Dietary Cation–Anion Difference of Timothy as Affected by Development Stage and Nitrogen and Phosphorus Fertilization

Dry dairy cows ( Bos taurus ) fed forages with a high dietary cation–anion difference (DCAD) are more likely to develop hypocalcaemia. We determined how development stage at harvest and N and P fertilization can be used to reduce the DCAD to <250 mmol c kg −1 dry matter (DM) for timothy ( Phleum pratense L.) grown on a soil high in K content. Stages of development (stem elongation, early heading, late heading, and early flowering) in spring growth, and treatments of P (0, 15, 30, and 45 kg ha −1 ) and N (0, 60, 120, and 180 kg ha −1 ) fertilization were evaluated. Concentrations of K, Na, Ca, Mg, Cl, S, and P of timothy were determined and the DCAD was calculated with a short (DCAD S ) and a long (DCAD L ) equation. From stem elongation to early flowering, DCAD S decreased from 326 to 196 mmol c kg −1 DM and DCAD L from 297 to 181 mmol c kg −1 DM; this reduction was attributed to a decrease in K concentration and a slight increase in Cl concentration with development stage. Nitrogen fertilization increased DCAD S and DCAD L only at stem elongation; the lack of a response at later stages of development is explained by the concomitant increase in both K and Cl concentrations with increasing N fertilization. Phosphorus fertilization did not affect DCAD S and DCAD L even though it increased timothy P concentration. Harvesting timothy at late heading, with an appropriate N fertilization to ensure adequate yield, is an option to produce a forage with a DCAD of <250 mmol c kg −1 . DM on a soil high in K.