Nationally, 90pc of our soils are deficient in either phosphorus (P), potassium (K) or lime, and our silage ground is no different.
The majority of soil sample results off silage ground that land on my desk are typically either index 1 or 2 for P and K, and it is probably no surprise either when you consider that we have halved the volume of phosphorus, potassium and lime being applied on Irish farms since the 70s (see Table 1).
0-7-30 was the product of choice for the majority of farmers fertilising silage ground in the 70s and 80s, but it was gradually replaced by Cut Sward (24-2.5-10) in the 90s and Noughties, and now we are seeing 'cheaper Cut Sward' products becoming popular such as 23-2.2-4.5.
Table 2 shows the difference in the units of P and K applied using these different fertiliser types. Spreading a combination of CAN and 0-7-30 contains double the amount of P and four times the amount of K as the cheaper Cut Sward type products. So why do farmers use these products?
Imagine the following conversation; Farmer goes to the local co-op/merchant to purchase his/her fertiliser for their silage ground, the first question they ask of the merchant is how much is a ton of Cut Sward. The person behind the counter replies €375 (or thereabouts), but I have a cheaper version of Cut Sward for €20 a ton cheaper, and the farmer says, I'll take that one so.
Sounds like a great deal? But the reality is that while you've saved €20 on the ton, you've also received 136 units less of fertiliser in that ton. This scenario was most likely also the reason why we shifted from 0-7-30 to Cut Sward in the 80s. As Table 2 shows, we are now spreading 100 units less of fertiliser on our silage ground by using these cheaper products.
But are they cheaper? If we crudely analyse the cost of these fertilisers, we see that the so called cheaper fertiliser are actually costing us more on a per unit basis.
The impact of this is that it is taking fields longer to grow a substantial crop and quality is deteriorating. Many farmers using the cheaper products have to wait until mid June for fields to bulk up, resulting in poor silage quality of approximately 55-65pc DMD whereas those that apply the recommended amounts of fertiliser will have fields ready for cutting from mid-May onwards resulting in top quality silage with DMDs in the 70s.
You may ask is there any place for Cut Sward on Irish farms. Cut Sward is an ideal companion to slurry (see Table 3), but it is not suitable as a silage fertiliser on its own.
You may notice on Table 3 that the recommendation for index 1, 2 and 3 soils are all the same. This is to avoid the luxury uptake of potassium occurring which happens when too much K is applied too near to cutting. For this reason, it is advised not to exceed 3 bags 0-7-30 in a single application. After the last cut of silage is taken, is the best time to rectify soil Potassium deficiencies.
For those on free draining sandy soils, Sulphur also needs to be considered. Replacing CAN with Sulpha CAN will deliver adequate sulphur for most. Fertiliser for silage ground needs to be spread in late March/early April. If you are buying it over the next few weeks, then perhaps we need to go back to traditional ways and spread what the previous generations were spreading.
Joe Kelleher is a Teagasc advisor based in Newcastle West, Co Limerick
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