Effects of varying monosodium phosphate intake on phosphorus excretion in dairy cows

Phosphorus excretion in faeces was measured by total collection using dairy cows fed five amounts of P in a 5 × 5 Latin square design with three week periods. Milk production was 23 ± 4 kg per day at the onset of the trial (DIM 185 ± 11). Different quantities of monosodium phosphate (MSP) were added to a basal diet to obtain daily P intake levels of 44, 67, 92, 117 and 142 g (2.4, 3.7, 5.1, 6.4 and 7.8 g P kg− 1of DM, respectively). Faecal P excretion increased linearly (r2 = 0.97) as dietary P intake increased and the apparent digestibility of P was calculated to be 0.37, 0.28, 0.22, 0.21 and 0.22, respectively. The relatively low apparent digestibility is explained by excess P intake in relation to the requirements. The concentration of P in faeces ranged from 4 to 23 g kg− 1 of DM corresponding to the lowest and highest level of P intake, respectively. Variation of P in faecal output was substantial among days, though total DM and P intake was constant throughout the collection periods. Results indicate a requirement for sampling over at least 5 consecutive days, even when using total faecal collection.