When it comes to transition dry cow nutrition I believe that theoretically the full Dietary Cation-Anion Balance (DCAB) system is by far the best approach. Practically though it can be one of the hardest to implement successfully.
Whenever I have been able to get the system working well the performance of the cows improves month on month as cows freshen into the herd. The fact that the system is very effective at preventing not only clinical but sub clinical milk fever is obviously one of the main attractions and benefits. As we all know clinical and sub clinical milk fever predisposes the transition cow to a range of other transition diseases and consequential losses. Interestingly, however, the benefit of the full DCAB is more than just prevention of milk fever; it positively effects mammary development. Work done in-vitro with mammary epithelial cells has shown that cells cultured in the presence of increased concentrations of calcium are far more responsive to growth factors and mitogens that stimulate mammary cell proliferation. Clearly the greater the mammary mass the greater the milk yield potential.
The benefits of the DCAB system can only be unlocked when we acidify the cows’ metabolism sufficiently to be in a position where we can add calcium in sulphate or chloride form. To say we are acidifying the cows metabolism is not strictly correct, what we are actually doing is placing an acid load on the metabolism via the diet in order to trigger the body’s own homeostatic regulatory system. The homeostatic regulatory process is in part achieved by mobilisation of calcium from the bone and the excretion of anionic substances or hydrogen ions via the urine; hence the drop in urine pH. As a side note the respiration rate can also be increased to correct acid-base imbalances in the blood. It is also worth noting that if we include carbonate sources of calcium in the diet we are reducing the need for and the effectiveness of the homeostatic mechanisms.
Knowing that you have placed sufficient anionic load on the metabolism to trigger the mobilisation of calcium is the key to being successful with the DCAB system. However, this can only be achieved if we monitor urine pH and continuously monitor the mineral balance of the dietary components, especially the forages.
Controlling or even just monitoring potassium levels in the diet is essential. For example on a grass silage based dry cow diet containing 6kg of straw a change of 1% in potassium concentration can raise or lower the DCAB of the overall diet by +/-150. This would be enough to reduce the anionic challenge on the metabolism from a level where calcium can be included in the diet to a level where the addition of calcium to the diet could be fatal.
Monitoring urine pH is essential to determine the effectiveness of the diet but the most difficult to do. I forget how many hours I have spent waiting for cows to urinate or stroking the area between the vulva and the udder in an attempt to persuade cows to depart with a small sample – in the majority of cases to no avail!
The problem is that without an indication of urine pH, an indirect indicator of just how effectively we are stimulating the blood acid-base regulatory mechanism we just do not know what is going on.
For example, if you are on a DCAB system and you are still getting milk fevers, is it that the anionic load is not high enough (?) and by adding calcium to the diet the process of mobilising calcium from the bone is redundant. In these situations we either need to reduce dietary calcium or increase the anionic salts. In many DCAB situations I often find that the main problem is the failure to increase the anionic load sufficiently. As you can see from the figure below it takes a greater anionic load to get urine pH from 8.0 to 7.5 than it does to get from 7.5 to 6.5. Quite often what happens is that we give up at around pH 7.5-7.8 when quite often a little more acidification would result in a successful DCAB regime being implemented.
Once you get the Urine acidified do not forget to satisfy the magnesium requirement. Many sources of magnesium have very low solubility so it is important to either feed a quality source or increase the feeding rate accordingly. It is worth remembering that the incidence of milk fever is correlated more strongly to magnesium ad potassium dietary concentration than it is to calcium. Magnesium is essential in the vitamin D3 pathway and for mobilising calcium from the bone.
If you would like more information on DCAB diets and how they could work for your herd please contact one of the Dairy Technical Services Team.