Avoiding Milk Fever
Ensuring transition cows get the right diet puts them on the path to absorbing more calcium and staying healthy
Ensuring your cows can mobilize calcium efficiently in the early stages of lactation will improve your herd's overall health status, and your farm's bottom line. Otherwise, a common metabolic disorder, best known as milk fever, even at the subclinical level, could keep your cows from reaching optimum milk production.
The most critical period of the year for a dairy cow is the transition from late gestation to early lactation. In these five to six weeks, many changes trigger substantial hormonal changes. At the same time, the cow's metabolism diverts nutrients toward milk production. To further complicate the early transition period, her feed intake is at its lowest around calving time, and will not reach its maximum until several weeks after she freshens.
Causing milk fever, or hypocalcemia as it is known clinically, is the cow's inability to meet calcium demand for colostrum and milk production. Even if you don't see a clinical case of milk fever in your herd, you could still have issues that impact a cow's overall health status, milk production and your bottom line. Lack of appetite, poor dry matter intake in early lactation or a flat lactation curve may indicate a subclinical case resulting in lost milk production.
When the condition is severe, clinical milk fever can even be fatal. Surviving animals, as well as those with subclinical milk fever, have increased risk of retained placentas, displaced abomasums and mastitis. In all cases, affected cows will produce less milk.
Numerous studies have confirmed that changing a cow's diet before she calves reduces the risk for clinical and subclinical milk fever. This involves lowering what is known as the dietary cation-anion difference (DCAD). A calculation is required to determine the DCAD value of a diet. Formulas are used to calculate DCAD, and results are often expressed in milliequivalents per kilogram of dietary dry matter (mEq/kg). Lowered DCAD generates a mild metabolic acidosis. This acidosis can be confirmed by testing urinary pH. A lower urine pH, which is more acidic, indicates whether the pre-calving ration is doing what it should be doing.
Many studies have researched the impact of lowering DCAD in the precalving diet. Recently, a group of researchers performed a meta-analysis using 22 published studies covering 75 treatment groups of cows.
A meta-analysis is a mathematical procedure combining results from a series of independent studies dealing with the same subject. This analysis allows more precise findings since the number of cases used to generate conclusions is greater. It lets researchers refine findings and generate clearer conclusions.
Among the objectives of this metaanalysis were comparing various DCAD equations for their impact on clinical milk fever and urinary pH, and evaluating the impact of lowering DCAD on clinical milk fever and urinary pH.
Results confirm the effectiveness of lowering DCAD to decrease urinary pH and clinical milk fever, and suggest a slight modification to the usual DCAD equation.
As well, lowering DCAD increases calcium available to cows before and at calving, the meta-analysis confirmed. This reduces the clinical milk fever risk. By measuring urinary pH of close-up dry cows, you can monitor the effectiveness of the ration at preventing milk fever.
A dairy cow's normal urine pH is around 8.0. When she is fed a low-DCAD diet, previous recommendations called for urinary pH in the 6.0 to 7.0 range. However, this meta-analysis concludes a urinary pH of about 7.0 would be a reasonable goal.
Reaching a urinary pH of 6.2 would require a very low DCAD and would only slightly further reduce clinical milk fever incidence, the analysis reveals. This excessive acidification would provide just a modest benefit in milk fever prevention compared with reducing urinary pH to about 7.0.
Another key aspect of acidification urinary pH below 7.0 is its effect on dry matter intake. As the DCAD value of the pre-calving ration decreases, so does dry matter intake. You have to weigh the benefits of milk fever prevention from lowering DCAD against problems that could arise from reduced dry matter intake before calving.
If low DCAD pre-calving rations are part of your herd's close-up dry cow program, using a pH meter or pH paper strips to monitor your cow's response can be invaluable.
The way a cow handles the transition period will define the success of her next lactation. The origin of several metabolic disorders, including milk fever, can be traced back to this period. Implementing sound management practices and understanding the challenges specific to a transition cow will help reduce the effect of these disorders.
Formulas for calculating DCAD in dairy feed
The usual formula to calculate DCAD is (K + + Na +) - (Cl- + S2-). Other equations are sometimes used. The meta-analysis showed a slight modification to the usual DCAD equation should better reflect the lesser acidifying ability of sulphur. The meta-analysis identified the equation [(Na + K) - (Cl + 0.6 S)] as the most highly correlated to clinical milk fever incidence and urinary pH. When this equation is used to calculate the DCAD value of a pre-calving ration, the usual target of -100 mEq/kg needs to become 0 mEq/kg instead.
E. Charbonneau, D. Pellerin, and G. R. Oetzel. 2006. Impact of Lowering Dietary Cation-Anion Difference in Nonlactating Dairy Cows: A MetaAnalysis J. Dairy Sci. 89:537-548. E. Charbonneau, P. Y. Chouinard, G. F. Tremblay, G. Allard, and D. Pellerin. 2009. Timothy silage with low dietary cation-anion difference fed to nonlactating cows. J. Dairy Sci. 92:2067-2077. G.B. Penner, G. F. Tremblay, T. Dow, and M. Oba. 2008. Timothy Hay with a Low Dietary Cation-Anion Difference Improves Calcium Homeostasis in Periparturient Holstein Cows. J. Dairy Sci.91:1959-1968.
This article appeared in the January 2010 ruminations column of the Ontario Milk Producer magazine.
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