Forage crops remove nutrients from the soil just as row crops. If the field is cut for hay and the hay transported elsewhere, then those nutrients are indeed removed. Table 1 presents some general uptake values by our most common forage species.
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Table 1. Approximate pounds of nutrients removed by various forage crops at specified yield levels when harvested as hay.1 |
| |
Species and assumed hay yield in tons per acre |
Tall fescue
3.5 |
Bermudagrass
6 |
Sorghum-Sudan
4 |
Alfalfa
5 |
| Nitrogen (N) | 135 |
258 | 160 | 280 |
| Phosphate (P2O5) | 65 |
60 | 61 | 75 |
| Potash (K2O) | 185 |
288 | 233 |
300 |
| Magnesium (Mg) | 13 |
18 | 24 | 25 |
| Sulfur (S) | 14 |
30 | -- | 25 |
1 Source: Potash & Phosphate Institute
These figures provide a fair estimate of the fertilizer requirements of these forage crops. These values are an under-estimate of actual fertilizer recommendations due to environmental loss and tie-up of added fertilizers. Of course, the nitrogen removed by alfalfa is fixed from the air but alfalfa will still remove the other nutrients from the soil.
If the hay is fed on the farm or on a unit where it was produced, then some of "dedicated" hay fields. Hay feeding practices can help to recycle nutrients more uniformly although it may not be reasonable in some operations.
The situation is quite different in a grazed pasture. Grazing livestock recycle 80 to 90% of the nutrients they consume or remove from pastures in their manure and urine. Proper grazing management can assist greatly in redistributing the nutrients more uniformly across the pasture. Large pastures and continuous grazing will definitely result in nutrient movement and concentration. Nutrients will be concentrated at watering points, salt and mineral feeders, and especially under trees where the animals lounge.
A cow, assumed to be that mythical 1000 lb. brood cow, really accounts for no net loss of nutrients from a farm until she is sold. Actually, as shown in Table 2, she does not account for much of the nutrient uptake by forages, although she processes many 100's of pounds of nutrients in her productive life time. The net loss of nutrients in a beef production operation is in the product that is sold; i.e., the calf/steer/heifer. A look at Table 2 shows that very little is removed when the calf is sold.
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Table 2. Approximate amount of nutrients contained in the bodies of two classes of livestock, in pounds.1 |
| | Mature cow |
500 lb Calf |
| Nitrogen | 26 |
13 |
| Phosphorus | 7 |
3.3 |
| Potassium | 2 |
1 |
| Calcium | 15 |
7 |
1 Calculations are approximations based on several
references of bodily percent composition.
If the nutrients being consumed by grazing livestock are not really being removed from the farm in "product", then we have to assume that they are either lost in another fashion or are poorly distributed around the farm. Typically losses are not that great, which leads us to the conclusion that good grazing management can have a direct influence on the amount of fertilizer needed in pasture systems by increasing the uniformity of nutrient distribution in manure and urine.
