Grassland Ecology and Ecosystem Management for Sustainable Livestock Performance

1. Introduction

Grassland ecosystems differ in plant and animal species composition and appearance depending on the location and climate. Grasslands cover 26 percent of the earth’s ice-free land area [1] and account for more carbon sequestration than forest land [2]. Plants within grasslands are primary producers, fixing carbon from the atmosphere to produce forage above ground for feeding livestock and wildlife and below ground for feeding soil organisms and maintaining soil health. Forage productivity is a function of the soil water holding capacity, drainage, soil fertility, soil health, and weather [3,4,5]. Secondarily, animal productivity from grasslands results from a complex system of interactions between grazing management and its impacts on the forage quality and availability [6,7]. Well-managed grassland systems can be highly sustainable due to nutrient cycling within the system, reducing the need for continuous commercial fertility inputs [8,9]. Properly managed grasslands also protect the soil from water and wind erosion and can decrease the overall erodibility [10].

Ruminant livestock has long been accused of being a significant contributor to global warming due to the production of methane through enteric and manure sources. However, methane produced by ruminants on grasslands cannot be compared to methane from industry and transportation, since ruminant methane is not a new greenhouse gas (GHG) entering the atmosphere but is instead a part of the terrestrial biosphere of the carbon cycle. Grasslands represent 23% of the total carbon storage on a global scale [11], with grazing ruminants playing an important role in both annual C flux and providing meat and milk to the world’s population. With increasing public and environmental pressures, graziers have focused on sustainability efforts in ruminant production through a number of interdisciplinary methods. In the United States, the total cattle herd has decreased from 131.8 million head in 1975 [12] to 92.1 million head in 2022 [13]. This represents a 30% reduction in the total cattle inventory, yet during that same time, the total production of beef increased by 15.6%, from 10.9 to 12.9 billion kg [14]. All ruminant and cecal digesting animals produce methane, but cattle and buffalo convert less than 10 percent of their gross energy intake into methane on a daily basis [15]. Other major carbon reservoirs and fluxes include the ocean and that which is found in rock [11]. Grassland soils also uptake and release methane depending on management and environmental conditions [16]. Nonetheless, pasture-based livestock production is the dominant means of converting vegetation produced on non-arable land into high-value human dietary products.

2. Overview of the Special Issue

This Special Issue of Agronomy contains 11 research articles pertaining to grassland management and management impacts on forage quality, animal productivity, and ecosystem health. These original research papers can be grouped into five categories:

(1)

Forage quality and animal performance [17,18,19,20]

(2)

Animal grazing behavior [21]

(3)

Fatty acid (FA) profiles of various forages [22,23]

(4)

Ecosystem health for production animals [24,25,26]

(5)

Technology for improving management [27]

2.1. Forage Quality and Animal Performance

Tracy et al. [17] found that adding nutritionally high-quality, summer-productive forages, such as alfalfa (Medicago sativa) or sericea lespedeza (Lespedeza cuneata (Dum. Cours.) G. Don), to swards containing tall fescue (Schedonorus arundinaceous (Schreb.)) infected with the endophytic fungus Epichloe coenophiala did not improve animal weight gain. Steers avoided sericea plants, resulting in the sericea cover increasing to over 82% of the sward. Steers selectively grazed alfalfa plants, resulting in the alfalfa cover decreasing to nearly zero by year 3. Neither summer-productive legume species improved animal performance in this tall-fescue-based grazing system.

Lauriault et al. [18] compared beef stocker growth on dormant, low-protein, perennial native grass pastures supplemented with protein to the high protein, cool-season annual forage triticale (Triticosecale Wittm. ex A. Camus (Secale_Triticum)) for three years. The triticale forage mass varied over the three years due to precipitation and the triticale planting date, which influenced the length of the grazing period. Triticale provided late winter gains, about twice that of cattle grazing dormant, protein-supplemented native grass.

Núñez et al. [19], using data from a long-term fertilization experiment, identified the main factors influencing the forage crude protein content of a basaltic native grassland in northern Uruguay. The authors found that fertilization and increased soil water availability were the main factors increasing forage crude protein. This information quantifies the main factors that drive the crude protein content of native grasslands, which can be used in prediction models for forage protein content in order to improve the grazing livestock nutrition of Campos native grasslands.

Holík et al. [20] studied the effects of 60 years of mineral and manure fertilization on lucerne leaf and stem starch and water-soluble carbohydrate (WSC) accumulation. Treatments were two levels of mineral N, P₂O₅ and K₂O application (0:0:0 and 91:71:175), each with or without manure. Intensive mineral fertilization reduced the stem and leaf WSC compared to the unfertilized control or manure alone. These changes were associated with a dilution effect caused by an increase in stem length with these treatments. Manure improved the leaf and forage WSC despite the associated increase in stem length and leaf weight ratio. This was probably due to the improved soil environment along with an increased presence of arbuscular mycorrhizal fungi.

2.2. Animal Grazing Behavior

Soder et al. [21] studied the effect of the pasture sward structure (herbage height, mass and vertical distribution of leaf and stem fraction) and nutritive value of vegetative meadow fescue (Schedonorus pratensis (Huds.) P. Beauv.), orchardgrass (Dactylis glomerata L.), quackgrass (Elymus repens (L.) Gould), and reed canary grass (Phalaris arundinacea L.) on the grazing behavior of dairy heifers. Vegetative-stage grasses were rotationally grazed during 5-day periods in the spring, summer, and fall. The herbage dry matter (DM) allowance was twice the expected daily intake (11 kg DM/animal/day). The sward characteristics were measured before grazing. The grazing behavior of the heifers was quantified using automatic jaw movement recorders. In this study, grass species and season had little effect on grazing behavior. The bite rate was negatively correlated with the herbage mass while the number of bites was positively correlated with the sward height and herbage mass when the herbage availability was not limited.

2.3. Fatty Acid Profiles of Various Forages

Whetsell et al. [22] studied the omega-6 and omega-3 fatty acid (FA) content in perennial ryegrass, tall fescue, orchardgrass, and Kentucky bluegrass samples collected on four harvest dates in the eastern region of the Appalachian Mountains. There was more variation in linolenic acid (C18:3) than linoleic acid (C18:2) concentrations across forage species and seasons. Perennial ryegrass had higher levels of linolenic acid compared to the other grasses. The linoleic acid concentrations changed less across seasons and were generally lower in tall fescue than in other species, which contained similar levels. There was a species by date interaction on FA concentrations. Kentucky bluegrass had a peak concentration of linoleic acid in August. The concentration in orchardgrass fluctuated across seasons, while concentrations in tall fescue and perennial ryegrass decreased as the season advanced. Managing plant species diversity in pastures to stabilize the content of omega-3 FA across seasons appears to be a valuable tool for managers to manipulate FA characteristics of meat and milk products from pasture-based systems.

Whetsell and Rayburn [23] studied the FA content in pasture grasses, legumes, and non-leguminous forbs in northeast West Virginia. Forage samples collected from rotationally stocked pastures were analyzed for the crude protein (CP), linoleic acid (C18:2), linolenic acid (C18:3), and total FA content. Species within botanical classes varied in their FA content. Forbs had the highest linoleic acid (C18:2) content followed by legumes and grasses. Grasses and forbs had the highest linolenic acid (C18:3) content. Forbs had the highest total FA content. The field data were combined with FA data from the research literature. The forage crude protein (CP) content is the forage quality measure most highly related to the forage FA concentration. After accounting for CP, the summer months caused a decrease while forbs caused an increase in the linolenic acid (C18:3) content. Vegetative growth and leafiness are the major determinants of the FA content in fresh forage. Grazing management to benefit vegetative growth and the presence of desirable forbs in tune with seasonal changes are valuable tools to increase desirable FA profiles in milk and meat products that may be of benefit to human health.

2.4. Ecosystem Health

Poudel et al. [24] studied hair cortisol as a non-invasive measure of chronic stress. A two-year study was carried out to compare behavioral and physiological responses of sheep (Ovis aries) grazing black walnut (Juglans nigra) silvopasture (BSP), honey locust (Gleditsia triacanthos) silvopasture (HSP), or open pastures (OP) treatments. The ewe sheep average daily gain was greater in HSP compared with OP but did not differ with BSP. Ewes on OP had higher hair cortisol concentrations than ewes on BSP or HSP treatments. Ewes on OP had higher intravaginal temperatures during the afternoon than ewes managed in silvopasture. Ewes on OP spent 500–700% more time standing and 125–150% less time lying down compared with ewes on silvopasture treatments. Hair cortisol measures are an effective, non-invasive technique for determining long-term chronic stress in grazing animals.

Keyser et al. [25] presented the status of grassland conservation and proposed solutions through a working lands conservation approach. Grassland management entails maintaining appropriate disturbances (management by grazing and fire) with an increased reliance on native grasses and forbs to improve the plant diversity within pastures. They provide examples of opportunities to achieve these goals, offer suggestions for agricultural and conservation policy, and provide a framework for evaluating tradeoffs that are inevitably required when pursuing a multi-purpose grassland management framework.

Reed and Morrissey [26] reviewed the scientific literature examining the current state of research regarding plant diversity and its potential benefits for soil health in pasture. Increasing pasture species diversity is a practice that is increasingly used by producers. Research shows that increasing plant diversity in pastures can lead to improved forage productivity, resilience, and soil health. It remains to be determined how these benefits improve grazed production given the limited experimentation in grazing systems. There is a disconnect between ecological and agronomic research regarding what qualifies as a “diverse” grassland or pasture.

2.5. Technology for Improving Management

Serrano et al. [27] studied the dynamic nature of a grassland savanna. The authors monitored soil and pastures in Southern Portugal for extensive grazing of cattle; they measured the soil apparent electrical conductivity (ECa), developed algorithms for defining homogeneous management zones (HMZ), used satellite imagery time series to characterize seasonal pasture quality along with soil and pasture sampling, and identified indicator botanical species. This provided a holistic evaluation of the soil–pasture–tree–animal ecosystem. This approach represents an important decision-support system for farm managers for smart sampling, differential application of fertilizer amendments or seeds, choosing the best spacing and density of trees, promoting dynamic grazing, and identifying the animal supplementation needs during the critical periods of the year.

3. Conclusions

Globally, grasslands are being degraded, more so than any other biome. In the eastern U.S., more than 20 million ha of grasslands and their native biota, wildlife, and agricultural production systems are under stress. Improving our understanding of the diverse relationships that take place at the soil–plant–animal interface can help us to become better stewards and managers of grasslands. Increasing the use of technology for monitoring ecosystems provides improved knowledge of the spatial and temporal ecology and leads to more profitable management strategies.