Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

J. D. Roussel


Twenty-four mid-lactation heat stressed dairy cows were paired in a completely randomized block design on basis of breed, age, lactation number, calving date, previous mild production and milk fat content in order to evaluate experimental responses. Test treatment was a 50:50 dry matter (DM) corn silage-concentrate diet supplemented with 4.75% DM specially-selected fish meal (FM). Animals were housed under natural summer conditions for the duration of a 70-d experimental period, commencing in mid-June. Low roughage diets were slightly below NRC requirements for both energy and crude protein. DM intake, milk fat content, and milk fat:milk lactose ratio were statistically significant, with means and SE of 14.56, 12.83 ($\pm$0.337) kg/d, 3.71, 4.20 ($\pm$0.04)%, and 0.79, 0.877 ($\pm$0.010) for fish meal supplemented (FMS) and control (CON) diets, respectively. Other milk component contents and yields and component ratios were not statistically significant. Though FM supplementation increased DM intake, it also significantly decreased gross milk production efficiency. Mean daily ETI and THI index values for experiment duration were 33.16 $\pm$ 0.86 and 78.02 $\pm$ 0.89, respectively. Respiration rates (RR) and rectal temperatures (RT) for FMS and CON cows were 99.6, 105.4 ($\pm$1.75) cycles per minute, and 39.7, 39.9 ($\pm$0.12) C, respectively. Significant correlations between RT and climate indexes were found only for the CON diet (r $>$ 0.21), while the FMS diet lacked significance (r $<$ 0.03). Plasma cortisol levels were not different between treatments, yet plasma aldosterone was significantly lower for the FMS group. FMS and CON diets mean plasma steroid concentrations were 8.65, 8.61 ($\pm$0.34) ng/ml and 217.5, 252.5 ($\pm$6.10) pg/ml for cortisol and aldosterone, respectively. Cortisol correlated negatively with daily maximum temperature in FMS cows, but not in CON cows. Cortisol correlated positively with RR in FMS cows, only. Plasma aldosterone did not correlate with RT, nor RR in either treatment group. Plasma aldosterone may provide an index for evaluating levels of adaptability on varying intakes of nutrients during times of heat stress in the lactating dairy cow. Productive performance was not enhanced from inclusion of dietary FM, yet non-productive physiological adaptability was improved.