Science Friday with Dr. Gustafson -- Choosing overly mature or poor hay
10/09/2026
This is our 2nd Science Friday post, where we discuss questions related to managing PPID and EMS in greater detail. If you have a topic you would like addressed, please drop it in a comment. Polite discussion, questions, comments are welcome!
So much emphasis has been placed on hydrolyzable carbohydrates (HC = ESC + starch) when it comes to managing hyperinsulinemia that the rest of the forage analysis is often overlooked. Let’s talk about structural carbohydrates because these will influence nearly every aspect of forage: digestibility, palatability, calories, protein, mineral content and simple sugars.
Structural carbohydrates (polysaccharides) make up the plant cell wall: cellulose, hemicellulose, pectin, beta-glucans. Lignin, a polyphenol, plays a role in structural support which is why mature grasses have that rigid, woody feel. When I’m looking at forage analysis results, I check moisture first and then immediately check acid-detergent fiber (ADF = cellulose, lignin) and neutral detergent fiber (NDF = cellulose, lignin and hemicellulose. [NDF – ADF = hemicellulose].
It’s not uncommon to hear recommendations to feed “poor” or “overly mature” hay to horses with high insulin. What brought me to this topic was listening to some growers chuckling about how they could offload their junk hay on “these crazy ladies looking for low sugar hay.”
The index for “poor” is different when determining market value vs. nutrient value and differs by grass or legume species. The American Forage and Grassland Council’s quality standards for hay set the lowest grade as less than 8% protein, with greater than 45% ADF and 65% NDF. Some recommend feeding straw*, up to 50% of the forage ration, but to monitor fecal output and watch for signs of colic (Equine Endocrinology Group). Data from Equi-Analytical Feed Composition Library shows US straw at 5% average protein, 50% ADF and 74% NDF. *This can be a useful strategy for donkeys, and not without merit in horses – but today’s topic is focused on not rejecting acceptable hay.
What justification is there for feeding poor quality forage and/or straw if nutritious and less wasteful options are safe? I wanted to understand how %NDF affects other nutrient variables, so I ran linear regression analyses using %NDF as a predictor variable. In other words, how does the amount of NDF affect the amount of other nutrients? I used one database specific to the Pacific Northwest (n=244) and another general to the US (n=780); all cool-season grasses, the latter selected from samples with >20% NSC. From highest to lowest effect, %NDF significantly lowered calories, magnesium, protein, calcium, copper, zinc, phosphorus and simple sugars (ESC). However, %NDF had no effect on starch or fructan and of all the variables ESC (simple sugars) was the least affected.
In practical terms, choosing poor quality forage to achieve “low NSC” is more costly to protein and minerals. It has no effect on 2 out of the 3 variables that make up NSC (starch, fructan) and only a minor effect on simple sugars. Yes, higher %NDF does mean lower calories, but average quality grass hay is not high in calories and can still be nutritious, safe, and less wasteful. If you’re choosing poor hay specifically to achieve “low NSC,” it’s not a good approach, but those growers with their junk hay will be happy to see you.
Kathleen Gustafson, PhD
President and Research Advisor, ECIR Group Inc.
