In addition to the composition of grass grown on “improved” (fertilized) soils, the grass plant itself has been transformed through selective breeding and genetic modification. The result is a grass with higher sugar content. Sugar increases palatability for increased consumption for rapid weight gain in cattle. Most USDA grass research is focused on livestock---cattle, sheep, hogs and goats. But little horse-related research is done on grass. Therefore, the hay we feed our horses has not been designed for them.
Much attention has recently been given to the nonstructural carbohydrate (NSC) (starch and sugar) content of hay. Higher levels of NSC are associated with increased levels of insulin and the development of laminitis. Native grasses are lower in NSC and would seem a better choice for horse owners because they are considered safer for horses to eat. But native grass is difficult to grow because it cannot compete with the newer grasses like fescue. In many instances, attempts by horse owners to grow a native pasture have been met with frustration and failure.
Are soil changes responsible for the increased incidence seen on “improved” pasture? Animals derive nourishment from the soil, and the soil affects the composition of all the food grown on it.