Dr Neil Miles, consulting soil scientist
Dung and urine are rich in plant nutrients, and clear evidence of this is the accelerated pasture growth where these products are deposited. Each animal is in a sense a “fertilizer spreader”, and farm management practices need to take this into account in order to make the best use of these nutrients and to avoid unnecessary expenditure on fertilizer and lime.
Nutrients in dung and urine
The feed consumed by animals – be it pasture, hay, silage, TMR or concentrates – is rich in plant nutrients. Only small proportions of these nutrients are used for meat and milk production, with the remainder being excreted in the dung and urine. In the case of dairy cows on pasture, the approximate percentages of N, P and K excreted are shown in Table 1.
The total amounts of nutrients excreted by a cow in a year are considerable (Table 2), with the amounts of nitrogen and potassium being the largest. Attaching a rand value to these nutrients underlines the economic significance of the recycling process. It should, of course, be borne in mind that the monetary value reported in Table 2 does not include that of the secondary nutrients, sulphur, calcium and magnesium, or of micronutrients such as zinc, copper, manganese and boron. And there is also the value of the dung in terms of its contributions to soil health (through improving soil organic matter levels, soil structure and biological health).
Why the high fertilizer requirements on livestock farms?
Given that the removal of plant nutrients from the farm in milk and meat is minimal, why is there an ongoing need for such large amounts of fertilizer in typical livestock operations? In the case of nitrogen, there is a partial explanation, in that large amounts of this nutrient may be lost by leaching from the rooting zone and by volatilization to the air from urine patches. But phosphorus and potassium are not lost in these ways, and one would expect long-term fertilizer requirements for these nutrients to be low on intensive livestock farms.
The principal reason for the ongoing high requirement for fertilizers is that because of the movements of animals and feeds, nutrients get depleted from certain areas of the farm and
concentrated in other areas. To illustrate this, let’s take a look at nutrient flows in a typical pasture-based dairy-farming operation (which includes silage and/or hay-making). On a farm of this kind, because of the movements of animals and feeds, there are large-scale flows of nutrients from one area to another. These flows are illustrated in the diagram, and result in a concentration of nutrients in areas where animals spend significant amounts of time and are fed (shown in blue), and a depletion of nutrients in more distant areas (yellow), and in particular in those areas in which feed is grown and removed. Plant nutrients are, of course, brought into the system in the form of fertilizers, lime and purchased feeds. Similar nutrient flows occur on intensive beef and sheep operations.
Depletion of nutrients from hay and silage fields
Large amounts of plant nutrients are contained in silage and hay (Table 3), and so their transportation represents a major flow of nutrients. The same applies to pasture utilized as ‘greenchop’. Potassium, in particular, is rapidly depleted in these ‘cut-and-removal’ type operations. This aspect is illustrated by the data presented in the graph, which shows the depletion of soil potassium levels in ryegrass pastures utilized either by grazing or for greenchop. Clearly, potassium loss when crops are mechanically removed can be very significant, and this must be taken into account in nutrient management programmes. A positive implication of this is that where nutrient excesses have built up in pasture systems, cutting-and-removal presents an effective means of addressing the problem.
Feeding on pastures
If feeding is confined largely to the same paddocks year after year, nutrients build up to excessively high levels in these paddocks. Soil test data from a kikuyu pasture (regularly used as a holding-camp for feeding) and maize silage land on a Northern KwaZulu-Natal dairy farm provide clear evidence of this problematical nutrient flow pattern (Table 4). The data reflect a build-up of potassium, phosphorus and organic carbon and nitrogen under kikuyu, while organic carbon and nitrogen levels in the maize field were extremely low (potassium and phosphorus supplies in the maize field were being maintained by annual fertilizer inputs).
Convenience is of course, usually the reason for restricting feeding to the same paddocks. However, this convenience comes at a price:
- The excessive fertility in the feeding paddocks represents a major waste of valuable plant nutrients which really ‘belong’ in the production fields (in the case of the example given in Table 4, a quick calculation reveals that the excesses of only potassium and phosphorus in this kikuyu pasture would have a value of about R14000 per hectare).
- Cows have a dislike for grass containing excesses of nitrogen and potassium, so intake of grass growing in the feeding paddocks is likely to be poor.
- Grass containing excess nitrogen and potassium has negative implications for animal health and reproduction.
Use of manure and slurry
On many farms there appears to be little appreciation of the value of manure and slurry in terms of its nutrient content, and these products, if used at all, are at best deposited onto the same patch of pasture year after year. This approach means that nutrients are not put to their best use and, furthermore, the quality of the treated pasture is eventually compromised (we frequently come across cases where animals will not eat pasturage because of nutrient overloads). Every effort should, therefore, be made to ensure that manure and slurry are distributed on those pastures in need of the extra nutrients, or better still, on silage or hay fields.
Practical pointers
- Make every effort to rotate fields on which animals are fed, or on which slurry is irrigated. If at all possible, feeding should take place on silage and hay production fields for at least part of the year. This will save enormously on fertilizer costs, and furthermore, should improve yields.
- Remember that ‘cutting-and-removal’ (hay, silage, green-chop) rapidly depletes nutrients, and in particular potassium. Fertilization and manuring programmes should take this into account.
- Target pastures with excessively high soil fertility levels. Bring the nutrients back into the system by either cutting-and-removing grass from them, or converting them temporarily to maize silage lands.
- Soil testing is a wise investment. Ensure that separate samples are taken from fields that are treated differently. Remember that fields furthest from the dairy or holding-yards usually have the lowest fertilities, so don’t neglect them. Stick with a competent adviser who samples meticulously and maintains comprehensive records for your farm.
- Save on fertilizers and optimize grass quality by varying fertilizer blends and rates on blocks of pasture with widely differing soil test levels. For example, to apply the same blend and rate to pastures testing 50 and 150 ppm potassium is inexcusable!
Please note: Consult a qualified person (Act 36 of 1947) for specific applications / recommendations
