Bos indicus cattle can maintain feed intake and fat reserves in response to heat stress better than Bos taurus cattle*

Bos indicus cattle are better adapted to tropical conditions than Bos taurus breeds and in part this may be related to differences in lipid and carbohydrate metabolism during heat stress. Bos indicus and Bos taurus cattle were subjected to heat stress and blood samples obtained to measure metabolism. Heat stress decreased feed intake and increased plasma non-esterifi ed fatty acids (NEFA) in Bos taurus but not in Bos indicus cattle. Also, Bos indicus appear to be more insulin resistant than Bos taurus cattle and this may explain some of the differences between breeds.


INTRODUCTION
Bos indicus cattle do not marble as well as Bos taurus and regardless of breed or diet, cattle fi nished in temperate regions develop more marbling than cattle fi nished under tropical conditions.Bos indicus cattle are better adapted to tropical conditions than Bos taurus breeds, in part because of a lower metabolic rate (Frisch and Vercoe, 1977).
Fat deposition in ruminants is largely the result of de novo synthesis in various adipose tissue depots.However, fatty acids can also be transported between depot sites as non-esterifi ed fatty acids (NEFA) or as lipoprotein triglycerides packaged in the lymph (from dietary sources) or liver.It is likely that different adipose tissue sites have different affi nities for the various forms of fatty acids and this may impact on the site of deposition.It is also possible that some of the marbling differences between breeds and temperature may be related to metabolism.The aim of this study was to determine the metabolic responses to heat stress and under nutrition in Bos taurus and Bos indicus cattle.

MATERIAL AND METHODS
Experiment 1 involving 18 heifers (12 Bos taurus and 6 Bos indicus; average weight 350 kg) in 3 replicates was conducted to determine the effect of breed and heat on feed intake and metabolism.Heifers were placed in a climate control room and offered feed at 2.25% BW and had ad libitum access to water.For the fi rst 2 days the wet bulb temperature was maintained at 20ºC before being increased (over 5 days) to 32ºC and then maintained at this temperature for 5 days.The wet bulb temperature was then lowered over 3 days.In an effort to separate the effects of temperature from the effects of the reduction in feed intake, Experiment 2 was conducted under temperate conditions (20ºC) with 6 Bos taurus heifers.Three of these heifers were fed ad libitum while the other 3 heifers were pair-fed (and watered) to the average levels consumed by the Bos taurus cattle in the study outlined above.Blood samples were taken via indwelling venous catheters and plasma was analysed for non-esterifi ed fatty acids (NEFA), triglyceride, glucose, lactate and insulin concentrations.
Data were analysed using Residual Maximum Likelihood (REML, Genstat 5) given the ability of this technique to deal with unbalanced data sets.Experiment 1 was analysed with breed and day as the fi xed factors and replicate and heifer as the random factors whereas Experiment 2 was analysed with feed level and day as the fi xed factors and replicate and heifer as the random factors.

RESULTS
Increasing the wet bulb temperature to 32ºC decreased feed intake in Bos taurus (2.0 vs 0.15 % BW; P<0.001) cattle but had no effect on feed intake in Bos indicus (1.8 vs 1.7 % BW) cattle (Figure 1).Increasing wet bulb temperature increased plasma NEFA in Bos taurus but had no effect in Bos indicus (Figure 2).Thus, at 20ºC plasma NEFA were slightly lower in Bos taurus than in Bos indicus cattle (136 vs 176 µM) whereas at 32ºC plasma NEFA was markedly higher in Bos taurus and unchanged in Bos indicus cattle (533 vs 165 µM).Although there was no overall effect (P=0.34) of breed on plasma glucose, increasing the temperature reduced plasma glucose in Bos taurus but had no effect in Bos indicus cattle as indicated by the signifi cant interaction (P<0.001) between day and breed.Thus, at 20ºC there was no difference in plasma glucose between Bos taurus and Bos indicus (4.69 vs 4.55 mM) whereas at 32ºC plasma glucose was lower in Bos taurus and unchanged in Bos indicus cattle (3.86 vs 4.51 mM).

EFFECT OF HEAT ON METABOLISM IN BEEF CATTLE
Plasma insulin was lower in Bos taurus than in Bos indicus cattle (22 vs 36 µU/mL, P<0.001) but was not affected by temperature.Similarly, plasma triglycerides concentrations were lower in Bos taurus than in Bos indicus cattle (0.17 vs 0.43 mM; P<0.001) but were not affected by temperature.Plasma lactate was not different between the breeds (0.85 vs 0.83 mM; P=0.76) and there was no consistent effect of temperature.Restriction of feed intake to a level similar to that observed under heat stress in Bos taurus cattle resulted in a similar increase in plasma NEFA (122 vs 733 µM; P<0.001) as observed under high heat conditions.BEATTY D. ET AL.Multiple regression analyses demonstrated that plasma NEFA concentrations were negatively related to feed intake (R 2 =0.85) but this relationship was not affected by temperature or breed.There was little effect of short term feed restriction under thermo neutral conditions on plasma glucose, triglycerides or lactate.There was an interaction between feeding level and day (P<0.001)such that feed restriction reduced plasma insulin whereas feed repletion increased plasma insulin when compared to the ad libitum fed heifers.

DISCUSSION
These data suggest that increased mobilization of body fat during heat stress may explain some of the effects of environment on marbling but not breed differences.Bos taurus cattle had lower plasma triglycerides and insulin than Bos indicus cattle regardless of temperature.Under temperate conditions Bos taurus cattle had higher plasma glucose whereas at higher temperatures, Bos indicus cattle had higher plasma glucose than Bos taurus cattle.These data suggest that Bos indicus cattle are more insulin resistant than Bos taurus cattle, particularly with respect to triglyceride hydrolysis and perhaps fatty acid uptake.

CONCLUSIONS
During heat stress Bos indicus cattle can maintain feed intake and body fat reserves better than Bos taurus cattle.Also, Bos indicus cattle are more insulin resistant than Bos taurus cattle and this may explain some of the differences in marbling between the breeds and environments.

Figure 1 .
Figure 1.Relationship between wet bulb temperature and feed intake in Bos indicus and Bos taurus cattle

Figure 2 .
Figure 2. Relationship between wet bulb temperature and plasma non-esterifi ed fatty acids (NEFA) in Bos indicus and Bos taurus cattle