In the present study, the effect of concentrate supplementation pre-weaning on the peripheral distribution of leukocytes, the functional activity of neutrophils and the acute phase response in abruptly weaned and housed beef calves was investigated. Many factors centred on the time of weaning may heighten the susceptibility of calves to bovine respiratory disease (BRD) or may exacerbate its outcome . The ability of calves to cope with weaning stress may affect their subsequent health and performance thus, reducing the negative impact of weaning stress through the use of management strategies designed to optimize their health and welfare are important considerations. Feeding concentrates is often advocated as a means of reducing weaning stress in calves through familiarisation to a palatable feed.
In the present study, the combined effect of weaning and housing resulted in neutrophilia and concurrent lymphopaenia, which is in agreement with other studies [11, 12, 15]. Provision of concentrates pre-weaning did not result in discernible differences in neutrophil number between treatments as it increased by approximately 50% post-weaning in both treatments. This increase is less than the magnitude of neutrophilia observed (164% increase) previously  in calves that were not supplemented with concentrates either pre- or post-weaning. Previous findings have indicated that increased total leukocyte number was represented by profound neutrophilia . In the present study, the total leukocyte number was unchanged, and this was related to a less marked neutrophilia response. In line with previous research , abrupt weaning and housing decreased the percentage of neutrophils performing phagocytosis and did not affect neutrophil oxidative burst activity, and moreover, the provision of concentrates pre-weaning did not affect these functions.
In the present study, examination of lymphocyte subsets revealed that calves offered concentrates pre-weaning displayed a lesser reduction in percentage WC1+ lymphocytes compared with those not supplemented. This reduction in percentage WC1+ lymphocytes was also less than that obtained previously  where calves were not offered concentrates either pre- or post-weaning. The present findings are in agreement with other studies that have shown that γδ T cells are most sensitive to physiological and pharmacological stressors with reductions in percentage WC1+ lymphocytes observed following weaning of beef calves , transportation of beef steers , parturition in dairy cows , and dexamethasone treatment in dairy and beef bulls [21, 28].
Additionally, calves that were offered concentrates pre-weaning had unaltered percentage of CD8+ lymphocytes and increased percentage CD4+ lymphocytes post-weaning compared with their non-supplemented counterparts. These findings suggest that offering concentrates may provide some level of immune protection, manifested as a lesser reduction in WC1+ (γδ T) cells and more stable αβ T cell population. An overall increase in CD4+: CD8+ ratio was reported by Degabriele and Fell (2001)  in sheep that were housed either in isolation or in groups. These authors attributed the rise in CD4+:CD8+ ratio to recovery of immune compensation following perturbation to lymphocyte subsets due to a stressful change in environment. Thus, CD4+:CD8+ ratio may provide more information on the recovery of homeostasis rather than occurrence of immunosuppression. Collectively, this may prove beneficial for calf health immediately post-weaning, particularly when pathogen exposure is heightened after social mixing and regrouping of unfamiliar calves during transportation and marketing as in commercial practice. Monocyte and eosinophil number were not altered by offering concentrates pre-weaning or by the combined husbandry practices of weaning and housing, which is in accordance with previous work . Alterations in haematological variables reported in this study were found to be within the normal physiological range for calves of this age, and thus no negative effects on animal welfare were determined using these measures. Furthermore, the difference in rectal body temperature between supplemented and non-supplemented calves was not of clinical significance .
In the present study, significant differences were not realised in acute phase protein response between supplemented and non-supplemented calves, concentration of haptoglobin increased post-weaning, in line with other studies [15, 31]. Hickey et al. (2003)  reported increased concentrations of fibrinogen in non-supplemented abruptly weaned bull calves.
Metabolic responses can provide valuable information on the nutritional status of an animal and can inform on global deficiencies and malnutrition. The diet of the calves pre-weaning was not restricted in either treatment as they had free access to their dams for suckling and grass herbage was freely available. As there is a period of dietary adaptation following weaning, whereby the calf ceases milk consumption and consumption of solids changes (from grazed pasture to grass silage (and concentrates)), examination of metabolic profiles is confounded by this. Hence, it can be difficult to conclude definitively on effects of stress on metabolic responses. Calves that were supplemented with concentrates had greater concentrations of total protein compared with non-supplemented calves prior to- and immediately post-weaning. Although differences in feeding behaviour, as time spent at the silage feed face or concentrate trough, did not differ between treatments post-weaning, because feed intake was not measured, it is difficult to directly attribute the increased concentration of total protein to increased intake. Increased concentration of total protein has been reported in abruptly weaned beef calves , however in that study animals were fasted for 24 h post-weaning, and the authors suggested that dehydration may have contributed to the elevated concentrations of total protein. Activation of the stress response is an energy-dependent process . However, due to aforementioned issues it is difficult to say whether increases in energy-related metabolites (glucose, NEFA and βHB) are attributable to weaning stress or adaptation to a new diet. It is most likely that both these factors contributed to the response observed. Further research is required in disentangle these elements and to provide additional information on the dietary adaptation of weaned beef calves. Consistent with the profile described by Boland et al. (2008) , NEFA increased initially and subsequently decreased in abruptly weaned beef calves. Elevated NEFA and βHB represent a shift in energy balance in cattle, and may suggest a greater mobilisation of energy reserves in weaning-stressed calves. Elevated concentrations of NEFA and βHB have been associated with reduced neutrophil function ex vivo  in dairy cattle under negative energy balance, and this may contribute to the reduced phagocytic capacity of neutrophils post-weaning in the present study.
In agreement with other studies [18, 19], on the day of abrupt weaning, calves in the present study spent approximately 32% of their time resting. Following the day of abrupt weaning, calves spent approximately twice as much time lying, which is consistent with Enríquez et al. (2010)  but contrary to Boland et al. (2008) . The discrepancies between studies may be attributed to differences in space allowance. Where excessive space is allocated, weaned calves spend more time active and less time resting post-weaning . Calves in the present study and those used by Enriquez et al. (2010)  were restricted to a space allowance of 3.7 m2 per animal in slatted floor pens and 2.1 m2 per animal in a corral, respectively, compared with large open space paddocks .
In the present study, the CS calves began to consume their full offered amount of concentrates (1 kg/day) by d -19 of the study. Feeding behaviour did not differ between treatments post-weaning with both groups spending similar percentage time at the silage feed face and concentrate trough. Although, calves that were not offered concentrates pre-weaning were initially slower to spend time at the concentrate trough on d 1, this difference was short lived and was not evident by d 2, with both groups fully adapted to the provision of concentrates in a feeding trough by d 7 post-weaning. Walker et al. (2007)  reported that paddock-weaned calves that were introduced to a feedlot were slower to find the feed bunk than their yard-weaned counterparts, however, similar to the present study, this difference in feeding behaviour was short-lived, persisting for only a few days and had no overall effect on performance. Moreover, Fell et al. (1998)  reported less morbidity in yard-weaned calves compared with pasture-weaned calves following entry into a commercial feedlot. Compared with beef calves that were weaned in paddocks, yard-weaned calves showed increased social interactions with their peers post-weaning .
In the present study, calves offered concentrates had a 0.09 kg numerically higher average daily live weight gain for the 26 d pre-weaning period compared with non-supplemented calves. Other authors have shown increased performance responses to ad libitum creep feeds with advancing days of supplementation , however, this is in contrast to the present study where calves received 1 kg/head daily pre-weaning. The relatively poor growth response to concentrate supplementation in comparison with other studies [4, 5] may be partly attributed to factors including milk yield of the cow, herbage allowance and nutritive value and, level of concentrate supplementation.