Date of Graduation

8-2025

Document Type

Thesis

Degree Name

Master of Science in Poultry Science (MS)

Degree Level

Graduate

Department

Poultry Science

Advisor/Mentor

Weimer, Shawna

Committee Member

Owens, Casey

Second Committee Member

Graham, Danielle

Keywords

Behavior; Broiler; Heat stress; Pasture; Performance; Tryptophan

Abstract

Pasture-raised broiler chickens face temperature extremes that can threaten welfare. In the study, we evaluated whether a 0.26 % tryptophan-supplemented diet (T) improved heat stress (HS) resilience in Ross-708 broilers raised on pasture. At D14, birds began the T or remained on the control diet (C) and were moved outdoors. On D20 and 41, half of each group underwent six-hour HS; the remainder stayed under thermoneutral (TN) conditions. Weekly body weight, feed intake on D20 and D41 were recorded, and fear was measured using the Tonic Immobility (TI) test. Neither diet nor HS affected body weight. However, tryptophan supplementation resulted in higher feed conversion ratios (FCR) in Week 6 and cumulatively from D14 to 42 compared to C, suggesting reduced feed efficiency. On D20 and 41, HS significantly reduced feed intake during HS, regardless of diet. At both ages, HS did not significantly affect fear behavior, though HS birds numerically remained immobile longer on D22, suggesting increased fearfulness. On D15, C birds showed shorter latency to first head movement compared to T birds, indicating heightened vigilance; these effects did not persist following HS on D22. Under HS, birds spent more time sitting, panting, and wing spreading, and less time standing and eating compared to TN. Within HS pens, T birds exhibited more simultaneous panting and wing spreading (PAWS) than controls, implying greater thermal stress. Age affected thermoregulatory behavior during HS, where younger birds on D20 were more active and utilized convective cooling (walking, wing spreading, PAWS), whereas older birds on D41 relied more on evaporative cooling behaviors, including panting and sitting behaviors. Overall, tryptophan supplementation did not positively affect growth performance or reduce fear before or after HS; instead, it may have exacerbated thermal stress.

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