The effects of heat stress and feed restriction on the microbiome of lactating dairy cows
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Elevated environmental temperatures and relative humidity can cause heat stress in dairy cattle which results in decreased feed intake, reduced milk production, and increased disease incidence. Our objectives were to evaluate the effects of heat stress and feed restriction on the rumen fluid, fecal, bedding, inguinal skin, teat skin, and milk microbiomes of lactating dairy cattle. Feces contribute bacteria to bedding and by evaluating rumen fluid, feces, bedding, inguinal skin, teat skin, and milk samples, we aimed to determine if elevated environmental temperatures induced changes in one microbial niche impacted the other sample types evaluated. We hypothesized that heat stress would result in increased shedding of Gram-negative bacterial phyla (Bacteroidetes and Proteobacteria) and mastitis-associated bacterial operational taxonomic units (OTU) (Enterobacter, Enterococcus, Escherichia-Shigella, Klebsiella, Staphylococcus, and Streptococcus). Our first study evaluated the effects of heat stress on one group of cows (n = 6) in a time course design through pretreatment, heat stress, and recovery periods. Our second study evaluated the effects of elevated and clamped rectal temperature (40[degrees]C [plus-minus] 0.5; n = 6) or pair-feeding (n = 6) in a sequential design through pretreatment, challenge, and recovery periods. A third group of control cows (n = 6) was maintained under thermoneutral conditions. During these studies, the relative abundances of dominant bacterial phyla, families, and genera in rumen fluid, feces, bedding, inguinal skin, teat skin, and milk often displayed different responses to heat stress and feed restriction in dairy cattle. Fecal samples were collected from heat-stressed cows in both studies but the relative abundances of Bacteroidetes and Firmicutes differed between the studies. Rumen fluid and fecal samples contained few of the OTU corresponding to mastitis associated bacteria. Enterococcus relative abundance appeared temperature responsive as shown by an increase in abundance in bedding, and on inguinal and teat skin during the heat stress challenge. Because inguinal skin was less exposed to the bedding than teat skin, the rise in the abundance of Enterococcus during heat stress highlights the skin's additional contribution to changes within the teat skin microbiome. These studies highlight that the skin microbiome responded to heat stress and may alter the bedding microbiome. Collectively, the results from these studies report specific bacterial OTU that appear heat stress-responsive and support the concept that mastitis-associated bacteria have different responses to heat stress. Additionally, the results from these studies highlight the variability within the dairy cow microbiome over time.
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