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

R. Puchala, I. Tovar-Luna, A. L. Goetsch, and T. Sahlu

E (Kika) de la Garza American Institute for Goat Research, Langston University, Langston, OK

Eight Boer (75%) × Spanish (BS) and eight Spanish (S) wether goats (155 ± 8 d of age and 19.2 ± 2.3 kg BW, initial) were used in a replicated crossover design experiment 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 the EE:HR ratio with ad libitum, near maintenance, and fasting levels of feed intake. Diets were 65% concentrate (CON; 13.8% CP, DM basis) and coarsely ground alfalfa hay (FOR; 18.5% CP). EE was determined by respiration calorimetry with head-boxes, expressed relative to average BW within the 2-d measurement periods, and HR was measured using Polar S610 heart rate monitors. EE ranked (P < 0.05) ad libitum > maintenance > fasting (499, 392, and 270 kJ/kg BW0.75), and differences (P < 0.05) in HR were similar (95.8, 71.5, and 54.2 beats/min, respectively). However, EE:HR was highest among levels of intake (P < 0.05) for maintenance (5.09, 5.61, and 5.31 kJ/kg BW0.75 per heart beat for ad libitum, maintenance, and fasting, respectively). HR with ad libitum intake tended to be higher (P < 0.07) for S vs BS goats (99.2 vs 91.9 beats/min), but genotype had no effect with maintenance intake or fasting. EE:HR with ad libitum intake was higher (P < 0.05) for BS vs S (5.57 vs. 5.07 kJ/kg BW0.75 per heart beat). Hour of the day affected EE, HR, and EE:HR at all levels of intake (P < 0.05). With ad libitum and maintenance intake, EE was highest during and after meals at 0800 and 1600. Conversely, EE during fasting was only higher during daytime hours (0800 to 2000) than at night. For ad libitum, maintenance, and fasting intake, the highest EE:HR ratio was at 1600 (5.60, 6.00, and 5.58 kJ/kg BW0.75 per heart beat, respectively). The lowest EE:HR ratio for ad libitum and maintenance intake was at 0700 (4.93 and 5.25 kJ/kg BW0.75 per heart beat, respectively), whereas during fasting the lowest value (4.69 kJ/kg BW0.75 per heart beat) was at 1000. In conclusion, for use of HR to predict EE, it appears desirable to determine the ratio of EE:HR over an extended period of time with a level of intake similar to that during prediction.


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