Effects of Management-intensive Grazing in relation to soil health and forage production in an irrigated perennial pasture system
Date
2019
Authors
Shawver, Casey, author
Brummer, Joe, advisor
Ippolito, James, committee member
Ahola, Jason, committee member
Rhoades, Ryan, committee member
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Abstract
Interest in Management-intensive Grazing (MiG) on irrigated, perennial, cool-season pasture has increased steadily in Colorado due to pressure to reduce public lands grazing, overall declining space available for pasture, and declining commodity prices. However, there are still many unknowns about how forage production, forage quality, plant diversity, and soil quality are affected by this management over the long-term. To investigate potential effects on these variables, a study was undertaken on a full-scale, 82 ha, center pivot-irrigated, perennial pasture that had recently been planted with four different species mixtures at the Colorado State University Agriculture, Research, Development and Education Center located 13 km northeast of Fort Collins, CO. During the first one and a half years of production, approximately 230 animal units (AUs) consisting of cow-calf pairs, yearling heifers, and yearling steers were grazed using management-intensive practices. Paddock size was dynamic and determined based on number of animals and forage availability with animals generally moved daily. Forage yield and quality, botanical composition and cover, animal unit days (AUDs), and soil quality parameters were measured. In both 2017 and 2018, species mixtures that were either hayed or grazed first in the rotation induced vegetative regrowth resulting in the greatest quality. Quality remained high enough through the grazing season to meet cattle nutrient requirements regardless of the species mixture. There was no significant difference between seasonal mean yields in 2017. The simple grass-legume mixture resulted in the greatest seasonal mean yield in 2018 with 3916 kg/ha-1 but the complex grass mixture resulted in greatest AUDs (7493.93 AUDs). Bare patches developed around patches of non-soft leaf tall fescue (Festuca arundinacea Shreb.) in this mix, which made it evident that cattle were selecting against this type of tall fescue. Botanical composition assessments did not align with the initial seeding rates in some cases, which suggests soil conditions, plant phenology, and competitive advantages contributed to differences between seeded and established composition of the mixtures. Ground cover showed that litter was similar among species mixtures that were planted and grazed earlier compared to the species mixture that was not. Mean plant cover (23.75%) was similar among all mixtures. Soil analysis included 11 biological, chemical, nutrient, and physical parameters that were aggregated into the Soil Management Assessment Framework (SMAF) program resulting in soil quality index (SQI) values. Positive soil quality effects were observed in the biological SQI with increases in microbial and enzymatic activities. Soil organic carbon remained relatively unchanged. Negative impacts occurred to the soil physical SQI, driven primarily by increasing bulk density. The nutrient SQI value declined due to the observed reduction in extractable soil P due to low concentrations occurring in multiple soil samples; similar observations occurred in all three extractions. In a separate soil physical property study, penetrometer resistance was measured due to grazing that occurred during wet conditions. Soil resistance was greater for paddocks grazed under wet conditions but was not yet at levels that would affect root growth and plant productivity. The overall results of this study suggest that this irrigated, MiG system has the potential for success in regard to forage production and quality and its ability to support grazing, but will need further research to determine the long-term impacts of grazing on soil quality and species composition shifts.
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Subject
grazing
MiG
soil quality
irrigated grazing
beef systems
pivot-irrigation