تاثیر استفاده از شکل‌های مختلف کروم در پیرامون زایش بر مصرف خوراک، قابلیت-هضم مواد مغذی، رفتارهای تغذیه‌ای میش‌های افشار و عملکرد بره‌های آن‌ها تحت تاثیر تنش گرمایی

Introduction: During the time around parturition, animals make many metabolic adjustments to support the transition from pregnancy to lactation. In addition, dairy cattle produce milk more than their ability to consume energy, as a result, they are in a negative energy balance at the beginning of lactation, which may reduce the longevity of the cattle in the herd and increase the rate of elimination of dairy cattle. Heat stress in late pregnancy is aggravated by energy restriction. In such conditions, the use of management and nutritional methods will reduce the problems during the transfer period and heat stress. One of the ways to optimize the productive and reproductive conditions of livestock, through improving the metabolism of nutrients and eliminating or reducing stress conditions, is to use chromium as a metabolic improver. Scientific sources suggested the amount of chromium needed for sheep 3 to 5 mg per day and for dairy cows 15 to 50 mg per day. Inorganic chromium has a bioavailability of about 0.5 percent. While organic chromium has more than 25 percent intestinal absorption, and therefore, consumption of inorganic chromium is not recommended due to its low bioavailability and toxic effects. Among the sources of organic chromium, chromium-methionine has been recognized by important global food and drug organizations as a compound with high bioavailability, impressive metabolic responses and no toxicity complications. Materials and Methods: This experiment was conducted to investigate the effect of using different forms of chromium around calving on feed consumption, nutrient digestibility, nutritional behavior of Afshar ewes and the performance of their lambs under the influence of heat stress. Forty pregnant Afshari ewes were assigned to four experimental treatments with ten replications from 42 ± 5 days before the expected birth in a completely randomized design. Experimental treatments include: 1) basic diet without chromium supplementation (control), 2) basic diet containing 3 mg of chromium in mineral form per kg of dry matter, 3) basic diet containing 3 mg of chromium in the form of chromium-methionine per kg Each kilogram of dry matter and 4) the basic diet contained 3 mg of chromium in the form of chromium nanoparticles per kilogram of dry matter. The length of the test period was 84 days. Weighing of ewes was done at the beginning, at the time of calving and at the end of the period. The rest of the feed is weighed every day and the daily feed consumption was calculated by deducting from the provided feed. In order to measure the digestibility of ewes, stool and feed samples were collected for 5 days from 10 to 6 days before possible calving and 31 to 35 days after calving. To determine the chemical composition of food and feces samples (dry matter, crude protein, crude fat, and organic matter), the methods of the official association of analytical chemists were used. The insoluble fiber in neutral detergent (NDF) and the insoluble fiber in acid detergent (ADF) were also determined by the Van Soest method. In relation to feed consumption behavior during 28 to 30 days after birth, it was measured as a 24-hour activity record. The time spent for the activities of eating, resting, standing and ruminating was recorded visually every 5 minutes and assuming that the activity continued in the last 5 minutes for all the animals during the day and night hours. After birth, the lambs were first dried and weighed after performing the necessary hygiene operations, and then they received the first portion of colostrum. Diarrhea status and duration of diarrhea and stool score were recorded daily. Stool scores were determined based on 1_ hard and consistent, 2_ soft and loose, 3_ loose and watery, 4_ watery with some blood and 5_ watery with blood and mucus. A digital thermometer was used to measure the rectal temperature. The thermometer was placed in the animal's rectum for 3 minutes and the temperature was recorded immediately. Respiration rate was measured by visual observation of the number of ups and downs of the animal's side in one minute. In order to measure the apparent digestibility of dry matter in lambs, feed and feces samples were sampled on the 35th day after birth using acid-insoluble ash (AIA) as an internal marker for six consecutive days. Results and Discussion: The results showed that there was no significant difference in the weight of the ewes before calving. But from the beginning of ewes giving birth (P=0.0466) to 6 weeks after giving birth (P=0.0484), the treatments receiving different forms of chromium performed better than the control group of ewes. Dry matter consumption of ewes also had an upward trend from the third to the sixth week after giving birth in the ewes receiving chromium-methionine and chromium nanoparticles (P < 0.05). The digestibility of dry matter and insoluble fibers in neutral detergent increased during the period before and after the delivery of ewes under the influence of chromium intake (P=0.0001). Addition of chromium supplement to the diet of ewes did not cause a significant difference in the parameters of feed consumption behavior, but it caused a significant difference in the resting behavior of the ewes, so that the lowest and longest resting time was related to the treatment receiving chromium nanoparticles and the control, respectively (P=0.0411). Adding chromium supplement to the diet of ewes improves the birth weight, daily weight gain, digestibility and diarrhea condition of the born lambs (P<0.05). Conclusion: In general, it is recommended to receive chromium, especially in the form of chromium-methionine and chromium nanoparticles, during the transition period of ewes under the influence of heat stress.