Fertiliser for hay: replacing what each cut removes

What a hay cut takes off the paddock

Every bale carries nutrients away. If they are not replaced, Agriculture Victoria warns pastures can underperform and low-fertility plants such as dandelions and sweet vernal grass may move in. Phosphorus (P) and potassium (K) are the two main nutrients to watch.

• Use the higher figure for early-cut or leafy hay and the lower figure for late-cut hay.
• Incitec Pivot's table (from national audit data) also shows nitrogen and sulfur: about 21 kg N and 1.7 kg S per tonne of grass/clover hay, and 28 kg N and 2.6 kg S per tonne of lucerne hay.
• The Corangamite Brown Book warns that paddocks cut for fodder year after year, with the fodder fed out elsewhere, are quickly 'mined' of potassium.

Work out the replacement rate

Agriculture Victoria's worked example for a late-season hay cut:

1. 30 round bales at about 350 kg each = 10,500 kg harvested (wet weight).
2. Hay is roughly 85% dry matter: 10,500 × 0.85 = 8.9 t DM.
3. Late-cut hay removes about 15 kg K and 2 kg P per t DM: 133.5 kg K and 17.8 kg P in total.
4. Divide by paddock area (3 ha): about 44.5 kg K/ha and 6 kg P/ha to replace.

Our arithmetic, using label analyses: 44.5 kg K/ha is about 89 kg/ha of muriate of potash (50% K), and 6 kg P/ha is about 68 kg/ha of single super (8.8% P). If about 30% of the hay is fed back on the same paddock, Agriculture Victoria says cut the replacement by 30%. Quote kg of nutrient per hectare when ordering so the supplier can match or blend a product. The NPK fertiliser guide explains label percentages.

Potassium: the big one

• Do not put on more than 60 kg K/ha in one go. Agriculture Victoria says higher rates could cause animal health issues such as grass tetany. If you need more, apply 60 kg K/ha after the cut and the rest with your normal maintenance dressing.
• The Brown Book rule of thumb: after hay or silage, apply about 15 kg K/ha for each tonne per hectare removed.
• Timing: a Dairy Australia (GippsDairy) article suggests potash-containing fertiliser either about six weeks before cutting or after harvest, especially if the fodder will feed cows close to calving (transition cows), because a high-potassium diet can upset calcium at calving.

Straight potash, such as Incitec Pivot MOP or Impact MOP, is the simplest way to replace K. Hay blends add N and S too: Summit lists Summit Hay at 13.6% N, 3.2% P, 16.5% K and 12.7% S. Check analyses side by side, for example Summit Hay vs Agfert Hay Booster, or browse the potassium fertiliser list. More on deciding whether potash pays: potash on pasture.

Nitrogen, phosphorus and sulfur

• Nitrogen: the GippsDairy article puts economic nitrogen rates at 0–50 kg N/ha on dairy pasture, with early-spring ryegrass responses of about 15–20 kg DM per kg N.
• When soil temperature drops below 8–10°C, it suggests sulphate of ammonia instead of urea, which also supplies sulfur. Compare them in urea vs ammonium sulphate and see when to apply urea on pasture.
• Phosphorus: on Victorian dairy soils there is little response above Olsen P 25; the article gives 18–22 as the ideal range.
• Replacing what hay removes is on top of the annual maintenance dressing, which Agriculture Victoria says is generally applied in autumn.

Lucerne hay and oaten hay

• Lucerne hay removes a lot of potassium and calcium and acidifies the soil: NSW DPI puts it at the equivalent of about 70 kg/ha of lime per tonne of hay removed. Keep testing pH in hay paddocks. See fertiliser for lucerne and how much lime per hectare.
• NSW DPI says 125–375 kg/ha of muriate of potash a year may become necessary where lucerne hay is cut regularly.
• Oaten hay: Agriculture Victoria lists 15–20 kg K per t DM; Soil Quality factsheets list about 25 kg K per tonne of oaten hay. See fertiliser for oats for the crop itself.

NSW Local Land Services suggests hay-cutting paddocks are among those to prioritise when fertiliser money is tight. Soil test hay paddocks regularly; see reading a soil test. Always check with your agronomist before acting.