Plasma Adrenocorticoid Response to Corticotropin in Dairy Cattle Fed High Silage Diets

Responses of plasma corticoids to adrenocorticotropic hormone (.32 U/kg body weight) administered during early lactation were measured in dairy cattle consuming large amounts of concentrate feeds and one of the following forage diets: alfalfa-timothy hay plus corn silage, alfalfa-timothy hay crop silage plus corn silage, or corn silage. Concentrations of plasma corticoids increased to maximum in all groups within 60 to 90 min. Concentrations remained at near maximum for 4 to 5 h and then declined to baseline. Maximum corticoid concentrations following adrenocorticotropic hormone injection differed among forage treatment groups. Responses of corn silage and hay crop silage groups (46.7 +/- 1.1 and 50.1 +/- 2.4 ng/ml) did not differ but were less than those in the hay group (65.5 +/- 2.9 ng/ml). Response measured as area under the curve of plasma corticoid response also differed among forage treatments: hay 443 +/- 17 h (ng/ml) is greater than hay crop silage 334 +/- 34 is greater than corn silage 245 +/- 36. Results are discussed in terms of stress susceptibility in dairy cattle consuming diets containing large amounts of silage.