Thermoregulatory differences in lactating dairy cattle classed as efficient or inefficient based on residual feed intake K. DiGiacomo A,D , L. C. Marett B , W. J. Wales B , B. J. Hayes C , F. R. Dunshea A and B. J. Leury A A The Faculty of Veterinary and Agricultural Sciences, The University of Melbourne, Parkville, Vic. 3010, Australia. B Agricultural Research Division, Department of Environment and Primary Industries, Ellinbank, Vic. 3821, Australia. C Biosciences Research Division, Department of Environment and Primary Industries, Bundoora, Vic. 3083, Australia. D Corresponding author. Email: Kristyd@unimelb.edu.au Abstract. It is suggested that one-third of the inter-animal differences in efficiency is explained by differences in digestion, heat production, body composition and activity; while the remaining variation is the result of energy expenditure due to biological processes such as ion pumps and mitochondrial function. Inefficient animals may be wasting energy on inefficient processes resulting in increased heat production that may be reflected by differences in skin and core temperature. While the association between heat production and residual feed intake (RFI) has been touched on, it is yet to be fully elucidated. It is hypothesised that more efficient animals will expend less energy as heat, which will be reflected by differences in core and skin temperature measures. Fifty-four primiparous, Holstein-Friesian cows previously assessed for RFI (26 inefficient/high RFI, 28 efficient/low RFI) were selected and drafted into outdoor holding yards for measurements on two occasions (once during lactation and once during the non-lactating ‘dry’ period). Measures of body temperature were obtained using an infrared (IR) camera to obtain skin (surface) temperatures at multiple locations [muzzle, eye, jaw, ear, leg (front and back), rump, shoulder, teat, udder, side and tail] and rectal temperatures were measured using a digital thermometer. Respiration rates (RR) were obtained by counting the number of flank movements in 1 min. A subset of 16 cows (8 efficient and 8 inefficient) were utilised for further IR imagery in an undercover environment (to eliminate the influences of external environments). Skin temperature measurement obtained using an IR camera during the outdoor period demonstrated that inefficient cows had higher (0.65  C) teat temperatures (P = 0.05). Rectal temperature and RR were not influenced by efficiency group. When IR images were obtained undercover inefficient cows tended to have higher shoulder (0.85  C) and neck (0.98  C) temperatures than efficient cows (P < 0.087); while udder temperature was significantly greater (1.61  C) for inefficient than efficient cows (P = 0.018). These data indicate that some of the differences in efficiency may be attributed to differences in thermoregulation, as reflected by differences in skin (but not core) temperature and that IR imagery is a suitable method for determining these differences in a non-invasive manner. Further research is required to further establish these relationships, and the measurement of skin temperatures should be undertaken indoors to eliminate external environmental influences. Additional keywords: efficiency, feed conversion efficiency (FCE), physiology, temperature. Received 13 March 2014, accepted 17 June 2014, published online 19 August 2014 Introduction The efficiency of energy utilisation is one of the most important determinants of the productivity of dairy production systems (Reynolds et al. 2011). Temperature regulation has been identified as an important aspect of physiological variation that could affect feed conversion efficiency (FCE) in dairy animals (Herd and Arthur 2009). The rationale for this is that animals that have a higher core body temperature than other similar animals under defined conditions (e.g. similar feed intake) must utilise more feed energy on producing metabolic heat at the expense of productivity (i.e. in this case milk synthesis) (Britt et al. 2003). Thermoregulation is identified as an important aspect affecting FCE in beef cattle. One path for energy loss in ruminants is via evaporative heat loss, which is mainly regulated by respiration rate (RR); however, any relationships between RR and residual feed intake (RFI) have yet to be elucidated (Herd and Arthur 2009). Scientific understanding of the physiological phenotype of low and high FCE cows is poor and further knowledge of the physiological differences between animals of different efficiencies is required to develop a suite of potential indices for improved selection of more efficient animals. Measures such as skin (surface) and core (rectal) temperatures (T R ), RR and heart rates are all important indicators of the thermal status of an animal. This study was designed to examine some measures CSIRO PUBLISHING Animal Production Science, 2014, 54, 1877–1881 http://dx.doi.org/10.1071/AN14311 Journal compilation Ó CSIRO 2014 www.publish.csiro.au/journals/an