DCAD - SAD or GLAD COWS!

In recent months there has been considerable press about anionic salts based on the results of a trial run (with a small number of cows) at Ellinbank in Australia. From this one trial the claim is that anionic salts do not work on pasture based systems in NZ and that magnesium is all that is required pre-calving.

Unfortunately just as we have seen many times in the past, particularly with the supplementary feed trials over the last five years, the message is being delivered and reported as a one liner or take home message. This leaves no room to take account of the different feeding systems farmers are now using in NZ.

Briefly, the theory behind feeding anionic salts is to get the cow mobilising calcium from her bones, as well as ensuring that she is absorbing calcium efficiently from her diet pre-calving. In this way when she calves and loses large quantities of calcium through the production of colostrum, she is able to draw on her calcium reserves thus avoiding hypocalcaemia. The result is increased in milk production, reduced retained membranes, reduced mastitis and reduced ketosis (Curtis et al.1985).

To stimulate the cow to mobilise calcium from her bones anionic salts are fed for 2-3 weeks pre-calving. Anions (i.e. chloride, sulphur and phosphate), when fed in high enough quantities lower blood pH as opposed to cations (i.e. potassium, magnesium, calcium and sodium), which increase the blood pH. The cow must keep blood pH within a very narrow band to stay alive. As the anionic salts lower the pH of the blood, the cow responds by producing parathyroid hormone which has two effects; firstly, it causes the re-absorption of bone, being the largest source of magnesium and calcium in the body. Secondly, this same hormone increases the efficiency of calcium absorption in the intestine of the cow from the diet. The calcium and magnesium being cations, increase the blood pH keeping the cow in a homeostatic state.

As the cow keeps her blood pH within a tight range (pH 7.2 - 7.4), it is not practical to measure blood pH to see if you are getting the effect from feeding anionic salts. However, a good indicator of this is the pH of the urine. Typically cows will have a urine pH around 8, before supplementing anionic salts. This should fall to less than 7.0 with 6 - 6.5 being a good target after 2 weeks of supplementation. If this occurs, then it is very likely that the cow is mobilising calcium and magnesium from the bones and will not develop milk fever, provided she is supplemented adequately with calcium and magnesium in her diet post calving.

Magnesium is very important as it is involved in the production of parathyroid hormone from the parathyroid gland among other important functions. For this reason the main anionic salts used in the past have been magnesium sulphate and magnesium chloride.

The trial from Ellinbank fed cows on pasture and hay. Pastures in Ellinbank are very high in potassium (above 4% per kgDM) as they are in many parts of New Zealand. High potassium intakes pre-calving are more likely to pre-empt milk fever than calcium (Horst 1997). The cows were administered with anionic salts by drenching and they were unable to lower the urine pH. This would indicate that insufficient anionic salt were fed to counter the high concentration of potassium (cations) in the diet.

In northern Australia early reports from two commercial (pasture fed) trials are indicating positive results from the use of anionic salts (Walker et al. 1998, McLachlan et al. 2000). This is probably due to tropical pastures having lower potassium levels than the temperate pastures.

In NZ many farmers are now strategically using maize silage or other supplementary feeds low in potassium that dilute the concentration of potassium levels of the diet pre-calving. With the addition of anionic salts, the DCAD of these diets are sufficiently low to get the fall in urine pH and reap the consequent benefits in animal health and production.

Our own experience over the last four years with our clients using anionic salts has been very positive. Although it has not eliminated milkfever in all herds, it has had significant benefits with the majority of clients. Fewer or no downer cows, increased appetite post calving, easier calving, healthier calves and cows reaching peak milk production earlier are common results from the use of anionic salts in our experience.

NZ dairy farmers have made good use of supplementing magnesium pre-calving, yet milk fever continues to be a major issue for many. So are the claims that all we need is more magnesium pre-calving correct? Recently a Scandinavian researcher (Pehrson et al. 1999) found that by feeding a concentrate pellet containing anionic salts twice daily pre-calving in a pasture silage/hay diet they were able to reduce the cows urine pH when compared to the control herd, with subsequent improvements in milkfever. Magnesium levels were equal in both the trial and treatment herds. Another researcher (Dishington et al 1982) significantly reduced milkfever in a herd with a history of milkfever (despite having high blood magnesium levels) by supplementing 3 kg of a concentrate pellet containing anionic salts for 3 weeks pre-calving.

Perhaps the strongest evidence that it is not just magnesium that is required was a trial by the researcher Oetzel et al 1988. In this trial, ammonium salts were used as an anionic salt in cows that had naturally high levels of magnesium. They found that those cows supplemented with ammonium salts had higher serum levels of calcium at calving. Ammonium salts do not contain magnesium or calcium; therefore the effect must be from the mobilisation of calcium from the bones and improved absorption of the calcium from the diet.

Our high potassium pasture are causing us issues with animal health, so strategies to reduce the potassium level of the diet pre-calving are necessary. The use of maize silage and other low potassium containing feeds can help here. Lower applications of potassium fertiliser may help address the high content in pastures and reduced nitrogen applications leading up to calving will help to lower potassium levels in pasture.

These are some of the strategies you can use to prevent milkfever in conjunction with anionic salts. Hopefully the dairy industry will not be held back for another five years waiting for the truth to prevail.

Warren Morritt B.Agr
Dairy Consultant/Manager
Intelact Nutrition Limited