PS10-11. Impact of Grazing on the Residue Structure of an Oat-vetch Cover Crop in Integrated Crop-livestock Systems.

Abstract Cover crops (CC) are essential tools for enhancing soil health and ecosystem services in agricultural rotations. Livestock integration into these systems represents a sustainable strategy that fosters synergistic effects within the agroecosystem. In the Pampa region of Argentina, integrated systems typically utilize annual cool-season forages between two cash crops to bridge the winter forage deficit while maintaining soil cover. This dual-purpose use of CC must sustain a balance between harvested and residual herbage. Indeed, cover crops residual herbage (CCRH) quantity and composition affect carbon (C) and nitrogen (N) inputs and dynamics, as well as nutrient availability for the subsequent crop. Therefore, the objective of this study was to evaluate how grazing CC affects CCRH, botanical composition, and dead material proportion within a crop rotation prior to soybean (Glycine max) sowing. The study was conducted in Azul, Argentina, across 8 paddocks (10 ha) no-till seeded with an oat-vetch mixture (75:25 ratio; Avena sativa L. and Vicia sativa L.) into wheat stubble. A completely randomized design was employed, with paddocks assigned to either grazed (G) or ungrazed control (UG) treatment (n = 4). Yearling beef heifers (183.6±14 kg BW) grazed the G paddocks under a rotational system at moderate intensity, targeting a residual sward height of 10-15 cm. The grazing period lasted 140-d (May-1 to Sep-17, 2025). Before chemical suppression, CCRH was determined by collecting two 0.16 m2 quadrat samples per paddock. Forage samples were harvested at ground level, and separated into grass, legume, and dead material fractions. Samples were oven dried at 60 °C to constant weight. Data were analyzed using a completely randomized design with the ANOVA procedure in InfoStat (v. 2020; Córdoba, Argentina). As expected, CCRH was affected by treatment (P = 0.0001; SEM=346,6) averaging 516.9 kg DM/ha for the G and 6,016.4 kg DM/ha for the UG treatment. Grass proportion was greater in G than in UG treatment (62% vs. 47%, respectively, P = 0.002; SEM=0.03) while vetch was 14% in average for both treatments (P = 0.423; SEM=0.04). On the other hand, dead material was higher (P = 0.009; SEM=0.03) in UG than in G, being 38% and 26%, respectively. Grazing reduced CCRH but also decreased the dead material fraction while maintaining the legume proportion. This shift improves residue quality and potentially modulate C and N mineralization dynamics. Finaly, while grazing CC provides high-quality livestock feed, grazing effects and the resulting CCRH quantity and quality remain the primary drivers of subsequent soil nutrient inputs.