Managing and achieving a successful fresh-cow transition is a critical component of a profitable and sustainable dairy business. Despite significant achievements through research and improved management strategies, there are still numerous on-farm and individual cow risk factors that come into play, resulting in a fine line between achieving a successful transition and one that always seems to be a work in progress.
Clinical and subclinical hypocalcemia has been well established as a gateway disease leading to a higher probability of other metabolic disease(s), lower milk production, reduced reproductive efficiency, compromised immune function and a subsequent higher cull rate. The effects of clinical hypocalcemia are financially significant; however, subclinical hypocalcemia is more costly because it affects a higher percentage of fresh cows.
Research to minimize fresh-cow hypocalcemia continues to be extensively studied. For the past 50-plus years, research has focused on evaluating different feeding levels of dietary macrominerals during the dry period with the goal of improving fresh-cow blood-calcium status. This approach has been largely based on the understanding of calcium metabolism as regulated by parathyroid hormone (PTH) and vitamin D. However, as we have learned through the years, research data does not always translate to “research-proven” success at the farm level.
Let’s review the last several decades of macromineral research and recommendations that have helped shape efforts to control hypocalcemia.
1980s: Dietary calcium restriction
The nutritional focus during this time period was to minimize dry-cow dietary calcium intake to “turn on” the calcium-regulating machinery (PTH mechanism) and prepare the fresh cow to meet the high calcium demand at calving. Research suggested feeding less than or equal to 20-25 grams of dietary calcium per cow daily to achieve the best results. However, it was very difficult to reach this low dietary calcium intake with typical on-farm feedstuffs. Based on the existing research, the industry moved toward lowering dietary calcium as much as possible. With the switch to lower calcium feedstuffs, nutritionists were able to formulate diets containing less than or equal to 60-80 grams of calcium, which was a marked improvement compared to many dry-cow diets that supplied well over 100-125 grams of calcium.
1990s: Minimizing dietary potassium
As we entered the 1990s, feeding lower-calcium diets continued to be the recommended strategy to address fresh-cow hypocalcemia. For the most part, feeding lower-calcium feedstuffs such as corn silage and grass hay, in contrast to high-calcium alfalfa forage, worked reasonably well. However, when producers started incorporating alternative low-calcium feedstuffs, such as small-grain forages, something went wrong on the way to the calving pen. Oftentimes, there was a big increase in hypocalcemia and down cows. Even though small-grain forage-based diets met the nutritional specs for lower dietary calcium, these forages were typically very high in potassium. At this time, new research identified that high potassium intake, not high calcium intake, was a significant risk factor triggering hypocalcemia. Retrospectively, the most likely reason the “low-calcium” corn silage/grass hay diet worked was due to the low potassium content, and the reason the “low-calcium” small-grain forage diet did not work was due to the high potassium content. Furthermore, high potassium intake can inhibit dietary magnesium absorption, which compounds the hypocalcemia risk. With the emphasis now on dietary potassium along with magnesium, researchers took it another step further by introducing the negative dietary cation-anion difference (DCAD) approach as a practical strategy for minimizing hypocalcemia.
2000-20s: Emergence and adaptation of negative DCAD diets
At the turn of the century, research really ramped up evaluating negative DCAD diets to find the optimal balance of the key cations (lower potassium and sodium) and anions (higher chloride and sulfur). Higher chloride and sulfur intake along with lower potassium and sodium intake was designed to create a mild metabolic acidosis in dairy cows. This metabolic acidosis could be monitored and adjusted by measuring urine pH. Research showed that the induction of a mild metabolic acidosis would increase the sensitivity of the bone PTH receptors, creating a more robust metabolic response to the high calcium demand at calving, thus reducing both clinical and subclinical hypocalcemia.
During these two decades, the dairy industry heavily adopted the negative DCAD approach. Pre-fresh rations focused on minimizing the potassium and sodium cations while increasing the chloride and sulfur anions through various combinations of feedstuffs and supplemental anion sources. In addition, dietary calcium and magnesium were monitored closely to provide higher amounts to enhance the fresh-cow response. However, on-farm challenges remained on how to optimize this negative DCAD approach. Common challenges included sourcing a consistent supply of low-potassium forage, monitoring urine pH, maintaining high dry matter intake (DMI) and keeping the negative DCAD diets “dialed” in.
2020s: Dietary phosphorus restriction
The focus today has evolved to dietary phosphorus. Historically, dietary phosphorus was not closely evaluated in dry-cow diets unless there were reports of “low phosphorus” milk fevers. In those cases, nutritionists often added more supplemental phosphorus to the diet. This was done to “help” prevent the nebulous low phosphorus milk fever syndrome. Additionally, it was rare that nutritionists reduced dietary phosphorus in dry-cow diets. When unexplained problems occurred with the fresh cows, our industry tended to increase dietary phosphorus with the perceived goal of mitigating the issue and improving fresh cow health.
The more we learn, the less we know
In a true paradigm shift, research now suggests that lower dietary phosphorus, not higher dietary phosphorus, enhances fresh-cow serum calcium levels independent of other dietary macrominerals. A strong body of research has demonstrated a significant increase in serum calcium levels in response to dry-cow diets balanced at less than or equal to 0.20% dietary phosphorus, versus the higher 0.30%-0.45% phosphorus that we have routinely fed to prepartum cows. Since it is difficult to achieve a less than or equal to 0.20% phosphorus diet with standard feedstuffs, dietary phosphorus binders have become commercially available. This strategy is a new approach in the pre-fresh-cow pen.
When fed correctly, dietary phosphorus binders can bind enough dietary phosphorus to mimic this phosphorus restriction approach. As we have traditionally understood how low-calcium diets can trigger bone mobilization through the PTH-regulating mechanism, it is now understood that a mild phosphorus restriction can also trigger bone mobilization. It is this bone mobilization response that results in improved fresh-cow blood-calcium status.
The study of each macromineral
In “Dairy Nutrition 101” classes, students learn that there are seven macrominerals required in a dairy cow diet. Each one of these macrominerals has been studied (some more than others) as it relates to mitigating fresh-cow hypocalcemia. It is interesting to note the timeline when each macromineral was researched and prioritized in the industry (Figure 1).
This is a “good news/bad news” story for our industry. The good news is there are no other macrominerals to study. Phosphorus is the last one. The bad news is that it would have been nice if low-phosphorus diets had been studied earlier since research today shows a very positive effect on fresh-cow blood calcium.
Better late than never
The latest dietary phosphorus research is important, regardless of which calcium mitigation strategy you are implementing in your transition program.
Today, there are three feeding options for mitigating fresh-cow hypocalcemia on dairy farms (Table 1).
It is important to note that all dietary macrominerals are important and should be closely evaluated. But, regardless of the pre-fresh feeding strategy used, do not overlook and overfeed phosphorus. The 2021 Dairy NASEM recommends 0.19% phosphorus in far-off diets and 0.21% phosphorus in pre-fresh diets to meet the nutrient requirements. Of course, final dietary concentrations will vary depending on the feedstuffs available and the nutritional strategy implemented.
There has been great progress in the last 50-plus years in the understanding and implementation of hypocalcemia mitigation strategies specifically as it relates to macromineral supplementation. As always, there is more research to be done and more to learn to improve current programs and to provide new and innovative strategies that address fresh-cow hypocalcemia challenges.
