Phosphorus homeostasis in dairy cows

Phosphorus (P) nutrition and homeostasis in dairy cattle has received increasing attention because excessive P excretion in animal waste has led to environmental concerns and a resultant decrease in the P content of dairy cow rations. Although current recommendations for dietary P supplementation are considered safe, the continuously increasing milk production of dairy cows presents an increasing challenge for the P homeostasis, particularly at the onset of lactation. In the present thesis four studies exploring potential mechanisms affecting P homeostasis in early lactating dairy cows are presented. A retrospective study conducted on over 1400 cows with displaced abomasum or abomasal volvulus revealed that decreased feed intake as seen in sick cows is likely to result in hypophosphatemia, whereas dehydration and decreased renal perfusion can cause hyperphosphatemia. A study exploring the effect of continuous intravenous dextrose administration on P homeostasis showed that dextrose infusion induces an insulin-dependent intracellular P shift resulting in a marked decline in plasma P concentration. The relationship between liver lipid and liver P content in dairy cows 14 days post partum was investigated in another study, revealing that liver P content decreases with decreasing cell water volume while the cytosolic P concentration remains constant. Furthermore, the liver P content in dairy cows throughout the transition period was studied and showed that cytosolic P concentration decreased by 20% in the first 2 weeks of lactation whereas the cytosolic electrolyte concentration of other intracellular electrolytes remained constant. The last study investigated the effect of fully compensated metabolic acidosis in dairy cows during late on P homeostasis before and after calving. Results indicate that compensated metabolic acidosis only mildly increases plasma P concentration and urinary P excretion, effects that are unlikely to be of clinical relevance. Moreover, neither insulin response to a glucose load nor the insulin dependent cellular P uptake was altered with compensated metabolic acidosis. Collectively, the results presented in this thesis provide an improved understanding of the mechanisms and factors affecting P homeostasis in periparturient dairy cows