Silage will be the main part of your herd’s diet for the next five or six months. Its mineral content changes from farm to farm, field to field and cut to cut. A silage mineral test before housing tells you what the diet is already supplying, and what’s missing. You can then feed only the minerals your cows actually need, instead of guessing.
This matters most for dry cows. What they eat in the last eight weeks before calving has a big effect on milk fever, retained cleanings, calf vigour and how well cows go back in calf. Testing now, in October, gives you time to get the right pre-calving programme in place before the first cows are dried off.
What a silage mineral analysis tells you
A standard silage test covers energy, protein and dry matter. A mineral analysis goes further. It shows the levels of the minerals your cows need, and the antagonists that stop them absorbing those minerals.
| What’s measured | Minerals | Why it matters in winter |
|---|---|---|
| Macro minerals | Calcium, phosphorus, magnesium, sodium, potassium | Milk fever, grass tetany, pica and dry cow balance |
| Trace minerals | Copper, zinc, selenium, iodine, cobalt, manganese | Calving, calf vigour, immunity, fertility and hooves |
| Antagonists | Molybdenum, sulphur, iron | These lock up copper and other trace minerals, even when levels look fine |
The antagonists are the part most farmers never see. Silage can have plenty of copper on paper, but if molybdenum or sulphur is high, the cow can’t use it.
What the results mean for your minerals
These are the results we see most often on Irish farms, and what each one means for your winter mineral programme.
| If your silage shows | What it means | What to do |
|---|---|---|
| High potassium | Potassium blocks magnesium and calcium uptake. This raises the risk of milk fever around calving. Silage from heavily fertilised or slurry-rich fields is often high in potassium. | Supply extra magnesium to dry cows, and increase it in the final week before calving. |
| High molybdenum, sulphur or iron | These bind copper in the rumen, so cows become copper deficient even with enough copper in the diet. Liming can raise molybdenum levels. | Set copper to overcome the lock-up, often using chelated copper. Confirm with a blood or liver test. |
| Low phosphorus | Cows may start eating stones, soil or wood (pica), and fertility can suffer. | Add phosphorus and sodium to the programme. |
| Low selenium | Linked to retained cleanings, weak calves and poorer immunity. Many Irish soils are low in selenium. | Include selenium in the pre-calving mineral. Don’t combine it with other selenium sources unless your vet advises it. |
| Low iodine | Leads to weak or stillborn calves, silent heats and retained cleanings. | Add iodine for the last four weeks before calving, and around breeding. |
| Low cobalt | Cobalt is needed to make vitamin B12. Young stock lose appetite and stop thriving. | Include cobalt for weanlings and replacement heifers. |
| Low sodium | Can reduce appetite and contributes to pica. | Add sodium to the mineral blend. |
One result on its own doesn’t tell the whole story. The balance between minerals matters as much as each individual level. Only about 1 in 10 cases of milk fever is clinical, so many cows are affected without showing obvious signs. A test shows the risk before you see problems.
Feeding more than grass silage?
Other winter feeds change the mineral picture again, so test or account for each one:
- Different cuts: first and second cut silage can differ a lot, and so can silage from different fields or years. Test each pit or batch of bales you’ll feed, especially whatever goes to the dry cows.
- Maize silage: low in phosphorus, sodium, calcium and magnesium.
- Fodder beet: low in phosphorus. Any soil eaten with it adds iron and molybdenum, which can drain copper over time.
- Kale, rape, swedes and turnips: high in calcium and potassium, and low in iodine, copper and selenium. They’re not ideal for cows close to calving.
- Weather: a wet summer or a drought changes how grass takes up minerals. Last year’s results may not match this year’s silage.
How to take a good silage sample
A test is only as good as the sample. Follow these steps:
- Sample each pit, or each batch of bales from the same field and cut, separately.
- Take samples from several points across the face of the pit or from several bales, not just one spot, and mix them together.
- Avoid mouldy, spoiled or surface material.
- Seal the sample in a clean bag, squeeze out the air, and label it with the pit or cut and the date.
- Keep it cool and send it off the same day or the next day.
- Ask for a mineral analysis, including molybdenum and sulphur. A standard feed-value test alone won’t show these.
Turning your results into a winter mineral plan
Once you have your results, here’s how TERRA NutriTECH builds them into a plan:
- Review: we look at your silage results alongside your herd history, such as milk fever, retained cleanings, weak calves or fertility problems in past years.
- Add blood tests where useful: silage shows what’s in the diet, and a pooled blood test shows what the cows are actually absorbing. Together they give the full picture.
- Formulate: we recommend a standard blend, or adjust one. Customising can be as simple as adding extra selenium, cobalt, copper or zinc to a standard product, or as complete as a mix made for your farm.
- Dose daily through the water: our AQUAMoov, AQUAHerd and OPISPro systems deliver the planned amount every day, with no hand-feeding and no missed days when you’re busy at calving.
- Check again: where possible, retest about 8 weeks after starting, and test again after each new cut you start feeding.
For dry cows, that often means a pre-calver such as Pre-Calver with Magnesium or Calving Xtra through the dry period, then Easy Calve in the close-up pens for the last week. The right products depend on what your silage shows.
Talk to us before you dry off. Send us your silage results, or ask us to arrange a silage and blood mineral analysis. Call Ronan on (087) 708 6714, email [email protected], or contact us through the website.