Soil and vegetation changes across a succulent Karoo grazing gradient.

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Soil and vegetation changes across a Succulent Karoo grazing gradient

This study describes soil and vegetation changes radiating out from a central watering point on a livestock farm in the Succulent Ceres Karoo (Tanqua). The aim was to identify possible relationships between long-term grazing pressure, and soil and vegetation properties. There are indications that continuous high-intensity defoliation of Succulent Karoo vegetation leads to a decline in species richness and perennial plant cover with a consequent loss of fertile topsoil. Over-grazed areas close to the watering point had shallow soils with a greater potential for crusting and therefore poorer water capacity. Mainly short-lived succulents (Mesembryanthemaceae) were recorded here, while under-utilised veld far from the watering point was identified by plant groupings dominated by Antimima hantamensis and Ruschia spinosa. Malephora crassa and Rhinephyllum macradenium are two key species which could be useful in veld condition assessment since they showed strong relationships with distance from the watering point. The results also have implications for restoration of degraded veld. Soils from degraded areas have been altered almost permanently and simply resting from livestock grazing may not achieve the desired vegetation recovery. Keywords: biodiversity; indicator species; piosphere; restoration; veld condition African Journal of Range & Forage Science 2003(1), 20: 11–1

Screening for diets that reduce urinary nitrogen excretion and methane emissions while maintaining or increasing production by dairy cows

Farmers face complex decisions at the time to feed animals, trying to achieve production goals while contemplating social and environmental constraints. Our purpose was to facilitate such decision making for pastoral dairy farmers, aiming to reduce urinary N (UN) and methane emissions (CH₄ ), while maintaining or increasing milk production (MP). There is a number of feeds the farmers can choose from and combine. We used 50 feeds (forages and grains) combined systematically in different proportions producing 11,526 binary diets. Diets were screened, using an a posteriori approach and a Pareto front (PF) analysis of model (Molly) outputs. The objective was to identify combinations with the best possible compromise (i.e. frontier) between UN, CH₄ , and MP. Using high MP and low UN as objective functions, PF included 10, 14, 12 and 50 diets, for non-lactating, early-, mid- and late-lactation periods, with cereals and beets featuring strongly. Using the same objective functions, but including ryegrass as dietary base PF included 2, 4, 8 and 4 diets for those periods. Therefore, from a wide range of diets, farmers could choose from few feeds combined into binary diets to reduce UN while maintaining or increasing MP. If the intention is maintaining pasture-based systems, there are fewer suitable options. Reducing UN will simply require dilution of N supplied by pasture by supplementing low N conserved forages. The results also evidence the risk of pollution swapping, reaching the frontier means arriving at a point where trade-off decisions need to be made. Any further reduction in UN implies an increment in CH₄, or reduction in CH₄ emissions increases UN. There is no perfect diet to optimize all objectives simultaneously; but if the current diet is not in the frontier some options can offset pollution swapping. The choice is with the farmers and conditioned by their context

Farm-scale carbon and nitrogen fluxes in pastoral dairy production systems using different nitrogen fertilizer regimes

The nitrogen (N) fertilizer application rate (kg ha−1 year−1) in pastoral dairy systems affects the flow of N through the soil, plant and animal pools of the system. With better understanding of the magnitude of these pools and their fluxes, dairy systems could be managed to improve N use efficiency, therefore reducing losses to the environment. A study with three levels of N fertilizer, 0 (N0), 150 (N150) and 300 (N300) kg N ha−1 year−1, was conducted in the Canterbury region of New Zealand from 1 June 2017 till 31 May 2018. Farm measurements, e.g. pasture and milk production, were used to calibrate three different farm-scale models, DairyNZ’s Whole Farm Model, DairyMod, and Overseer®. The models were used to extrapolate periodic farm measurements to predictions of carbon (C) and N pools and fluxes on an annual basis. Pasture and milk production per hectare increased from N0 to N300 by 70 and 58%, respectively. There was a concomitant increase in farm-gate N surplus (input–output) of 43%, resulting in predicted increases in N leaching and greenhouse gas emissions of 72 and 67%, respectively. By increasing N fertilizer from 0 to 300 kg N ha−1 year−1, 53% more feed N flowed through the dairy herd with surplus N deposited as urinary N increasing by 49%. Plant uptake and soil immobilization increased by 58 and 343%, respectively, but not enough to avoid substantial increases in leaching and emission losses. Carbon flux through the soil system increased through increased litter and faecal deposition, but with very little C sequestration because of accelerated microbial respiration rates

Dynamics of forage ingestion, oral processing and digesta outflow from the rumen: A development in a mechanistic model of a grazing ruminant, MINDY

Detailed representation of ingesta inflow to and digesta outflow from the rumen is critical for improving the modelling of rumen function and herbage intake of grazing ruminants. The objective of the current work was to extend a mechanistic model of a grazing ruminant, MINDY, to simulate the dynamic links between ingestive and digestive processes as affected by forage and sward features (e.g. sward structure, herbage chemical composition) as well as the internal state of the animal. The work integrates existing aspects of forage ingestion, oral physiology and rumen digestion that influence ingesta characteristics and digesta outflows from the rumen, respectively. The paper describes the structure and function of the new development, assessing the new model in terms of dynamic changes of oral processing of ingesta and rumen dilution rate under different grazing contexts. MINDY reproduces characteristics of ingesta inflow to and digesta outflow from the rumen of grazing ruminants, achieving temporal patterns of occurrence within and between meals, similar to those for grazing animals reported in the literature. The model realistically simulates changes in particle size distribution of the ingestive bolus, bolus weight and rumen dilution rate in response to contrasting grazing management regimes. The new concepts encoded in MINDY capture the underlying biological mechanisms that drive the dynamic link between ingestion and digestion patterns. This development advances in the understanding and modelling of grazing and digestive behaviour patterns of free-ranging ruminants

A model of diurnal grazing patterns and herbage intake of a dairy cow, MINDY: Model description

Estimates of herbage intake and parallel measurements of ingestive and digestive behaviors of grazing ruminants pose considerable experimental and technical difficulties, owing to dynamic interactions between the plant, the rumen and the animal. As a consequence, advances in the area have been slow and costly. Model simulations that capture such interactions are critical for research and management decisions involving the grazing process. This work describes MINDY, a mathematical, mechanistic and dynamic simulation model of the diurnal grazing pattern of a dairy cow. MINDY is based on a cluster of three models: (1) Molly (Baldwin, 1995), a model of ruminant digestion and metabolism; (2) a model representing feed consumption as a function of diurnal fluctuations in the internal state of the animal; and (3) a sward structure, herbage quality and grazing behavior model. The objective of the work was to describe the diurnal grazing pattern, including ingestive actions and rumination behaviors, herbage intake, and nutrient supply to the animal in response to the animal's internal state and grazing environment. The model was coded in ACSL and simulations were conducted using ACSLXtreme. In addition to dietary nutrient composition required by Molly, MINDY requires sward surface height and mass, and grazing area offered to the cow. Key sub-model parameters were identified by sensitivity analyses and parameterized using two data sets from mid-lactation Friesian and late lactation Holstein dairy cows breeds under set stock conditions. The parameterized model predicted realistic estimates of ingestive behavior for different cow genotypes managed under set stocking and rotational grazing. It also predicted a realistic number of steps taken while eating and searching and sward defoliation dynamics as well as diurnal fluctuations of digestion and metabolism. Additional evaluations are required and further data may be needed to better define some parameters, but the model offers promise as an heuristic tool for feed intake and grazing process research and as an informative tool for grazing and cow management decisions

Diurnal patterns of urination and drinking by grazing ruminants: A development in a mechanistic model of a grazing ruminant, MINDY

Measurement of water consumption and urinary nitrogen (UN) excretion of individual grazing ruminants is difficult, time-consuming and expensive. Therefore, prediction and modelling are critical for research to improve N and water use efficiency. The objective of the current work was to use a mechanistic model of a grazing ruminant, MINDY, to represent drinking and urination diurnal patterns, and the resulting pattern of UN excretion. This work is primarily an integration of existing knowledge of basic urination physiology and water dynamics in ruminants. MINDY reproduces observed patterns of urination achieving the correct temporal occurrence, relative volumes and nitrogen (N) concentration of individual urination events for a grazing dairy cow, comparable with those reported in the literature. The model simulates daily water imbibed and UN realistically, as well as ingestion rates for herbages with different protein content and contrasting grazing managements. Results of a cross-validation indicate that the root mean square prediction error and mean absolute error as % of the observed mean, respectively, were 26 and 23% for daily water imbibed, 26 and 27% for urination volume, and 25 and 19% for the frequency of urination. Although further parameterization and validation are needed, for a new development in an exploratory model like MINDY, these numbers are encouraging and reflect that the concepts encoded capture many of the underlying biological mechanisms that drive the diurnal pattern and daily UN excretion, as well as thirst, acceptable

Development of an improved representation of rumen digesta outflow in a mechanistic and dynamic model of a dairy cow, Molly

© 2015 Elsevier B.V. Accurate predictions of outflow of digesta from the rumen are critical for improving modeling of feed intake, rumen function and fermentation patterns of ruminants. The main objective of this work was to develop an improved representation of rumen digesta outflow in the Molly model. The work is primarily an integration of existing knowledge of rumen digestion responsible for variations in digesta outflows in ruminants, and describes the structure and function of the new development, assessing the new model in terms of ruminal outflow and fermentation in response to different feeding scenarios. The present development includes three changes to the model: (1) a medium-size particle pool was added to the rumen which was assumed to ferment and pass from the rumen; (2) particulate passage was made a function of particle size, particle concentrations in the rumen, and liquid passage rate; and (3) fermentation rate was made a function of particle surface area in the medium and small particle pools. Although prediction accuracy of digestive functions was not substantially improved by the change in model structure, the model now reproduces observed patterns of variation in rumen function as affected by the food intake and dietary particle size compared with those reported in the literature, which was not previously the case. It also reproduces more realistic trends in rumen fermentation patterns, digestion and methane yields. The concepts embedded in the new development capture underlying biological mechanisms driving the variation in digesta outflows from the rumen that were not captured before