Evaluation of heat stress and glucose precursor supplementation on performance and blood metabolites of fattening Makoei lambs

Heat stress (HS) compromises efficient animal production and although difficult to accurately quantify, the economic impact on the global livestock industries is likely greater than $100 billion annually (Baumgard and Rhoads 2013). The economic decline in the dairy industry is primarily associated with lowered milk production, reduced growth, compromised milk quality, reduced reproduction, increased metabolic disorders and poor immune function. Dry matter intake per kilogram live weight was lower and the maintenance requirements of sheep were higher at high ambient temperatures (Marai et al. 2007). Pair-feeding study in dairy cows indicated that during heat stress, reduced dry matter intake accounted for only 35- 50% of the decreased in milk production (Wheelock et al. 2009). A variety of amelioration strategies for HS are available and can be implemented alone or in a coordinated manner that include physical modifications to the environment, management adaptations, genetic selection, and dietary modifications. Nutritional strategies are among the easiest and cheapest to alleviate the negative effects of HS. Nutritional tactics used during HS generally focus on enhancing the energy density of the diet and decreasing the ration forage based on the reduction of incremental heat (Mahjoubi et al. 2016). Providing gluconeogenic substrates or supplements that metabolized into glucose precursors may increase productivity during the heat stress (Baumgard and Rhoads 2012). The objective of this study was to evaluate the heat stress effects and also glucogenic supplement during heat stress on performance, heat stress indices and blood metabolites of fattening Makoei lambs.

In this experiment, 10 male Makuei lambs with 3 months age and average live weight of 32 ± 3.2 kg in 2 treatments and 5 repeat per treat at two experimental periods were used. 10 days before beginning of period 1, the lambs were adapted to 2×1 meter individual cage and experimental diets. Lambs received water and feed individually. Feed was delivered to the animals as a total mixed ration and offered in the morning (8:00) and evening (16:00). Diet was formulated to meet or exceed NRC (2007) recommendations (Table 1) using the Small Ruminant Nutrition System. In period 1 that lasted 10 d, lambs at thermal neutral (TN) condition (24.5±3.2 °C and temperature-humidity index [THI] of 66.1 ± 2.5) and heat stress (HS) (33-41°C and THI of more than 83) were fed ad libitum. In period 2 (lasted 10 d) the lambs of HS condition continued to ad libitum feeding and lambs in TN condition pair fed (feed intake reduced according to HS group feed intake in period 1) to eliminated the confounding effects of unequal nutrient intake. Both groups received 10 gr/d of a top-dressed glucogenic product (GL-Part 100®) in two equal meals. All production and blood data were collected daily through both periods. Three thermostat-regulated heaters were used to generate the heat load and 2 fans were utilized to evenly distribute the heat load within the room and fans were angled to prevent hot air blowing directly on the animals. In the beginning of the experiment and at the end of each experimental period, lambs were weighed before morning feeding. During each period, DMI and water intake were recorded daily and body temperature indices were obtained at 1400 h. Blood samples were collected during both periods from the jugular vein (5 mL in collection tubes containing of sodium heparin) before and 4 h after morning feeding on d 3 and 8 in P1, and d 3, 8 in P2. Blood was centrifuged at 2000 x g for 15 min to obtain plasma and then, stored at -20°C for later analysis.

HS significantly reduced DMI in both periods than TN group (P<0.05). Glucogenic supplement in period 2 significantly increased DMI than period 1 (P<0.01). Glucogenic supplement increased average daily gain and feed efficiency significantly (P<0.05). HS significantly increased rectal temperature in both periods, but Glucogenic supplement did not effect on body temperature. At a.m. feeding in both periods, blood glucose concentration of HS group significantly lower than TN lams (P<0.05). After feeding, HS did not affect any of the blood parameters (Mahjoubi et al., 2016), But Glucogenic supplement affected NEFA blood concentration significantly (P<0.05). Lower blood urea nitrogen in glucogenic supplementation treatments was probably due to a higher supply of glucose precursors and a less need for muscle tissue degradation to supply glucose precursors, leading to increased nitrogen efficiency (Donkin et al., 2009). After feeding, no increase in blood glucose after glucogenic supplementation was interesting because we expected that glucose precursor consumption would increase blood glucose concentration after feeding. The results differ on the effects of using glucose precursor supplements. Glucose precursor supplementation numerically increased blood insulin levels compared to the first period, which may be another reason for the lack of increased blood glucose in lambs after glucogenic supplementation. Differences in the type and composition of glucose precursor supplements, how much they are used in the diet and the physiological stages of livestock are the reasons for the differences in the results of different researchers.

As expected, HS reduced DMI than TN lambs, but glucogenic supplemented lambs have more DMI than lambs in period 1. Dietary glucose precursors improved feed efficiency at HS. These results showed that in HS the efficiency of converting dietary nutrients into tissue accretion was enhanced as glucose precursors fed. Also, HS significantly increased rectal temperature in both periods. At a.m. feeding in both periods, blood glucose concentration of HS group significantly was lower than TN lams. But glucogenic supplementation significantly affected NEFA blood concentration. In overall, improvement of some important functional and metabolic indices in lambs fed with glucogenic supplement in HS condition can indicate the beneficial effects of this nutritional strategy on lambs during HS, but this needs more research to prove.