Mineral nutrition of ruminants

Mineral requirements for ruminants depend on stage of pregnancy, bodyweight, milk yield and composition (milk contains an average of 1.2 and 0.9 g/l calcium and phosphorus respectively and colostrum can contain twice these levels). A complicating factor is the fact that mineral requirements need to be balanced for the complete lactation cycle. Minerals mobilised from the bone during early lactation must be replenished towards the end of the cycle to maintain cow health and performance. However, most systems do not account for this phenomenon. Current nutritional recommendation systems refer to an "average animal" under "normal" feeding conditions and in a state of good health. Recent improvements in animal production means that "normal" conditions might be sometimes outdated. But also there can be a substantial variance around this "average" animal.
The factorial method is the most efficient and reliable method to assess mineral requirements. This method is based on establishing estimates for net requirements for maintenance, growth, pregnancy and lactation together with a true absorption coefficient resulting in dietary requirements for the mineral elements.
To arrive at practical allowances a safety margin is normally included. This safety margin usually allows for the variation within flocks or herds, fluctuations in feed intake and in feed composition.

                                        Maintenance + Pregnancy + Growth + Lactation
Dietary Requirement = ----------------------------------------------------------
                                                            Absorption coefficient

Mineral element allowances (g/d) of a lactating cow (600 - 650 kg BW, 35 kg milk/d)