Genomic evaluation of diet digestibility in US Holsteins and correlations with nationally evaluated traits

The objectives of this study were to estimate genetic parameters for diet digestibility in Holsteins and to determine genetic correlations of digestibility with production traits. A total of 1,494 fecal samples were collected in the morning and afternoon from 1,065 lactating cows (1,314 samples) and 178 nulliparous heifers (180 samples) from 5 commercial herds and the Penn State University herd. Fecal and TMR samples were dried in a forced-air oven for 48 h at 55°C and composite fecal samples made from dried morning and afternoon samples on an equal dry-weight basis. The fecal and TMR samples were sent to a commercial laboratory to evaluate neutral detergent fiber (NDF) and indigestible NDF (iNDF) on an OM basis following a 240-h in vitro digestion. Three digestibility traits were derived including fecal potentially degradable NDF (pdNDF) concentration defined as fecal NDF - fecal iNDF, total-tract NDF digestibility (NDFD) and total-tract DM digestibility (DMD); high values for NDFD and DMD are favorable whereas a lower fecal pdNDF concentration is preferred because it indicates that there was less fiber in the feces that could have been digested. Yield data included 67,717 test-day milk yield and 67,764 test-day fat % records on 4,950 animals and genotypes were available from 947 animals with fecal samples, 2,796 cows with yield data only, and 1,838 ancestors. Digestibility traits were evaluated with single-step multi-trait animal models that considered fixed effects of lactation, contemporary group, biweekly stage of lactation, random additive genetic, random permanent environment, and error. Heritability estimates were 0.16 ± 0.04, 0.12 ± 0.02, and 0.06 ± 0.01 for fecal pdNDF, NDFD, and DMD, respectively. Additive genetic standard deviation was higher for fecal pdNDF and NDFD (1.21 ± 0.16 and 1.39 ± 0.15%, respectively) than DMD (0.67 ± 0.05%). The genetic correlation of fecal pdNDF with NDFD and DMD was -0.95 ± 0.02 and -0.34 ± 0.15, respectively, and the genetic correlation between NDFD and DMD was 0.60 ± 0.12. There was an unfavorable genetic correlation of fecal pdNDF, NDFD, and DMD with milk yield (0.63 ± 0.10, -0.77 ± 0.06, and -0.72 ± 0.05, respectively). However, the genetic correlations of fecal pdNDF and NDFD with fat % were favorable (-0.40 ± 0.10 and 0.31 ± 0.09, respectively). Correlation of EBV for fecal pdNDF, NDFD, and DMD with gPTA for milk yield were unfavorable, whereas the correlation of EBV for fecal pdNDF, and NDFD with national evaluation for fat % and protein % were favorable. In conclusion, diet digestibility is a heritable trait and can be evaluated using commercial farm animals through fecal sampling. There is genetic variation for diet digestibility among cows, and favorable genetic association with milk components, suggesting direct genetic selection for digestibility particularly NDFD is possible without compromising milk fat and protein contents.