Long term effect of reduced dietary phosphorus on feed intake and milk yield in dry and lactating dairy cows

Abstract

Eighteen multiparous Holstein cows were used to study the long term effect of reducing dietary P concentration on intake of DM and milk yield, on blood levels of inorganic phosphate (Pi), Ca, vitamin D3, parathyroid hormone (PTH) as well as assessing bone turnover by the use of bone formation marker osteocalcin (OC) and bone resorption marker cross-linked carboxy-terminal telopeptide of type I collagen (CTX). Treatments were initiated 3 weeks before expected calving, and cows were followed until week 36 after calving. In dry period treatments were Low P (LP) (1.7g P/kg of DM), Medium P (MP) (2.1g P/kg DM) and High P (HP) (2.5g P/kg DM). After parturition and throughout lactation treatments were LP (2.3g P/kg DM), MP (2.8g P/kg DM) and HP (3.4g P/kg DM). Differences in dietary P were obtained by adding 0.60 and 1.19% mono-sodium phosphate per kg DM to MP and HP, respectively. The cows were fed restrictively pre-partum and for ad libitum intake from one week before expected calving and throughout the experiment. Due to a high number of health problems, LP treatment was terminated after sampling in week 12, and cows were shifted to HP and continued on this treatment until week 36. Reduction in P concentration from 3.4 to 2.3g P/kg DM reduced DMI, milk yield, milk protein yield and plasma Pi concentration, and increased serum concentration of CTX, but did not affect serum concentration of OC in early lactation. Reduced dietary P concentration from 3.4 to 2.8g P/kg DM did not affect DMI or milk production in early lactation. Dry matter intake and milk yield were not affected by dietary P concentration in late lactation. Plasma Ca increased in LP, whereas there was no effect on serum concentration of PTH or on plasma concentration of vitamin D3 pre-partum, in early or late lactation. Despite an estimated mobilization of P in early lactation, on the basis of results of CTX and OC, it is not possible to reduce P concentration from 3.4 to 2.3g P/kg DM in dairy cows in this period without compromising DMI and milk production. The results indicate a potential for reducing P concentration in early and late lactation from the current recommendations of 3.4 to 2.8g P/kg DM without effects on performance, but this level of dietary P might not allow for repletion of bone P mobilized in early lactation.