Effect of Different Fat Sources on some Blood Metabolites, Hormones, and Enzyme Activities of Lambs with Different Residual Feed Intake in Heat-Stressed Condition

Most efficient animals in heat-stressed condition, intelligently regulate their metabolism for maximizing their productivity and fats play an important role in reducing heat stress in these animals but the underlying mechanisms remain elusive. The effects of different fat sources on some blood metabolites and hormones and enzyme activities of heat-stressed lambs with different residual feed intake (RFI) were studied. In the preliminary trial, 48 male lambs of four different breeds (Zel, Dalagh, and their hybrids with Romanov; BW=16.87±0.55 kg) in a block randomized complete design used to assay RFI for 67 days. After that, 32 lambs of the same breeds as a block (BW=30.74±1.21 kg) with two different RFIs in heat stressed condition were used in a 2 × 4 factorial trail over 84 days. The treatments included four rations: 1) basal ration (control), 2, 3, and 4) rations supplemented with calcium soap fatty acids (FA), beef tallow, and canola oil, respectively. Dry matter intake (DMI) was high in control and high RFI groups (low efficient) (p < 0.05). Lipid sources had significant effect on serum glucose, cholesterol, triiodothyronine (T3), and Insulin, pulse and respiration rate (p < 0.05). No differences found between treatments for triglyceride, thyroxine (T4), low-density lipoprotein (LDL), and Very-low-density lipoprotein (VLDL). Glucose concentration had strong correlation with the RFI (p < 0.01). The Low RFI lambs (LRFI) had high T3 and low LDH levels. These lambs had high respiration and pulse rate (p < 0.05). Lambs fed with fat supplemented rations had higher concentration of serum aspartate aminotransferase (AST), alkaline phosphatase (ALP), and lactic dehydrogenase (LDH) enzymes than control (p < 0.01). Lipid supplementation in heat stressed lambs markedly alters glucose, cholesterol, T3 and respiration and pulse rate that independently of reduced dry matter intake (DMI) through coordinated changes in fuel supply and utilization by multiple tissues. Even more challenging the most efficient lambs (LRFI) had high physiologic rate and high activity for removing heat from tissues.