Technical Note: The relationship between heart rate and energy expenditure in growing crossbred Boer and Spanish wethers

Eight Boer (75%) x Spanish (BS) and 8 Spanish (S) wethers (155 ± 8 d of age and 19.2 ± 2.3 kg BW, initial) were used in a replicated crossover design with a 2 x 2 factorial arrangement of treatments to determine effects of genotype, diet quality, and time of the day on energy expenditure (EE), heart rate (HR), and EE:HR with ad libitum, near maintenance, and fasting levels of feed intake. Diets were 65% concentrate and coarsely ground alfalfa hay. Energy expenditure ranked (P < 0.05) ad libitum > maintenance > fasting (500, 390, and 270 kJ/kg BW0.75). Heart rate did not differ between genotypes when fasting and with maintenance intake, but was greater (P < 0.05) for S vs BS when intake was ad libitum (BS: 55, 71, and 92; S: 52, 72, and 100 beats/min for fasting, maintenance, and ad libitum, respectively (SE = 2.0)). There was an interaction in EE:HR (P < 0.05) between level of feed intake and genotype (BS: 5.31, 5.59, and 5.00; S: 5.07, 5.57, and 5. 22 kJ/kg BW0.75:beats/min for ad libitum, maintenance, and fasting, respectively (SE = 0.13)), without an effect of diet. The effect of time on EE, HR, and EE:HR differed among levels of intake (P < 0.05). General patterns of change in EE and HR as time of day advanced did not differ, but increases near meals followed by decreases were of slightly greater magnitude for maintenance vs ad libitum intake. The ratio of EE:HR was greater for the maintenance level of feed intake than for ad libitum intake at most times. These results indicate similar potential for use of HR to predict EE of different genotypes of growing meat goats and that establishing EE:HR with different diets or levels of intake may not be crucial. Magnitudes of difference among hours suggest that when EE:HR is used to predict EE of confined goats from full-day measurement of HR, EE:HR should be determined over an extended period of time, such as 24 h.