A Model of Phosphorus Digestion and Metabolism in the Lactating Dairy Cow

Abstract

A dynamic, mechanistic, compartmental model of phosphorus (P) digestion and metabolism was constructed in the Advanced Continuous Simulation Language using conservation of mass principles and mass action kinetics. Phosphorus was assumed to exist in 3 forms: inorganic (Pi), phytic acid (Pp), and organic (excluding phytic acid; Po). All 3 forms were assumed to be present in the digestive tract with absorption of Pi into blood. Inputs to the model were total P intake; Pp, Po, and Pi as proportions of total P; milk yield; rate of salivation (fixed at 239 L/d); and rate of liquid passage from the rumen (fixed at 198 L/d). The model was fitted to 2 experiments from the literature. Derived parameters were well defined by the data. With a mean observed P intake of 75 g/d, total tract P digestibility was 38%. Phytic acid P digestibility in the rumen was 74%, with no additional Pp digestion in the lower tract. Inorganic P and Po digestibility in the lower tract were 48 and 89%, respectively. Flows of Po and Pi from the rumen were 2.4 and 3.0 times greater than intake, respectively. The increase in Po was apparently due to microbial growth. The increase in Pi arose primarily from secretion of Pi into the rumen via salivation where 65% of absorbed P was recycled to the rumen. Milk synthesis used 30% of absorbed Pi, and 1% was excreted in urine. This research suggested that the primary regulation points for maintaining blood P were bone deposition and resorption and absorption from the intestine. However, because bone P balance was related to both dietary P intake and ruminal phytase activity, it is critical to achieve a better understanding of phytate digestibility across several feeds if dietary P is to be reduced below current requirements.

Nitrogen and Phosphorus Utilisation and Excretion in Dairy Buffalo Intensive Breeding

“…Buffaloes that were at Day 50 of lactation at the start of the study had a lower P digestibility compared with buffaloes at later stages of lactation and also non-lactating buffaloes. This may have been due to the transitioning of buffaloes at Day 50 of lactation to a new diet in which the source of P was mainly phytates which have relatively low digestibility (Hill et al, 2008). Non-lactating buffaloes showed the highest P digestibility and, since the amount of ingested P was similar for the four groups of buffaloes, it is likely that non-lactating buffaloes were able to meet their demand for P by reducing kidney clearance and the amount excreted.…”

Meta-analysis of factors that affect the utilization efficiency of phosphorus in lactating dairy cows

“…Increasing interest in environmental issues has stimulated further development of nutrient-based mechanistic models within an environmental framework, and the need exists to include variation at the animal level in such nutrient-based mechanistic models (Dijkstra et al, 2007). In one extant mechanistic model of P metabolism in dairy cows (Hill et al, 2008), dietary P is the only dietary component included. The results of the present meta-analysis suggest that including effects of other nutritional factors, in particular dietary NDF and CP content, would benefit such a mechanistic model.…”

“…The results of the present meta-analysis suggest that including effects of other nutritional factors, in particular dietary NDF and CP content, would benefit such a mechanistic model. Moreover, in the Hill et al (2008) model, a constant concentration of Pi in milk was assumed. Comparing this with the study that contains the model input data (Knowlton et al, 2001) shows that the concentration of Pi in milk appears equal to the total P concentration in milk.…”

Phosphorus Nutrition in Dairy Animals: A Review

“…The inorganic P occurs as phosphate while organic P occurs in plants as phytic acid, phospholipid, nucleic acid etc (Mcdowell and Sharpley, 2003). P is one of the macro mineral found in every cell of the body and is used in several body processes including critical energy pathways (ATP), cell signalling and synthesis of cell membranes, RNA, DNA and bone (Hill et al, 2008). It is hard to think of any physiological function which would not involve directly or indirectly phosphoric acid compound therefore P is now widely acclaimed as master mineral.…”