer would cost $102.

Amortized over our 4-ton yield, that fertilizer would add $25.50 to the cost of producing each ton of hay. But this amount of fertilizer doesn’t even supply all the N and K we need, and we’ve also not included the costs for labor and machinery.

Fertility replacement is part of the true cost of making hay. If we harvest our hay and we try to save money by not replacing this fertility, we are actually mining the soil. And over time, our field will “wear out” and our forage yields will decline. Sound familiar?

So this summer, before we climb on our tractors and crank up the haying equipment, we might want to consider the true costs of this activity. We do have some alternatives. If we make hay, we can use that field as a winter-feeding area, and thus in a crude way recycle some of the fertility.

Or we can use that field only for grazing, where the animals just recycle the nutrients and charmingly deposit manure, which ultimately will help improve the soil’s organic matter.

If we need hay, we can buy it (someone else’s hay with someone else’s fertility), which we can then spread across our fields during the winter feeding period. And we’ll probably buy that hay for less than what it really costs someone else to make.  FG

Woody Lane is a livestock nutritionist and forage specialist in Roseburg, Oregon. He operates an independent consulting business and teaches workshops across the U.S. and Canada. His book, From The Feed Trough: Essays and Insights on Livestock Nutrition in a Complex World, is available through.

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