IN general the spring calving suckler cow gets very little compound ration and therefore gets no opportunity to get supplementary minerals in her regular diet.
This is as it should be, as the nutritional requirement of a suckler cow in late pregnancy is low and should be completely supplied by silage of moderate quality and to a restricted allowance of high quality silage. There are seven “major” minerals (calcium, phosphorus, potassium, sodium, magnesium, chlorine and sulphur) and 15 “minor” or trace elements regarded as essential for good animal health and performance. The most important trace minerals are copper, selenium, manganese, cobalt, iodine, iron, zinc and molybdenum.
Normally grass and good quality silage are reasonably well balanced for the major minerals but deficiencies of trace minerals mainly copper, selenium, iodine and cobalt are common in Irish pastures. In addition, a surplus of some minerals can lead to a poor absorption of another mineral, e.g., excess molybdenum or sulphur causes copper deficiency.
Cows fed on poor hay or straw should get supplementation of the calcium, phosphorus, sodium and magnesium. However, excess calcium combined with low magnesium in the diet of pre-calving cows can result in milk fever immediately after calving. Therefore, pre-calver minerals should contain little or no calcium (depend on the forage) but up to 15% magnesium in the mineral mix.
Diets containing high levels of cereals are deficient in calcium and those containing high levels of pulps and molasses are deficient in phosphorus while having adequate to excess calcium.
The most common trace mineral deficiencies in cattle on forage are copper, iodine, selenium and cobalt. Deficiencies can cause a range of health problems that affect growth, performance, disease resistance and reproduction. Some of the chemical signs of deficiencies in copper, iodine and selenium are common to each other. These are stillbirths, abortions, lowered immunity to diseases such as calf scours, pneumonia, mastitis and below normal survival rates in calves around the time of birth.
Getting a definitive diagnosis of trace mineral deficiencies can be tedious. Doing a large range of tests and/or giving a large selection of mineral supplements can be costly. Sometimes a blood test could show a deficiency but if there are no clinical signs and performance is satisfactory there is unlikely to be a response to extra mineral supplementation. Blood and feed tests do not give a full diagnosis of themselves but are best used to support a clinical diagnosis by your vet. For a start, try to find out if there is a problem, i.e., is the productive or reproduction performance below average or what it could be expected to be within the normal range. If the answer is yes, then check out the obvious causes such as inadequate feeding, parasites, infectious diseases or poor environmental conditions. Next check out the mineral status of the feed and if there appears to be a mineral imbalance this should be corrected. Blood tests on a sample of animals can give an indication of possible deficiencies.
Research, over several years, has established the mineral requirements and given recommendations for different categories of stock based on the mineral content of the common feeds. A sensible policy is to give a pre-calver mineral supplement to suckler cows on silage as the sole diet.
More specialized supplements are needed for animals on maize or whole crop silage or where fodder beet or brassicas makes up the main proportion of the diet. The mineral is best fed at a fixed rate per day and with silage the loose mineral can be spread on the silage. The recommended rate is 100 grammes per cow per day. If all cows can feed at the one time, applying the mineral once per day (e.g., after the silage is put in) should be adequate, otherwise spread on the silage twice per day. Free access mineral licks or blocks can also be used but those are more expensive and intake among the cows is more variable than loose mineral spread on the silage.