Drinking water intake of grazing steers: the role of environmental factors controlling canopy wetness

A comprehensive meta-analysis of the impacts of intensified drought and elevated CO2 on forage growth

Forage crops are used worldwide as key feed sources for dairy systems. However, their productivity and quality are limited due to intensified drought events, elevated carbon dioxide (CO₂), and their interaction with climate change, with consequences for the security of animal husbandry and the agricultural economy. Although studies have quantified the impacts of such stresses on forage growth, these impacts have been less systematically investigated in a global context due to differences among various forage groups, regional microclimates, and environmental factors. Herein we employed nine forage growth-related variables involving three perspectives, i.e., photosynthetic parameters, production, and quality, from research articles published between 1990 and 2021 via a meta-analysis. A linear mixed-effect model was then used to explore the quantitative relationship between these factors in a restricted dataset. Decreasing trends in all four photosynthetic parameters were detected across different eco-geographical regions with increasing drought stress. The maximum decrease in DMY occurred in the Mediterranean, with 52.8% under drought conditions. Globally, eCO₂ significantly increased photosynthetic rate (P_n) and instantaneous water use efficiency (WUE_i) by 40.8% and 62.1%, respectively, which also had positive effects on forage dry matter yield (DMY) (+25.1%), especially for forage in Northern Europe. However, this stress would significantly decrease forage quality by decreasing crude protein (CP) (-19.7%) and nitrogen content (N content) (-13.5%). These negative impacts would be aggravated under the co-occurrence of drought and eCO₂, including a significant increase in WUE_i (+111.1%) and a decrease in DMY (-12.3%). Gramineae showed a more sensitive response to drought stress in photosynthetic parameters and DMY than Leguminosae, but the latter exhibited a better response in photosynthetic parameters and production under eCO₂. Our analysis provides a consensus concerning how the growth parameters of forage have changed under environmental stresses.

Effects of Wilting and Dew on the Water Isotope Composition of Detached Grass in Temperate Grassland

Understanding the water isotopes in feed products derived from grass is fundamental for tracing domestic animal products. Grass silage water was reported to have fewer heavy isotopes than fresh grass, but it is still unknown whether dew formation (either dewfall or dewrise), exchange with soil water, or other processes override the expected enrichment of heavy isotopes due to wilting. The isotopic variations of water (δ²H, δ¹⁸O) in fresh grass and cut grass during wilting on soil and on plastic were compared in this study. Drying enriched heavier isotopes, but this was overridden by three processes that finally caused low δ²H and δ¹⁸O values: (i) the adsorption of humidity from the surroundings, (ii) the exchange with humidity, and (iii) the depletion of heavy water isotopes close to organic surfaces, called the surface effect, which was the most dominant effect at the end of drying when the water content became low.

Ambient Conditions and Feeding Strategy Influence δ18O of Milk Water in Cows (Bos taurus)

There are increasing concerns by consumers regarding agricultural product traceability and authenticity. Oxygen isotope composition (δ¹⁸O) has been used in this context based on the relationship between δ¹⁸O of animal products and annual precipitation. However, in dairy products this relationship is affected by the seasonality of δ¹⁸O in milk water which in turn depends on the feeding system used. We measured 608 milk samples from 28 farms with various feeding strategies in southern Germany throughout the year, investigating the influences of ambient conditions, drinking water source, and feeding strategies on seasonal variation of δ¹⁸O in milk water (δ_milk). The mechanistic Munich-Kohn model reflecting these influences predicted the seasonal and farm-specific variation of δ_milk well. The relationship between δ¹⁸O of precipitation and δ_milk varied in different feeding strategies. The interplay of ambient conditions and feeding strategy on δ_milk should thus be carefully considered when identifying the origin of milk.

Model explanation of the seasonal variation of δ18O in cow (Bos taurus) hair under temperate conditions

Oxygen isotopes (δ18O) in animal and human tissues are expected to be good recorders of geographical origin and migration histories. However, seasonal variation of δ18O may diminish the origin information in the tissues. Here the seasonality of δ18O in tail hair was investigated in a domestic suckler cow (Bos taurus) that underwent different ambient conditions, physiological states, keeping and feeding during five years. A detailed mechanistic model was built to explain this variation. The measured δ18O in hair significantly related (p < 0.05) to the δ18O in meteoric water in a regression analysis. Modelling suggested that this relation was only partly derived from the direct influence of feed moisture. Ambient conditions (temperature, moisture) also affected the animal itself (drinking water demand, transcutaneous vapor etc.). The clear temporal variation thus resulted from complex interactions with multiple influences. The twofold influence of ambient conditions via the feed and via the animal itself is advantageous for tracing the geographic origin because δ18O is then less influenced by variations in moisture uptake; however, it is unfavorable for indicating the production system, e.g. to distinguish between milk produced from fresh grass or from silage. The model is versatile but needs testing under a wider range of conditions.