Impact of level of spontaneous heating during storage of bermudagrass hay on rumen in situ disappearance kinetics in steers

Spontaneous heating reduces forage quality and nutrient digestibility in stored alfalfa (Medicago sativa L.) and cool-season grasses. However, research describing the impact of spontaneous heating on disappearance kinetics of warm-season grasses is limited. Five crossbred ruminally cannulated steers were used in a randomized complete block design to determine the impact of spontaneous heating during the curing process on in situ disappearance kinetics of bermudagrass hay. Bermudagrass hays were baled at various moisture levels and bale densities that resulted in spontaneous heating that produced 5, 119, 201, 273, or 401 heating-degree days (HDD; >35 °C) during a 60-day storage period. Hays were evaluated for rumen dry matter (DM), neutral-detergent fiber (NDF), N, and neutral-detergent insoluble nitrogen (NDIN) disappearance kinetics. The immediately soluble DM fraction and effective ruminal DM disappearance generally decreased (P<0.05) as HDD increased. The fraction of DM that disappeared at a measurable rate was lower (P<0.05) in 5 HDD hays and the disappearance rate of DM (Kd) was faster (P<0.05) from 5 and 119 HDD hays compared with forages subjected to higher HDD. The Kd for NDF was faster (P<0.05) from 5 and 119 HDD than from 201 and 273 HDD hays and effective NDF disappearance was higher (P<0.05) from 5 and 119 HDD than from higher HDD hays. The Kd for N was faster (P<0.05) from 5 HDD, and effective N disappearance was greater (P<0.05) from 5 and 119 HDD than from 201 or higher HDD. The Kd for NDIN was faster (P<0.05) from 5 and 119 HDD and effective NDIN disappearance was higher (P<0.05) from 5, 119, and 201 HDD than from higher HDD hays. Moderate levels of spontaneous heating have negative effects on rate and effective ruminal disappearance of both DM and N in bermudagrass hay, thereby reducing the digestibility of DM and protein from heated forages.