Dexter cattle have two primary genetic defects: Chondrodysplasia (also known as dwarfism) and Pulmonary Hypoplasia with Anasarca (PHA). Chondrodysplasia results in disproportionate dwarfism, with affected animals having a heavy body on short legs. PHA is a lethal defect that causes incomplete lung development and fluid accumulation, leading to stillbirths or abortions.

The text above is copied from an internet search. Dexter cattle have two genetic defects of concern. The dwarfism defect usually does not cause health problems but does cause them to be small and not grow well, even compared to the normal Dexters without the dwarfism gene. As a breed Dexters do not grow as rapidly as many of the more common beef breeds.

The other defect is of serious health concern. Pulmonary Hypoplasia is a recessive gene the if an animal is born with two recessive genes, they are usually going to be stillborn. I have a neighbor with Dexters that had a calf born alive with this defect. They tried to nurse and assist this calf for a couple days before it died. Being a recessive gene, if an animal is a carrier and bred to animals know to be free of this gene, the offspring should be healthy and have a 50% chance of wither being clean or being a carrier. There is a genetic test that can be performed on animals to determine if they are a carrier. Same for dwarfism. 

Regarding grasses of open fields. a common recommendation of a good forage mix is to combine grasses (examples - timothy and orchard grass) and legumes (examples - birdsfoot trefoil, red clover, alsike clover, white clove). if forage stands are between 25-40% legume, they will provide enough nitrogen into the soil for both themselves and the grasses and nitrogen fertilizer does not need to be applied. If only grasses are in the forage stand, yields will be much improved with annual nitrogen fertilizer applications.