While there is consensus that increasing bio-diversity in our horse’s diets is desirable, there are two schools of thought as to the types of plants that should be encouraged.
Our perspective comes from dealing with all the problem horses we are presented with from all over the world. We have an extensive data-base and it reveals a 100% correlation between the incidence of these issues, everything from digestive, behaviour, neuro-muscular, staggers, respiratory, separation anxiety, mud-fever, obesity, EMS, laminitis, head-flicking, aggressiveness to hormonal and reproductive and the predominance of short or lush green forage especially when it contains legumes like clover and Lucerne (alfalfa).
If horses were meant to eat legumes, they would have a ruminant style digestive system, not a mono-gastric one. This is because they evolved on LOW NITROGEN diets *
They have ‘thrifty genes’. Their feral and wild cousins survive and thrive in semi-arid and high altitude environments where forage is much sparser and requires more energy to find.
Less than 10% of wild/feral horse diets consist of anything other than grasses, bushy shrubs and sedges.*
Sowing various species of grasses (steer clear of any strains of rye-grasses and tall-fescues) is a great step in the right direction. Avoid mixes which contain clovers, plantain and chicory even if you are growing hay – soaking does not remove the undesirable aspects of these legumes. They may be lower in sugars but they are too high in potassium and crude protein/nitrogen to be safe for horses, especially those with issues; to graze.
On the other hand grass either at a mature stage of growth (gold & stalky) or fed as hay, is suitably lower in potassium, crude protein/nitrogen and sugars and therefore is suitable for all horses to consume all day long without developing health and behaviour problems. In fact it tends to resolve the problems listed above.
*The Evolutionary Strategy of the Equidae and the Origins of Rumen and Caecal Digestion. Evolution 757:754 (Janis, c. 1976) **Wild Equids – Ecology Management and Conservation - Jason I Ransom and Petra Kaczensky - Multiple observational studies have shown that grass species make up 83% -91% of the feral horse diet in all seasons, shrubs (about 8%) and herbaceous plants (1%) play a limited role, primarily in winter.