The transition from the dry period to lactation is one of the most stressful parts of a dairy cow’s life. Physiological and hormonal changes accelerate during the eighth month of gestation as the milk secretion glands enlarge in the udder and the cow prepares to give birth. Nutritional requirements increase significantly and play a pivotal role in the cow’s energy status and health at the time of birth and in the early weeks of lactation when milk production is reaching its peak.
Managing calcium metabolism and reducing prevalence of hypocalcemia (milk fever) in transition cows continues to challenge even the best dairy farmers and managers. Calcium is a macromineral necessary for bone formation and muscle function. Cows that are hypocalcemic for extended periods of time have poorly functioning rumens that lead to other metabolic problems including ketosis, displaced abomasums, laminitis, retained placentas, metritis and mastitis. Research has shown that cows with even subclinical levels of milk fever are likely to produce several hundred pounds less milk over the course of lactation.
During the dry period, calcium requirements are minimal and these mechanisms for replenishing plasma calcium are relatively inactive and are slow to start up again at the time of calving. The beginning of lactation, however, places a sudden and large demand on the calcium supplies and the mechanisms that keep it in balance in the dairy cow. A cow producing 22 pounds of colostrum will lose 23 grams of calcium in a single milking. This is about nine times as much calcium as is present in the entire plasma calcium pool of the cow. Calcium lost from the plasma pool must be replaced through increased calcium absorption in the intestine and calcium resorption from the bones.
Cows normally do a good job of keeping calcium balanced through a complex interaction of vitamin D and parathyroid hormone — except at the time of calving. Because the replenishing mechanisms needed to metabolize calcium are slow to respond at the time of calving, nearly all cows experience some degree of hypocalcemia during the first days after calving. Intravenous calcium treatments have been the treatment of choice for decades to help the cow along while intestinal and bone mechanisms have time to adapt.
In the 1980s dairy researchers began to recognize that macrominerals and their ions play a critical role in cellular metabolism. We know that at the time of freshening it’s necessary for calcium to be pulled from the bones because dietary calcium and the mechanisms required to metabolize it from the diet are not operating at full capacity. For this to occur the cow’s system must be slightly acidified — which involves a negative ionic charge referred to as anionic — to draw out the positively charged cationic calcium ions and get them into the bloodstream.
All ions have a negative or positive charge. To understand the anionic-cationic dynamic, think about the poles of a magnet. One pole is positive and the other is negative. Like poles repel each other and opposite poles attract each other. The same type of thing happens with ions in the bloodstream. In the case of the positively charged calcium ions, there needs to be a negatively charged environment to pull them out of the bones and get them into the bloodstream. If the bloodstream is overloaded with other positive ions — remember, like charges repel each other — the calcium ions can’t get to the bloodstream because they are held back. This is often the case in a close-up cow’s diet and the troublemakers are most often sodium and potassium ions, which also have powerful cationic charges.
When balancing diets for dry cows that are close to calving we need to get the system slightly acidified for approximately two to three weeks prior to calving. This acidification requires knowing the values of the two cations, potassium and sodium, and the two anions, sulfur and chloride, for the entire ration. Therefore, an analysis of all the feedstuffs in a close-up diet is required. The determination of whether a diet is anionic or cationic is calculated using a formula that measures the acid/base balance in the feed.
The cation-anion balance is most often known as the dietary cation-anion difference or DCAD. The DCAD formula will result in a positive or negative value when the cations are added together and subtracted from the sum of the anions. A positive value indicates that the diet is alkaline (more cations) or, if negative, acidic (more anions). A properly formulated anionic diet will result in a negative value typically around –5 to –10. (There are variations of this formula in use that incorporate other dietary minerals including phosphorus, calcium and magnesium, all of which have small additional impacts on the resulting calculations.)
The importance of having accurate values for the four main minerals in the dry cow diet cannot be overstated. Particularly in forages, mineral values vary considerably. Grass and legume forages are notoriously high and variable in potassium, which is the primary cation affecting DCAD, resulting in a positive value. Chloride and sulfate salts added to a dry cow/close-up diet are the means by which a positive DCAD becomes a negative DCAD.
All milk cow diets are positive DCAD diets due to the high levels of potassium in forages. When formulating close-up diets, look for low potassium forages. The higher the potassium in the diet, the more chloride or sulfate that must be added and there will come a point where the ration becomes unpalatable to the cow and too expensive to be practical.
Monitoring whether an anionic diet is being accomplished requires testing the pH level of the urine of these cows. A pH of 7 indicates neutrality; < 7 = acidic and > 7 = alkaline. The normal urine pH of a milk cow is around 8. So any pH value approaching 7 indicates that there is some acidification going on. The target for urine pH in anionic diets should be about 6.5–6.0. However, a close-up diet with a pH in the low 7 range will still have some minimal anionic affect.
Transition dry cow and close-up diets that incorporate anionic salts have been shown to reduce the propensity for milk fever and improve calcium metabolism at the time of calving. Those cows are less likely to experience other metabolic problems that often rob them of high milk peaks and overall production during their lactation.