Diet digestibility is a heritable trait in US Holsteins, and cows can be genetically evaluated for it using routine fecal sampling on commercial farms, according to a genomic study of 1,494 fecal samples from 1,065 lactating cows and 178 nulliparous heifers across six herds.

Heritability estimates reached 0.16 for fecal potentially degradable NDF (pdNDF), 0.12 for total-tract NDF digestibility (NDFD), and 0.06 for total-tract DM digestibility (DMD). Those figures, combined with meaningful additive genetic variation, mean selection for digestibility is biologically feasible — something producers cannot get from management changes alone.

How the study was built

Researchers collected morning and afternoon fecal samples from 1,065 lactating cows (1,314 samples) and 178 nulliparous heifers (180 samples) on 5 commercial herds plus the Penn State University herd. They dried fecal and TMR samples in a forced-air oven for 48 hours at 55°C and composited dried morning and afternoon fecal samples on equal dry-weight basis.

A commercial laboratory analyzed neutral detergent fiber (NDF) and indigestible NDF (iNDF) on an organic matter basis after 240-hour in vitro digestion. From those results, the team derived three digestibility traits:

• Fecal pdNDF concentration (fecal NDF minus fecal iNDF) — lower is better, because it indicates less potentially digestible fiber passed undigested
• Total-tract NDF digestibility (NDFD) — higher is favorable
• Total-tract DM digestibility (DMD) — higher is favorable

Yield data included 67,717 test-day milk yield and 67,764 test-day fat percentage records on 4,950 animals. Genotypes covered 947 animals with fecal samples, 2,796 cows with yield data only, and 1,838 ancestors. The researchers fit single-step multi-trait animal models with fixed effects of lactation, contemporary group, and biweekly stage of lactation, plus random additive genetic, random permanent environment, and error terms.

What do the genetic correlations show?

The strongest signal among the digestibility traits was negative: fecal pdNDF correlated genetically with NDFD at −0.95 ± 0.02, confirming the two measures capture essentially the same underlying biology from opposite directions. The correlation between NDFD and DMD was 0.60 ± 0.12, while fecal pdNDF and DMD correlated at −0.34 ± 0.15.

Additive genetic standard deviation was higher for fecal pdNDF (1.21 ± 0.16%) and NDFD (1.39 ± 0.15%) than for DMD (0.67 ± 0.05%), indicating more selectable genetic variation in the fiber-digestibility measures than in overall DM digestion.

The milk yield relationships were unfavorable across all three traits: fecal pdNDF at 0.63 ± 0.10, NDFD at −0.77 ± 0.06, and DMD at −0.72 ± 0.05. In practical terms, genes for higher milk volume tend to travel with genes for poorer fiber digestion in this population.

The fat and protein trade-off changes the picture

Fat percentage told a different story. Fecal pdNDF and NDFD showed favorable genetic correlations with fat percentage of −0.40 ± 0.10 and 0.31 ± 0.09, respectively. Estimated breeding values for fecal pdNDF and NDFD correlated favorably with genomic PTA for national evaluations of fat percentage and protein percentage, even though correlations with milk yield PTA were unfavorable.

That pattern matters at the bull-buying and ration-formulation level. Selection for NDFD appears compatible with maintaining or improving milk fat and protein content, even if it pulls against raw milk volume — a trade-off many component-priced dairies already accept.

What changes at the farm level?

For food-animal practitioners and nutrition consultants, the study demonstrates a workable phenotype-collection pipeline: dried, composited fecal samples from commercial herds, analyzed by a commercial lab for NDF and iNDF after 240-hour in vitro digestion, fed into national-scale genomic evaluation. No research-farm infrastructure is required.

The practical payoff is feed efficiency. A cow that extracts more energy from the same TMR lowers feed cost per unit of milk solids and reduces manure fiber output. Because digestibility is heritable, those gains compound across generations rather than resetting with every ration change.

The study did not report a regulatory filing or an active national selection index for digestibility, so implementation would require integration into existing evaluation programs such as CDCB-style pipelines. Producers and veterinarians should treat these results as research findings, not yet a purchasable trait on a sire proof.

Where this goes next

The authors conclude that direct genetic selection for digestibility — particularly NDFD — is possible without compromising milk fat and protein contents. As fecal-sampling workflows scale across commercial herds, digestibility could move from a nutrition-only metric toward a routinely evaluated, genomically predicted trait in Holstein improvement programs.