Supplementation of rumen-protected choline during warm summer days reduces body surface temperature and improves dry matter intake of Holstein dairy cows.

Elevated ambient temperature negatively affects dairy cows' productivity. The present study investigated the efficacy of rumen-protected choline (RPC) supplementation in alleviating the adverse effects of heat load and improving the productive performance in multiparous Holstein dairy cows at elevated ambient temperatures. Two multiparous cow groups, each enrolled since preceding calving, underwent a 2-phase experimental design consisting of a 1-wk pre-experimental period (wk -1) and a 4-wk experimental period (wk 1-4) in the same free-ranging barn. At the start of the pre-experimental period, cow groups were randomly assigned to a control group (CON, n = 9, 77 ± 26 DIM) receiving a basal diet throughout the study, whereas the RPC group (RPC, n = 9, 77 ± 8 DIM) received the same basal diet during the pre-experimental phase and was supplemented with RPC (60 g/cow per d) during the experimental phase. During the study in July 2025, the weekly mean ambient temperature ranged from 18.3°C ± 1.2°C to 20.9°C ± 1.8°C, with corresponding temperature-humidity index (THI) values between 64.1 ± 1.7 and 67.9 ± 2.6. Respiration rate was measured during the pre-experimental period (wk -1, d -6). Rectal temperature was determined using a digital thermometer and surface temperatures (muzzle, ear, eye, and neck) by aFLIR T620 infrared camera, during both the pre-experimental (wk -1, d -6) and experimental (wk 3, d 20) phases. Additionally, DMI, water intake (WI), milk yield (MY), ECM, BW, and milk composition were recorded for each cow individually in both periods. No significant differences were detected between the 2 groups during the pre-experimental phase, except for DMI, WI, BCS, and milk lactose percentage; however, pre-experimental differences were accounted for in the model. During the experimental period, RPC supplementation reduced eye temperature (RPC 34.7°C ± 0.1°C vs. CON 35.1°C ± 0.1°C) and tended to reduce ear (33.6°C ± 0.2°C vs. 34.4°C ± 0.4°C), muzzle (33.0°C ± 0.3°C vs. 33.6°C ± 0.2°C), and neck (34.3°C ± 0.2°C vs. 34.8 ± 0.2°C) temperatures. Moreover, RPC supplementation increased DMI (22.5 ± 0.47 vs. 18.6 ± 0.47 kg/d), WI (95.3 ± 1.05 vs. 71.6 ± 0.72 L/d), and BW (676 ± 3.53 vs. 657 ± 2.38 kg) relative to the CON group. Furthermore, RPC supplementation significantly increased milk fat yield (1.24 ± 0.04 vs. 1.16 ± 0.04 kg/d). Overall, the findings show that RPC supplementation may serve as an effective nutritional intervention to alleviate the negative effects of high ambient temperatures and to improve productivity in dairy cows.