The skeleton is comprised of 35 % calcium. In addition, calcium is required for the normal functioning of muscle and glands. In particular, in young horses calcium deficiencies and excesses can lead to abnormal skeletal development (see the online help for calcium). Too much calcium will reduce the uptake of a number of minerals including iron and zinc. Good sources of calcium for the horse are clover hay, alfalfa and most commercial concentrate mixes which contain 6-14 g Ca/kg. Grass hay varies in calcium content from 1.5 - >15 g Ca/kg (if you are buying hay in bulk, consider having it analysed).
Maintenance: Ca (g/d) = 0.04*BW
Young growing horses: calcium requirements are calculated from the maintenance requirement with an added allowance for growth. Typically, a young horse will deposit 16 g Ca per kg of weight gained. With a 50 % absorption digestibility of calcium, the diet must provide 32 g of Ca/kg of weight gained.
Ca (g/d) = 0.04*BW + 32*DWG (Daily Weight Gain)
Young horses which are exercised need an additional calcium intake which is related to the maintenance requirement:.
Ca (g/d) = (0.04*BW + 32*Daily gain) * (DE for horses in training / DE for horses not trained)
Pregnancy: Pregnant mares require extra calcium during the 9th-11th months.
Months 1-8: Ca = 0.04*BW
Months 9-11: Ca = 7.95 * DE (MJ)
Milk Production: The calcium content of mare's milk declines during lactation from circa 1.2 g/kg to 0.8 g/kg. When producing 15 kg/d of milk, a mare that absorbed 50 % of the calcium in her diet will need, in addition to 20 g/d for maintenance, 36 g/d of Ca for milk.
1st - 2nd month of lactation:
Mare 200 kg: Ca(g/d) = 0.04*BW + (0.04*BW*1.2/0.5)
Mare above 400 kg: Ca(g/d) = 0.04*BW + (0.03*BW*1.2/0.5)
3rd month of lactation to weaning:
Mare 200 kg: Ca (g/d) = 0.04*BW + (0.03*BW*0.8/0.5)
Mare above 400 kg: Ca (g/d) = 0.04*BW + (0.02*BW*0.8/0.5)
Exercise: The calcium requirement for horses in work is given as a proportion of the energy requirement:
Ca (g/d) = 5.1 * DE (MJ)