Hmmm. Lots of things go into that equation, so let’s work on some answers.
In fact, let’s describe a hypothetical field and work out the exact amounts of nutrients removed by haying and by grazing, step-by-step. Since this is a magazine article and our field is hypothetical, we can include some attractive features. We’ll select a reasonably fertile area of 5 acres. Our field is flat, well-drained and, of course, tightly fenced. Under our good management, this field has produced reasonable forage yields in the past. We will assume that the forages are growing nicely by May 1 and that forage growth ends in mid-October.
Our forage mixture is a nutritious combination of grass and clover. We’ll also assume that our forages grow evenly in a vegetative state throughout the growing season, with no peaks or valleys, and the plant nutritive value remains consistent and high. Remember, this is a hypothetical field, so we can dream, can’t we? Actually, none of these ideal characteristics alter the underlying principles or calculations; they just make the calculations easier and cleaner.
What is our annual forage yield? Let’s say that over the entire growing season, our field produces a respectable 2.5 tons of hay per acre. At 90 percent dry matter (DM), this equals 2.25 tons DM per acre. Therefore, our 5-acre field yields a total of 11.25 tons DM (= 5 x 2.25), which equals 22,500 pounds DM. This is an important number.
Now let’s calculate the nutrients lost in hay. We will assume that our hay is hauled to a barn or sales arena, which means the nutrients are trucked away from the field. For this exercise, we are interested in the primary plant nutrients of nitrogen (N), phosphorus (P) and potassium (K). (Note for experienced graziers: We are interested in the elements N, P and K, and not the fertilizer bag levels of P2O5 and K2O.) Our hay is vegetative forage, so let’s also assume that it analyzes at 14 percent crude protein, 0.25 percent P and 2 percent K. These are reasonable values for this type of forage.
The hay calculations are relatively straightforward – we’ll just apply those percentages against the total DM yield of 22,500 pounds. But there is one quirky exception: The crude protein value must first be converted back to nitrogen because we are interested in the nitrogen as a plant nutrient. The standard conversion factor is 6.25. You may recall that crude protein is a calculated number: We first test for nitrogen and then multiply that value by 6.25 to get crude protein.
For the purposes of this article, we must go back the other way. Therefore, 14 percent crude protein in the hay equals 2.24 percent N (= 14 ÷ 6.25). So by applying the percentages of 2.24 percent N, 0.25 percent P and 2 percent K against a 22,500-pound yield, the soil nutrients removed in the hay from these 5 acres are 504 pounds N, 56 pounds P and 450 pounds K.
Now for the comparison: We must calculate the amount of nutrients that leave the field when we graze sheep. Unfortunately, this is not a straightforward calculation, so bear with me. First, we need to describe the flock management, which will tell us how many sheep leave the field at the end of the season and their weights. Then we must identify the percentages of nutrients in those animals. But these calculations include an important proviso: We are only interested in the “net loss” of nutrients from the field, not the entire weight of animals that walk off the field.
Net loss? Net loss means the amount of soil nutrients contained in the additional weight animals gain while they are in the field – above the amount that entered the field when we first opened the gate in the spring. To be specific, grazing sheep that only maintain their weight through the summer represent zero net loss of nutrients from the field. During the summer, soil nutrients simply recycle through those sheep while they graze, going into one end as grass and coming out the other end as pellets.
For example, if I graze 20 dry ewes in a field for four months, and every ewe weighed 160 pounds when it entered the field and still weighed 160 pounds when it left the field four months later, the net nutrient loss from that field is zero. But you ask, what about gut fill? Well, sure, animals leave the field with a full rumen, but they also enter the field with a full rumen. Full equals full, so we consider that amount a wash.
Back to our calculations. First, we’ll calculate the number of sheep that can graze that field for the season and how much they will gain. Then, as we did with the hay, we’ll identify the percentages of nutrients in the animals and apply those percentages to the additional weight gain. Still with me? Here goes …
Our 5-acre field produces 22,500 pounds DM for the growing season. Let’s assume our ewes each weigh 160 pounds. They drop a 200 percent lamb crop in April, lose no lambs at lambing and enter the field on May 1 each raising two 20-pound lambs. How many of these ewes can we graze for the season? Normally we would assume a DM intake of 3 percent bodyweight for the ewes, but because these ewes are rearing twin lambs that are growing, and we don’t want to use complex simultaneous equations or daily iterations of feed intake in our calculations, let’s include an extra intake cushion to account for the feed consumed by the lambs.
We’ll simply increase the DM intake of the ewes to 6 percent bodyweight, which means that a 160-pound ewe would eat 9.6 pounds DM per day. Our grazing period is May 1 until Oct. 7, which is 160 days (160 days is a very convenient number, as you’ll see; remember, with our theoretical field, we can create all sorts of adjustments to make our calculations easier).
So 22,500 pounds of DM at 9.6 pounds per day gives us 2,343.8 ewe-days. Dividing this by 160 days gives us 14.6 ewes for the entire period. Let’s round this off to 15 ewes.
Fifteen ewes with a 200 percent lamb crop means 30 lambs. Let’s say that no lambs die; coyotes never find them; the lambs have no worms; and on Oct. 7, all the lambs weigh exactly 100 pounds (I’m beginning to like our theoretical field). When the flock leaves the field on Oct. 7, the ewes all still weigh 160 pounds, so their net nutrient loss to the field is zero. The lambs have each gained 80 pounds (= 100 - 20), representing a respectable daily gain of 0.5 pound (= 80 ÷ 160). Therefore, the total net live weight gain of the lambs is 2,400 pounds (= 80 x 30). This is the additional weight gain of the entire flock in that field during the grazing season.
Now the big step: We need to calculate the net loss due to these lambs. How much soil fertility is in that additional weight gain? Well, body composition numbers are not exactly found on feed tags, so I searched the scientific literature, including some older physiology textbooks like the legendary Duke’s Physiology of Domestic Animals (1977) as well as many recent research articles. I finally came up with some solid numbers: On a live-weight basis (i.e., as they walk across the scale) 100-pound lambs contain approximately 16 percent crude protein and 5 percent ash. Using the same conversion factor of 6.25 to convert crude protein back to nitrogen, their nitrogen content is 2.56 percent (= 16 ÷ 6.25).
The ash is nearly all bone, and bone contains 27 percent calcium and 13 percent phosphorus. For the purposes of soil fertility, we’re really interested in the phosphorus. Therefore, 13 percent of the 5 percent ash equals 0.65 percent. I found the potassium value in a 1968 book on body composition. No calculations here, just a lookup number. The potassium content of lambs at that weight is 0.17 percent.
Now we have our three percentages of soil nutrients: 2.56 percent N, 0.65 percent P and 0.17 percent K. When we apply these percentages against the 2,400 pounds of additional body gain, we get the following net nutrient loss from grazing 15 ewes and their 30 lambs for the season: 61 pounds N, 16 pounds P and only 4 pounds K. These are the nutrients removed from the field in the sheep.
There we have it. In our example, grazing sheep and selling the lambs removes 88 percent less nitrogen, 71 percent less phosphorus and 99 percent less potassium compared to making hay from the same field.
It’s interesting that grazing removes so little potassium. Potassium is usually the nutrient most depleted in a long-term hay field, primarily because it constitutes a significant percentage in forage and many tons of hay are hauled off year after year. But in animals, although potassium is critically important metabolically, the actual amount in the body is relatively small when compared to phosphorus, which is a major component of bone.
We made lots of assumptions for our theoretical field, so these numbers are hardly exact. The hay numbers are pretty good, but the sheep numbers? Well, even if we had overestimated feed intake and could increase the flock size by 50 percent, or had a lambing percentage better than 200 percent or allowed the ewes to gain body condition through the season (and there are other possible variations as well), all these changes would have indeed increased the net loss of nutrients through grazing – but not by much. The difference between removing nutrients in hay is so much greater than by grazing that these refinements really don’t change the relative patterns of the numbers.
Finally, we can also express the nutrient losses on a “per-acre basis” by simply dividing by five. On a per-acre basis, hay removed 101 pounds N, 11 pounds P and 90 pounds K. Grazing removed 12 pounds N, 3 pounds P and 1 pound K. And when we place real costs on these numbers (namely, the fertilizer needed to replace lost nutrients), the financial differences are stunning. So from a no-nonsense business perspective, how much is it worth to find ways of increasing grazing and reducing the need to make hay? That may be something to ruminate on. 
PHOTO: Grazing sheep that only maintain their weight through the summer represent zero net loss of nutrients from the field. During the summer, soil nutrients simply recycle through those sheep while they graze, going into one end as grass and coming out the other end as pellets. Photo by Walt Cooley.
Woody Lane is a certified forage and grassland professional with AFGC and teaches forage, grazing and nutrition courses in Oregon, with an affiliate appointment with the Crop and Soil Science Department at Oregon State. His book, From The Feed Trough: Essays and Insights on Livestock Nutrition in a Complex World, is available through Woody Lane.
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Woody Lane, Ph.D.
- Lane Livestock Services
- Roseburg, Oregon
