Some mares are not “good milk-producers”.
A “French saddle” mare should produce between 15 and 18 litres of milk per day, and a “heavy” mare between 20 and 25 litres.
Many breeders underestimate mares’ nutritional requirements during the first three months of lactation.
These requirements practically double in terms of protein and energy content compared with those at the end of gestation.

For example, for a mare weighing 500 kg
Daily requirements :
First month of lactation : 10 to 11 EFU (equine forage unit) (32000 to 35200 kilocalories ) and 950 grams of EDNM (equine digestible nitrogen matter) 
- 2nd and 3rd month : 9 to 10 EFU and 770 to 880 g of EDNM
- 4th month : 7.5 EFU and 660 g of EDNM.

Mares which are underfed during this period will draw on their own body reserves to try to feed their foals properly and will start to lose condition.
She will very soon struggle to produce sufficient milk and her foal will have difficulty developing smoothly.
In contrast, overfed mares, particularly those receiving rations with excessive amounts of cereals (excess starch) will be prone to fat deposits around the teat which does not encourage good milk production.
Highly specific breeding feeds like Elevage/breeding from. adjust® range by ROYAL HORSE should be used for mares during the lactation period.
Consider contacting your veterinary surgeon and specialist technician.