Heat stress cut milk yield by 35% in a 12-cow trial, and a clay-based mycotoxin binder added to the ration failed to reverse that loss, a research team reports in the Journal of Dairy Science.

The study, available online September 28, 2026, enrolled 12 primiparous Holstein cows averaging 148 ± 25 days in milk and 600 ± 50 kg of body weight. Investigators balanced cows by DIM and milk yield, then assigned them to one of two ad libitum treatments top-dressed onto an individually fed TMR:

• Control (CON, n=6): 500 g of ground corn per day
• Clay mineral (CM, n=6): 500 g of ground corn plus 50 g of Biofix® per day

The naturally occurring mycotoxin content of the TMR was low. After a 3-day acclimation to individual pens, cows entered two sequential periods. Period 1 ran 10 days under thermoneutral conditions. Period 2 ran 7 days under heat stress induced by individual electric heat blankets (Thermotex Therapy Systems Ltd., Calgary, AB, Canada).

What did heat stress do to the cows?

Across both diets, the protocol raised rectal temperature by 1.9°C and respiration rate by 54 breaths per minute. Dry matter intake fell 45% and milk yield dropped 35% relative to Period 1. Circulating glucose fell 6% and glucagon fell 21% in both groups. Plasma non-esterified fatty acids climbed 70% and blood urea nitrogen climbed 41%, a pattern consistent with mobilization of body reserves.

Circulating cortisol did not move, but fecal cortisol metabolites rose 3.4-fold, indicating the adrenal axis responded even when serum cortisol looked flat. On day 1 of heat stress, lipopolysaccharide binding protein increased 1.4-fold and haptoglobin spiked 27.8-fold relative to Period 1, signs of acute immune activation.

Did the clay additive change production?

No. The team, led by S. Rodriguez-Jimenez with co-authors J. Opgenorth, E.J. Mayorga, M.A. Abeyta, B.M. Goetz, N. Reisinger, J. Faas, R. Palme, and L.H. Baumgard, found the additive did not rescue either the DMI or the milk-yield drop. Both losses tracked identically between CON and CM cows throughout Period 2.

What about the immune and metabolic signals?

Two signals diverged between diets. Insulin tended to run higher in CM-fed cows (36% in Period 1, 54% in Period 2). On day 1 of heat stress, serum amyloid A jumped 7.0-fold in CM cows but stayed flat in controls.

"Circulating serum amyloid A (SAA) increased in CM-fed cows (7.0-fold) on d 1, while it was unaffected by HS in CON, which suggests CM might have immunomodulatory effects," the authors wrote.

By day 2, SAA in the CM group had returned to Period 1 levels, and both groups tracked together for the remainder of Period 2. Neither NEFA, BUN, LBP, nor haptoglobin differed between diets.

What are the caveats?

The trial is small (n=6 per group) and brief. Heat stress was acute, severe, and delivered by external heating blankets rather than ambient barn temperature. The diet carried low background mycotoxin contamination, so the product's binder capacity was barely tested. None of the production variables suggested a benefit at this dose and duration.

The authors call for follow-up work "during a longer and less severe HS and when cows are fed a more mycotoxin-contaminated TMR."

What does this mean on the farm?

For dairy producers weighing Biofix® as a heat-stress intervention, this dataset offers no production lift. The product, marketed as a mycotoxin binder, may still earn a place on operations with contaminated feed, but the n=12 design cannot support that claim.

The acute-phase protein signal hints that subclinical immune effects of clay-based additives deserve a closer look under field conditions, where herd-level outcomes matter more than small-group means.

Larger, longer field trials under realistic summer heat loads will determine whether the insulin and acute-phase protein shifts translate into measurable herd-level benefits.