The objective of this thesis was to study phosphorus balance in dairy cows when fed a sub-optimal level of phosphorus in the diet. Further exploration into whether phosphorus depleted animals showed a predilection to seek a dietary phosphorus source and how motivated they were to attain that source were also conducted. To deplete cows of phosphorus a sub-optimal phosphorus diet (0.24%) was formulated. These animals were compared to cows fed at the optimal NRC (2001) level of phosphorus (0.36%) over a period of ten weeks of lactation, beginning three weeks postpartum. Results indicated that when cows were fed the sub-optimal phosphorus diet, blood plasma phosphorus concentration was lowered and faecal phosphorus output was reduced. Milk production, milk phosphorus, salivary phosphorus, body weight and dry matter intake were not significantly different between the two dietary groups. No appetite or motivation for dietary phosphorus was shown in the sub-optimal phosphorus fed group. Lowering phosphorus in the diet proved beneficial to reducing faecal phosphorus output whilst not adversely affecting dairy cow health or production parameters.