Reappraisal of the phosphorus requirement of lactating dairy cows

Abstract

Phosphorus (P) is an essential macro-mineral necessary for many body functions and needs to be supplied in sufficient quantities to optimize animal performance. In practice cows are commonly fed excess P. A high dietary P concentration leads to high P excretion via feces which can result in freshwater eutrophication. Therefore, overfeeding dietary P should stop. In the Netherlands, fecal P excretion of cattle tends to lower over time from 2017 to 2021, however, further P reduction will be needed. Due to P essentialness, there are some limitations and criteria which need to be understood before making the decisions to lower the P concentration in the diets. First study, 60 pregnant multiparous Holstein Friesian (HF) dairy cows were assigned to a RCBD with repeated measurements. The experimental diets contained either 3.6 (Dry-HP) or 2.2 (Dry-LP) g P/kg DM during the dry period, and either 3.8 (Lac-HP) or 2.9 (Lac-LP) g P/kg DM during 56 days of post-calving period. The proportion of cows being hypocalcemic in the first week after calving was lowest with both Dry-LP and Lac-LP groups. Before calving, plasma Carboxy-terminal collagen crosslinks concentrations (CTX) decreased with Dry-HP and increased with Dry-LP. After calving, plasma CTX concentrations increased but this increase was more pronounced when Lac-LP instead of Lac-HP was fed. Overall, the results suggest that when feeding diets containing low P postpartum, cows excreted less P in the feces than at sufficient dietary P without negative impact on feed intake, milk production and fiber digestion. Moreover, feeding postpartum diets containing low P compared with recommended P increased bone resorption, and increased plasma CTX concentrations, without change in plasma concentrations of PTH and 25(OH)D3. Second study, 18 multiparous, mid lactating HF dairy cows were used and fed high dietary P with coarse artificial dried grass (HPCG), low dietary P with coarse artificial dried grass (LPCG) and low dietary P with grass pellet (LPGP). The diets contained 2.79 (HP), 2.06 (LPCH), and 2.23 (LPGP) g P/kg DM. The cows fed with grass pellet (GP) spent less time than the coarse artificial dried grass (CG) group on ruminating, but no effects were observed on saliva P, plasma Pi, PTH, 25(OH)D3, 1,25(OH)2D3, CTX and osteocalcin (OC) concentration, OM and NDF digestibility, and fecal P excretion. Cows fed the LP diets were in negative P balance however, no upregulation of P absorption was observed. Instead, those cows resorbed P from bone to fulfil their requirement. Third study, every week, from week 14 in 2017 up until week 13 in 2018, tank milk samples from 14 dairy plants across the Netherlands were collected. In total, 52 weekly milk samples were analyzed for macronutrients and minerals. Results indicated that the milk P content is changing upon changes in macronutrients and minerals content in the milk. The mean P content of milk was found to be 101.2 mg/100 g which is 1.2% greater than the value commonly used to calculate the P excretion with manure.