In the previous article, “The Challenges of Modern Dairy Cow Management” phosphorus excretion from dairy cows and the consequent eutrophication was briefly mentioned. Phosphorus is an essential major nutrient for plants, hence the rapid growth of algae and removal of oxygen when it enters water courses (Mekonnen and Hoekstra, 2018).
Phosphorus is also essential for the dairy cow. Deficiencies of phosphorus commonly manifest in young cattle as poor growth rates, rickets, and anorexia. In adult dairy cattle it leads to reduced feed intake and overall poor performance and the onset of the condition known as Pica, where animals have a desire to eat objects of poor nutritional value such as soil, fence posts, string and even stones (Elshahawy and Aly, 2016). Pica can be attributed to other mineral deficiencies but in the case of phosphorus, it describes a long-term lack of dietary phosphorus, although it is not overly common in the UK. The type of hyperphosphatemia dairy producers will be more familiar with occurs around calving. Often mistaken for milk fever, cows suffering low blood phosphorus will become recumbent due to impaired nerve and muscle function and suffer downer cow syndrome. It is difficult to study downer cow syndrome attributable to phosphorus deficiency as it’s not easy to induce within a research setting (Grunberg 2008; Rodehutscord et al., 1994). Nevertheless, many vets such as Blowey (2016) report that downer cows suffering phosphorus deficiency have a tendency to be more alert than the milk fever cow, some will even eat feed placed in front of them. Oral administration of inorganic phosphate salts is an increasingly common and generally effective treatment strategy for the disorder.
Clearly a lack of phosphorus is extremely undesirable, and a common strategy is to provide plentiful amounts in dairy cattle diets which often leads to overfeeding. A study by Sinclair and Atkins (2015) found that in 50 farms across the midlands and the north of England, on average phosphorus was overfed by 20% within cattle diets, based on the recommendations provided by NRC (2001) see Figure 1.
Figure 1 Phosphorus feeding recommendations for lactating dairy cattle g/kg DM.
Where P = phosphorus. Adapted from Ishler and Wu (2003).
As discussed in the previous article, cattle generally only utilise up to 40% of the phosphorus consumed (Salazar et al., 2012), dependent on the source e.g., organic or inorganic which effects the bioavailability. With the potential to excrete 60% of the phosphorus consumed, the amount excreted is further exacerbated if the amount fed is above the animal’s requirements. It is also worth noting that inorganic phosphorus such as phosphates are derived from a limited resource, and must be used wisely (Sharpley et al., 2018).
It is well established that feeding phosphorus close to the animals’ requirement is an effective way of reducing excretion. Beyond precision feeding however, using other methods are somewhat limited. Although, Goff (2006) suggests that in maintaining good calcium homeostasis, phosphorus excretion can be reduced. The reason this occurs is that in early lactation the dairy cow is commonly in a negative calcium balance, due to the demands of calcium for milk production and a limited dry matter intake. Consequently, the dairy cow will mobilise additional calcium from the bones and replenish this reserve in later lactation. As 98% of the calcium and 80% of the phosphorus is found within the bone and teeth; phosphorus is also subsequently released as a by-product when calcium is mobilised from the bone (Taylor, 2007). To release and use calcium from the bone requires the action of parathyroid hormone (PTH). However, when PTH is released, it also increases urinary and salivary losses of phosphorus which is undesirable. Therefore, reducing hypocalcaemia by a nutritional route is an effective means of reducing phosphorus losses from the animal.
Feeding Glycal Forte® is potentially an effective nutritional strategy to reduce losses of phosphorus as it can provide up to 55g of bioavailable calcium chloride. This would contribute to an increased dietary calcium content of the diet post-calving, avoiding hypocalcaemia and also losses of phosphorus. Additionally, the calcium in Glycal Forte® contributes to a negative DCAD at around -65 mEq/250g dose. Feeding a negative DCAD diet to cows prior to calving causes the blood of the cow to become slightly acidic, this increases the sensitivity of tissues to PTH. This is a proven effective strategy in reducing the incidence of hypocalcaemia, by improving uptake of dietary calcium and release from the bone. Reducing the need for prolonged and increased levels of PTH to combat low blood calcium levels (Goff et al., 2014).
Elizondo Salazar, J. A., Ferguson, J. D., Beegle, D. B., Remsburg, D. W. and Wu, Z. (2013). Body phosphorus mobilization and deposition during lactation in dairy cows. Journal of Animal Physiology and Animal Nutrition, 97(3): 502-514.
Elshahawy, I. I. and Aly, M. A. (2016). Some studies on deviated appetite (Pica) in cattle. Alexandria Journal for Veterinary Sciences, 51(1): 97-101.
Goff, J. P. (2006). Macromineral physiology and application to the feeding of the dairy cow for prevention of milk fever and other periparturient mineral disorders.Animal Feed Science and Technology, 126(3-4): 237-257.
Grünberg, W. (2008). Phosphorus homeostasis in dairy cattle: some answers, more questions. In:Tri-State Dairy Nutrition Conference, pp. 29-35.
Goff, J. P., Liesegang, A. and Horst, R. L. (2014). Diet-induced pseudohypoparathyroidism: A hypocalcemia and milk fever risk factor. Journal of Dairy Science, 97(3): 1520-1528.
Ishler, V. and Wu, Z. 2003. Reducing Dietary Phosphorus in the Dairy Herd. Penn State College of Agricultural Sciences Cooperative Extension.
Mekonnen, M. M. and Hoekstra, A. Y. (2018). Global anthropogenic phosphorus loads to freshwater and associated grey water footprints and water pollution levels: A high‐resolution global study. Water Resources Research,54(1): 345-358.
National Research Council, (2001). Nutrient requirements of dairy cattle: 2001. National Academies Press.
Rodehutscord, M., Pauen, A., Windhausen, P., Brintrup, R. and Pfeffer, E., (1994). Effects of drastic changes in P intake on P concentrations in blood and rumen fluid of lactating ruminants. Journal of Veterinary Medicine Series A, 41(1‐10): 611-619.
Sharpley, A., Jarvie, H., Flaten, D. and Kleinman, P., (2018). Celebrating the 350th anniversary of phosphorus discovery: A conundrum of deficiency and excess. Journal of Environmental Quality, 47(4): 774-777.
Taylor, M. S. (2007). Calcium and phosphorus metabolism in Jersey and Holstein cows during early lactation (Doctoral dissertation, Virginia Tech).