Effects of Feed Additives on Performance, Nutrient Digestibility, Feed Intake, and Health of Weaned Calves: A Review

Abstract

The weaning transition period, spanning 3-6 months of age, is a critical physiological phase marked by rumen development, immune system maturation, and heightened susceptibility to nutritional stress in beef and dairy calves. In recent years, phytogenic and bioactive feed additives, including tannins, saponins, essential oils (EOs), amino acids (AAs), and phenolic compounds, have emerged as promising alternatives to antibiotic growth promoters (AGPs). This review systematically examined the effects of these additives on average daily gain (ADG), feed intake, nutrient digestibility, and health parameters in weaned calves aged 3–6 months. Low to moderate doses of hydrolysable tannins (0.1-3% dry matter intake) enhance nitrogen retention and either maintain or slightly increase average daily gain without compromising nutrient digestibility. EO blends at optimal doses (150–600 mg/kg DM) improved feed intake, dry matter digestibility, and immune markers, including immunoglobulins and antioxidant enzyme activity. Rumen-protected lysine and methionine supplementation are critical for maximizing growth and nitrogen retention. The combined use of these additives shows synergistic potential, although controlled trials in the 3–6-month age group remain limited. Optimizing the dose, form, and combination strategy is essential for practical application in calf nutrition.