Q: This summer, we have two horses supported on 6 acres of grass pastures. One is an easy keeper and continues to be chubby. The other has always been an easy keeper, (ridden 5-6 times a week) but is quite thin now. Shouldn't the other horse be thin as well? We have no bare spots in our pasture, but the pasture is grazed down. Is there a way to know how much pasture is needed per horse beyond the vague 2 acres per horse guideline?
A: It is very difficult to measure the amount of grass a horse is actually eating on pasture. On average, good pasture produces about 3 tons of grazeable dry matter (DM) per acre per year (June - September) or about 50 pounds a day per acre. A horse needs to eat about 2% of its body weight to maintain a healthy body condition (about 20 pounds a day for a 1,000 pound horse). Each day, each horse should be eating about 40 pounds of grass. We recommend 2 acres per horse because the pasture grasses need about 30 days of rest (no grazing) for every 7 to 10 days of grazing (this will vary depending on time of year and weather conditions). Is your 6 acre pasture divided up to provide a rest period for a section of the pasture, while another section is being grazed (i.e. rotational grazing)? When the pasture is eaten down or over grazed, the amount of grass available is very minimal or zero.
I'd recommend getting an exam on the "thin" horse to ensure its teeth are in good chewing condition and that there are no underlying health problems contributing to the weight loss. Riding 5 to 6 times a week is considered more than "recreational", and your horse's energy requirement is beyond the base 2% figure. Are you providing vitamins and minerals as well?
Even though your pasture looks green, it's highly unlikely there is enough available grass if its "eaten down" to support both horses.
It's very common for individual horses to respond differently to grazing in regards to weight loss and gain. Some horses in the same pasture will consume more grass than others. It would not be surprising or uncommon to have one thin and one over weight horse in the same pasture.
We recommend horse owners with horses on pasture monitor their horse's body condition score on a weekly basis, both to monitor weight gain and loss. If a horse is gaining weight, restrict the amount of time allowed to graze. If a horse is losing weight, supplement hay or grain. Monitoring body condition score is one way to estimate the amount of grass a horse is actually ingesting when on pasture.
By Krishona Martinson, PhD & Paul Peterson, PhD, University of Minnesota