Double-muscled and conventional cattle have the same net energy requirements if these are related to mature and current body protein mass, and to gain composition

The hypothesis tested in this paper is that double-muscled (DBM) and conventional 16 cattle, 17 considerably differing in body composition, have similar NE requirements when: a) NEm is scaled as a 18 function of current (Pi) and adult (Pm) protein mass; and b) ME for gain (MEg) is estimated from protein (Pr) 19 and lipid (Lr) retention and their partial ME use efficiencies, the kp and kl values, respectively. First, two 20 databases were examined; one was developed combining well known literature information from 21 comparative slaughter trials conducted on British beef steers; the other was based on a trial conducted using 22 extremely lean DBM Piemontese bulls. From the first database, NEm was calculated to be 1.625 × Pi ÷ Pm × Pm 0.73 (MJ/kg0.73). From the second database, the daily MEg was determined as 22.8 MJ × Pr ÷ kp + 38.74 MJ 23 24 × Lr ÷ kl, assuming (from prior reports) that kp = 0.20 and kl = 0.75. Thereafter, MEm was defined as ME intake minus MEg, and, hence, NEm was predicted as 1.625 × Pi ÷ Pm × Pm 0.73 (where 1.625 was the value 25 26 obtained from the first dataset). The resulting km (NEm/MEm) averaged 0.67. This km value did not differ 27 from that (0.65; P = 0.12) predicted by Garrett’s equation, which uses dietary ME content as the only 28 predictive variable. Second, the procedure was tested for the ability to detect effects on km caused by 29 increasing BW and dietary factors not estimable from the dietary ME content only. Data were gathered from 30 a trial involving 48 DBM Piemontese bulls divided into 4 groups fed one of 4 diets differing in CP content 31 (145 or 108 g/kg DM); with or without addition of 80 g/d of rumen-protected CLA (rpCLA). Bulls were 32 examined at 3 consecutive periods of growth, corresponding to 365, 512 and 631 kg of average BW. All 33 energy balance items were influenced by increasing BW, except km (P = 0.61), in agreement with the expectation that NEm requirement depends on the degree of maturity (Pi/Pm) and the Pm 0.73 of an animal, 34 35 whereas km reflects characteristics of the feed provided. The km value was also influenced by the CP × 36 rpCLA interaction (P = 0.013). We conclude that DBM and British beef steers have similar NE requirements 37 when these are scaled as a function of Pi and Pm, and gain composition, considering Pr, kp, Lr and kl. The 38 proposed procedure will be useful to predict the energy requirements and feed utilization in cattle of different 39 types that vary in BW, provided that body and gain compositions are known or accurately predicted. 40