This study aimed to evaluate the effects of dietary organic selenium (Se) supplementation on growth
performance, hematological parameters, and selected blood metabolites in fattening Baluchi lambs. A total of 24
male lambs (initial body weight; BW, 18.4 ± 1.7 kg) were assigned to a completely randomized design with four
dietary treatments and six replicates per treatment over a 75-day period. Prior to the experimental phase, lambs
were acclimated to individual cages and a basal diet for two weeks. The dietary treatments included a control group
(basal diet without organic selenium supplementation) and three experimental groups receiving 0.25, 0.5, and 0.75
mg Se/kg feed in the form of an organic selenium-methionine supplement. The results indicated that selenium
supplementation had no significant effect on average daily feed intake, final body weight, average daily gain
(ADG), or feed conversion ratio (FCR). Furthermore, there were no notable alterations in hematological
parameters, including red and white blood cell counts, hematocrit values, or hemoglobin concentrations, attributed
to selenium supplementation. Likewise, the levels of thyroid hormones, specifically triiodothyronine (T3),
tetraiodothyronine (T4), and thyroid-stimulating hormone (TSH), did not exhibit any significant changes in
relation to dietary selenium intake. Importantly, the activity of blood glutathione peroxidase (GPX) demonstrated
a linear increase corresponding to increased selenium supplementation. Additionally, a quadratic influence of
dietary supplemental Se on blood urea concentration was observed. Specifically, lambs receiving 0.25 mg and 0.5
mg of Se/kg of diet revealed elevated concentrations of circulating urea compared to those on the control diet.
However, no significant differences were detected in malondialdehyde, creatinine, triglycerides, or cholesterol
levels among treatment groups. In conclusion, the administration of dietary organic selenium at levels up to 0.75
mg/kg did not influence growth performance nor demonstrated any negative effects on blood cells and metabolic
status of the lambs. Nevertheless, it resulted in a notable enhancement of glutathione peroxidase activity,
underscoring the pivotal role of organic selenium in enhancing the antioxidant system of fattening lambs.