Predicting spatial-temporal patterns of diet quality and large herbivore performance using satellite time series

Authors: Kearney, Sean; Porensky, Lauren; Augustine, David; Derner, Justin; Gao, Feng

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/26/2021
Publication Date: 2/28/2022
Citation: Kearney, S.P., Porensky, L.M., Augustine, D.J., Derner, J.D., Gao, F.N. 2022. Predicting spatial-temporal patterns of diet quality and large herbivore performance using satellite time series. Ecological Applications. 32. Article e2503. https://doi.org/10.1002/eap.2503.
DOI: https://doi.org/10.1002/eap.2503

Interpretive Summary: In order to implement adaptive rangeland management efforts, rangeland managers need to know, among other things, how forage conditions are changing across space and time within a single grazing season, and how these changes affect livestock season-long weight gains, as this impacts ranch profitability. Information on changes in forage conditions and livestock performance within a grazing season is not often available for rangelands because forage conditions are strongly influenced by precipitation and temperature in ways that are not consistent from year to year. This study used 20 years of satellite data and ground observations to estimate diet quality and forage production at a daily time step, and then used this information to predict weight gains for yearling steers in northeastern Colorado, again at a daily time step. The study was able to produce reasonably accurate estimations of diet quality for individual pastures throughout the grazing season. Importantly, diet quality strongly affected animal performance, even when forage production was high. Diet quality was highest when forage was actively growing - either early in the season or during 're-greening' events. These findings can help rangeland managers better understand how adaptive management strategies such as rotational grazing may affect livestock performance.

Technical Abstract: 1. Adaptive management of large herbivores requires an understanding of how spatial-temporal fluctuations in forage biomass and quality influence animal performance. Advances in remote sensing have yielded information about the spatial-temporal dynamics of forage biomass, which in turn have informed rangeland management decisions such as stocking rates and rotations for free-ranging cattle. However, less is known about the spatial-temporal patterns of diet quality and their influence on large herbivore performance. This is due to infrequent concurrent ground observations of forage conditions with performance (e.g., weight gain), and previously limited satellite data at fine spatial and temporal scales. 2. We combined multi-temporal field observations of diet quality (weekly) and performance (i.e., weight gain, monthly) with satellite-derived phenological metrics (pseudo-daily) to model daily weight gains of free-ranging yearling cattle in shortgrass steppe. We used this model to predict grazing season (mid-May to October) weight gains – a key management indicator – across 40 different paddocks grazed over a 10-year period (n=138). 3. We found strong relationships between diet quality and the satellite-derived phenology metrics, especially related to the timing and rate of green-up and senescence. 4. Satellite-derived diet quality metrics were strong predictors of monthly weight gains (R2 = 0.68) across a wide range of aboveground net herbaceous production. 5. Predicted season-long average daily weight gains had a root mean squared error of 0.11 kg day-1 and an R2 of 0.59. The model performed better temporally (across repeated observations in the same paddock) than spatially (across all paddocks within a given year), highlighting the need for accurate vegetation maps and robust field data collection across both space and time. 6. Synthesis and applications. Free-ranging cattle performance in rangelands is strongly affected by forage quality, which is related to the timing of vegetation green-up and senescence. Senescing vegetation suppressed weight gains, even if adequate forage was available. Remotely-sensed changes in forage quality and quantity together during the growing season explain a large proportion of the variation in weight gains for this large herbivore.