Volatolomics of peripheral matrices as a potential tool to assess rumen function and host biology in lactating dairy cows fed diets contrasting in fiber and starch content

Noninvasive biomarkers of rumen function are required to develop precision nutrition with no constraints on animal welfare. Alterations in rumen function caused by diets with high acidogenic potential may be used as a model where fermentation affects the profile of volatile organic compounds (VOC) of the rumen fluid headspace and other matrices of the host ruminant, such as breath, urine, feces, milk, or sweat. We used selected-ion flow-tube mass spectrometry (SIFT-MS) to test the potential of the volatolome of breath, rumen fluid, urine, feces, sweat, and milk to discriminate contrasting diets in lactating dairy cows. Sixteen lactating Holstein cows (76 ± 10 DIM, 22.6 ± 2.21 kg/d of milk yield, mean ± SD) with a rumen bolus were enrolled in a 10-wk longitudinal experiment based on 2 contrasting diets with different proportions of the same feeds: (1) a high-fiber and low-starch diet (HFIBER, n = 8, 8% starch, DM basis); and (2) a high-starch (HSTARCH, n = 8, starch increasing to 24% in wk 9 and 10). Cows were sampled at the end of wk 9 and 10, and the samples were stored at -80°C until analysis. The volatolome of each sample was determined by SIFT-MS in full scan mode (positive and negative ions). Performance and rumen variables were analyzed as repeated measures in wk 9 and 10. Volatolomic data were analyzed by partial least square discriminant analysis for each sampling week independently. The HSTARCH cows showed higher milk production and milk protein levels, which were associated with slightly greater DMI, than the HFIBER cows. Rumen fluid concentrations of VFA were lower in HSTARCH cows than in HFIBER cows, regardless of the week, except for propionate, which was higher in HSTARCH cows only in wk 9. Similarly, the protozoa count was lower in HSTARCH cows than in HFIBER cows, with a more marked difference in wk 9 than 10. These differences in rumen traits were associated with increased subacute ruminal acidosis risk in HSTARCH cows, regardless of the week. The number of ions initially tested as potential predictors ranged from 177 to 253 depending on the matrix. Breath yielded a valid model discriminating the diet only in wk 9, whereas urine yielded valid models with moderate to high predictive ability to discriminate the diets in both weeks. The milk volatolome also yielded valid models in both wk 9 and 10, but its predictive ability was low. Meanwhile, rumen fluid, feces, and sweat did not yield predictive models at any time. Our results suggest that breath, urine, and milk rather than rumen fluid or feces have greater potential for the development of noninvasive biomarkers to monitor the diet and rumen function of dairy cows.