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Aonghusa Fahy November/December 2023

Animal Nutrition

Animal Nutrition

  • Silage cut in mid May is tested
  • Mineral analysis highlights the need for protected copper


  • Aonghusa weights on December 1st to monitor performance indoors
  • All weanling are front-loaded with 2 kgs of meal/head/day

Animal Nutrition

Aonghusa sent off a second sample of silage cut in mid – May and he is happy enough with the results. The DMD stands at almost 71% which is good and the crude protein is 13.6%. Ideally, he would be target mid 70’s for DMD and closer to 15% for crude protein.

Aonghusa also got a mineral analysis of the silage fed to the suckler cows pre calving. It highlighted high levels of Molybdenum which is a copper antagonist . As a result the copper in the mineral fed needs to be in a protected form.

Table 1: Aonghusa's mineral analysis

Macro Minerals

Magnesium (Mg):  Magnesium plays a crucial role in mobilising calcium from bones and increasing gut absorption, to reduce milk fever. Low levels of magnesium can also cause slow calving. Magnesium is not stored in the body, so it is very important it be fed right up to calving.

After calving Mg supplementation is required especially on lush spring or autumn grass to prevent grass tetany.  

Phosphorous (P): Makes up approx. 30% of total minerals in the body. A phosphorous deficiency can severly affect reproduction, causing silent heats, irregular oestrus and low conception rates. Low phosphorous is also associated with pica – i.e. eating stones etc.  

Sodium (Na):  Aids in nutrient transfer, waste removal, involved in muscle and heart contraction, rumen and blood pH. Deficiency signs are urine licking, reduced male fertility, lower milk production and depraved eating behaviour.

Calcium (Ca):   Calcium maintains normal muscle function and a deficiency can cause difficult calving’s and retained placenta. Obviously once a cow is lactating the requirement for calcium increases dramatically, it is a deficiency in calcium that cause milk fever.  A cow is unable to physically consume her calcium requirement, she has to mobilise it from her own bones. In order for her to be able to do this post calving, she has to start pre calving. Magnesium aids in this process and this is why minerals high in magnesium are fed pre calving. If calcium is fed pre calving, it meets her pre-calving requirement and she won’t have started the process of mobilising calcium from her bones and this will lead to a deficiency after calving, therefore - Do not feed calcium pre calving.


Trace elements

Copper (Cu): Deficiency can lead to small weak calves, scours and decreased milk. In weanlings, it can cause poor growth rates.

Selenium (Se): One of the few elements that can pass through the placenta from the cow to the calf. It is important as a deficiency can cause muscular dystrophy (weakening and wasting of muscle). Some areas are high in Se, so you should test your silage to ensure you do not cause a toxicity. A deficiency can also cause an issue with retained placentas.

Iodine (I): Deficiency can cause small weak calves, dead or hairless calves, or calves that do not want to suck. The animal will have low immunity. It can also lead to poor reproductive performance. It is also not stored in the body and needs to be fed right up to calving.

Cobalt (Co): Involved in the synthesis of B12 by the rumen, deficiencies more often seen in sheep. Can cause a rough coat, poor appetite and anaemia.

Manganese (Mn): A deficiency can affect growth, bone formation and the nervous system leading to poor growth, reproduction and bowing of the joints.

Zinc (Zn): Plays a role in the immune system and repair of damaged tissues. while it is also involved in the synthesis and metabolism of proteins and carbohydrates, teat keratin formation. A deficiency can lead to poor skin, mastitis, slow healing of wounds, bad hoofs and stiff joints. It can also lead to lower conception rates.



Vitamin A: Increases disease resistance and stimulates the immune system. Cows that have a deficiency in vitamin A can also produce dead, weak or blind calves because vitamin A is needed for normal growth and development including growth of the foetus. Again it is passed through the colostrum. Can also cause retained placenta.

Vitamin D3: Essential in calcium and phosphorous metabolism. Promotes growth and mineralisation of healthy bones, therefore plays an important role in the prevention of milk fever

Vitamin E: Required for good health and immune function. If fed pre calving it elevates the level in colostrum and help to keep your calves heathy. It does not pass through the placenta, so calves must get adequate colostrum.

Oxide v’s Sulphate Minerals

Bioavailability of sulphates and chlorides is generally greater than bioavailability of oxides. One exception is magnesium oxide, which is absorbed well enough to be used in beef cattle minerals. However, avoid mineral supplements that use copper oxide, which is poorly absorbed. Iron oxide is also poorly absorbed and is generally used to add colour to the mineral mix.

Boluses: Boluses will only cover trace elements, they do not cover macro minerals as macro minerals are fed in grams, the requirement is too large to be covered by boluses or injectables.

Catch crops, bolus with a high Iodine bolus to cover length on crop, have a good mineral thereafter. Catch crops are high in calcium, have cows removed at least a month pre calving.

What are protected minerals?

Minerals interact with one another in the diet and some may cause others to be bound in the rumen and therefore they are not able to be absorbed or is unavailable to the animal. A common example is high molydenum locking up copper. In this scenario, you can use what is called protected cooper, which basically means that it is already bound at manufacturing to another compound allowing it to pass though the rumen and be absorbed in the small intestine. These are call organic or chelated forms.










Aonghusa weighed the male weanlings on December 1st and they averaged 330kgs. This is an average daily gain of 1.09kgs. The 13 weanlings gained 0.91kg per day since their last weighing which is very good performance . The weanlings were castrated during the summer and will be kept for a second grazing season.

The heifers were also weighed on December 1st. They average weight was 341kgs giving an average daily gain of 1.04kgs since birth. They gained 0.64kgs per day since their last weighing . This is a good weigh gain while indoors on silage and meal.