Interacciones del estrés térmico con la etapa posparto para remodelar la composición de ácidos grasos de la leche en vacas Holstein en clima oceánico.

Elena Niceas Martínez Díez¹, Cristina Castillo Rodríguez¹, Joaquín Hernández Bermúdez¹, Rodrigo Muiño Otero¹

(1)-1Department of Animal Pathology, Campus Terra-IBADER, University of Santiago de Compostela, 27002 Lugo, Spain.

Climate change is increasing the frequency and intensity of heat stress in dairy systems, with potential consequences for animal physiology and milk quality [1,2]. This study investigated the independent and interactive effects of season (winter vs summer) and postpartum stage (1 week vs 1 month postpartum) on the bovine milk fatty acid (FA) profile in an intensive Holstein herd in NW Spain monitored in 2023 (227 lactating cows; mean yield 33.7 kg/day; mean milk fat 3.80% and protein 3.36%). Multiparous cows were sampled across two periods: winter (January-March) and summer (July-September), yielding 40 milk samples (n=20/season; within each season n = 10 at day 7 and n=10 at day 30 postpartum). Thermal exposure was quantified using the temperature-humidity index (THI). Summer conditions included intermittent mild-to-moderate heat load (THI 72-78 for 49 days and THI 79-88 for 4 days), whereas winter THI remained consistently below 68. Milk FA composition (25 FAs; mg/100 mL) was quantified by gas chromatography 6flame ionization detection (GC 6FID) and analyzed using two-way ANOVA. 

Seasonal comparisons indicated that summer was associated with higher de novo-related (short/medium-chain) FAs, including C10:0 (102.26 vs 128.25 mg/100 mL, p < 0.05) and C14:1(n-5) (28.05 vs 38.86, p < 0.01), and higher C15:0 (25.75 vs 42.06, p < 0.01), while cis-11 C18:1 decreased (7.40 vs 4.08; p < 0.01). Conjugated linoleic acids (CLAs) were higher in summer (24.02 vs 32.16, p < 0.05), whereas C18:2(n-6) 9t,12t decreased (2.36 vs 1.69, p < 0.05). Total FA, saturated FA (SFA), monounsaturated FA (MUFA), polyunsaturated FA (PUFA), omega-3, and omega-6 did not differ by season alone (p > 0.05). 

Across postpartum stage, multiple de novo FAs increased from day 7 to day 30 (C6:0: 64.09 to 82.29, p < 0.05; C8:0: 51.05 to 65.35, p < 0.05; C10:0: 99.95 to 130.57, p < 0.05; C12:0: 149.93 to 190.04, p < 0.05), and α-linolenic acid (ALA; C18:3 n-3) increased markedly (10.10 vs 15.57 mg/100 mL, p < 0.01). Total FA content remained unchanged between time points (2753.93 vs 2919.80 mg/100 mL, p=0.5337). 

Critically, season-postpartum interactions were significant for all short/medium-chain FAs (p < 0.01) and for lipidic indicators including total FA (p < 0.01), SFA (p < 0.01), MUFA (p < 0.01), PUFA (p < 0.01), omega-3 (p < 0.01), omega-6 (p < 0.01), and CLAs (p < 0.05). The summer-1 month group generally showed the highest concentrations for broader lipid fractions (total FA: 3559.46 mg/100 mL (summer-1 month) vs 3141.09 (winter- 1 week), 2280.13 (winter-1 month), and 2366.77 (summer-1 week)). 

Overall, these results demonstrate that milk lipid composition is shaped not only by season or postpartum stage independently, but by their non-additive interaction, indicating stage-specific metabolic sensitivity to intermittent summer heat load, consistent with evidence that heat stress can modify milk lipid composition [3] and that milk FA profiles are influenced by lactational stage and metabolic-energy status [4]. These supports incorporating lactation stage into heat-mitigation and milk-quality management strategies in temperate, oceanic dairy systems.

References:

1. IPCC. AR6 WGI: Summary for Policymakers 6 Headline statements (2021).

2. West JW. Effects of heat-stress on production in dairy cattle. J Dairy Sci. 2003;86(6):2131 2144. doi:10.3168/jds.S0022-0302(03)73803-X.

3. Liu Z, Ezernieks V, Wang J, et al. Heat Stress in Dairy Cattle Alters Lipid Composition of Milk. Sci Rep. 2017;7(1):961. doi:10.1038/s41598-017-01120-9.

4. Gross J, van Dorland HA, Bruckmaier RM, Schwarz FJ. Milk fatty acid profile related to energy balance in dairy cows. J Dairy Res. 2011;78(4):479 488. doi:10.1017/S0022029911000550 .