Introduction

Milk fever, both clinical and subclinical, can cost dairy producers approximately $150 per case and may lead to other metabolic issues [1]. A valuable strategy to reduce milk fever cases on farms is the use of DCAD diets. Dietary Cation Anion Difference (DCAD) refers to the balance between cations (potassium and sodium) and anions (chloride and sulfur) in a cow’s diet. By feeding prefresh (21 days from calving) animals a low DCAD diet, dairy producers can help prevent the occurrence of milk fever. 

How it Works: Calculating DCAD

DCAD is a calculation used to determine the balance of positively and negatively charged ions in a diet.  Anions (negatively charged ions) include chloride (Cl) and sulfur (S), while cations (positively charged ions) include sodium (Na) and potassium (K).  

To calculate DCAD, we subtract the total amount of anions in the diet from the total amount of cations.  If there are more anions than cations, the DCAD value is negative, indicating we have created a negative DCAD diet. 

How it Works: Negative DCAD and the Cow’s Response

A negative DCAD diet helps prevent milk fever by increasing the availability of calcium that cows can access when needed at calving.  

This starts with formulating a negative DCAD diet.  By feeding minerals that contain chloride and/or sulfur – commonly referred to as anionic minerals – we introduce a high level of anions into the cow’s system.  In response, the cow’s body enters a state of metabolic acidosis [2], triggering two key changes: 

1. Urine pH decreases 
2. Blood pH decreases 

Lower blood pH makes bones and kidneys more responsive to Parathyroid Hormone (PTH).  This is important because PTH interacts with the bones to increase its release of calcium from bone stores.  Additionally, PTH works within the kidney to increase signals to the digestive tract to increase absorption of calcium from the diet [3]. 

Meanwhile as urine pH decreases, the body responds by excreting more calcium from the kidneys into the urine.  This may seem like a “waste” as the calcium leaves the body in urine, but it actually triggers a helpful response.  This calcium excretion sends signals to the bones to release more of its calcium stores into the bloodstream [4]. 

These three mechanisms – calcium excretion by the kidney, calcium release by the bones, and increased dietary calcium absorption – work together to boost the amount of calcium available to the cow [5].  With more available calcium, cows can better meet the calcium demands of the mammary system after calving. 

Guidance for Formulating a Negative DCAD Diet

To properly achieve metabolic acidosis, research suggests that diets should be formulated to a negative DCAD value between -100 and -200 mEq/kg (-10 to 20 mEq/100 g) [6,7].  Aiming closer to -100 mEq/kg (-10 mEq/100g) may be preferable in many cases, as values near -200 mEq/kg (-20mEq/100g) may trigger too severe of a metabolic acidosis response.  An adequately negative DCAD diet should result in an average urine pH of 5.8 to 6.8, which can be monitored on-farm to confirm whether the diet is working as intended.  Keep in mind that urine pH may take up to 48 hours to drop to these levels after starting the diet. 

How a Negative DCAD Diet Helps Cows Get More Calcium for Calving

Effectiveness

Negative DCAD diets for dairy cattle have been well researched over the past few decades.  Studies have shown that feeding a negative DCAD diet to dairy cows during the pre-fresh dry period (21 days from calving) successfully increases blood calcium levels before and immediately after calving [2,8,9].  As a result, many studies have reported lower incidences of clinical and subclinical milk fever [2,8,9,10].  Additionally, cows fed negative DCAD diets have shown a reduced risk of other transition-related diseases including retained placenta and metritis [7].  Feeding negative DCAD diets have been shown to decrease dry matter intake during the dry period, but intakes returned to normal after calving [2,7,9].  Overall, milk and component yields have been reported to be similar or higher in cows fed negative DCAD diets during the pre-fresh dry period compared to cows who were not [7,9,11].  It is important to note that feeding negative DCAD diets for the entire dry period (40-60 days) instead of the pre-fresh dry period (21 days) has had neutral to negative influences on milk production [12,13]. 

Although feeding negative DCAD diets to heifers prior to freshening has been associated with reduced uterine disease post-calving, studies have shown their impact on productive performance have been either neutral or negative [7,11].  Heifers have a much lower risk of developing milk fever than multiparous cows, therefore, feeding them a negative DCAD diet is likely unnecessary. 

On-farm considerations

To successfully feed a DCAD diet on your dairy, there are some considerations you should keep in mind. 

Palatability

Choosing the right DCAD product is crucial for maintaining your cows’ health and productivity. Some DCAD products might not be very tasty, and if the cows don’t like the taste, they might eat less, leading to a drop in dry matter intake. To ensure your cows are getting the most out of their diet, it’s essential to pick a DCAD product that is palatable. Integrating DCAD products into a total mixed ration (TMR) is more effective at maintaining dry matter intake compared to those feeding ingredients individual or component feeding.  

Monitor Urine pH

Regularly monitoring urine pH is key to making sure the DCAD diet is working as intended. This helps you adjust the diet as needed to keep your cows in optimal health and performance. An adequate negative DCAD diet should result in an average urine pH of 5.8 to 6.8.  The incidence of milk fever is predicted to be less than 5% when urine pH is maintained below 6.5 [7]. Cows will need to be fed the diet for approximately 48 hours before urine pH will reach these levels. 

Test Forage Mineral Levels

Balancing a diet for DCAD is like fine-tuning a musical instrument – precision is key! To hit the right targets, you need to know the exact mineral levels in the forages you’re feeding your animals. When you send in forage samples, make sure to request a wet chemistry analysis with a DCAD mineral package. This detailed analysis ensures you have accurate mineral levels, preventing any missteps that could throw off the DCAD diet and affect your animals’ health and performance. 

Create a Low Stress Environment

Effective cow and feed management are like the backbone of a healthy herd. To ensure your cows are happily eating on the diet formulated by a nutritionist, it’s vital to avoid overcrowding in the dry cow and prefresh pens. This helps them access their food without stress or competition. 

Keeping a close eye on the diet being fed and ensuring it matches the formulated plan is crucial for meeting their nutrient requirements. Stress-free cows are cows that will eat well. Limit pen moves, provide plenty of fresh and clean water, and protect them from heat and cold stress to keep them comfortable and thriving.