2. Forage Grass Research—A Historical Overview
The research with forage plants and pastures in countries with developed animal husbandry systems started in the early 1900s. Among the scientific publications that came out at that time, some became important contributions and references for all the progress and achievements obtained by grassland scientists, even at the present time. Graber , cited by Volenec et al. , was one of the first to report that the concentration of total non-structural carbohydrates (TNC) in roots decreased soon after defoliation during the regrowth phase of alfalfa (Medicago sativa L.). Watson , cited by Black , demonstrated that a measure of the size of the plant photosynthetic apparatus was important in making comparisons of crop yield and productivity, and proposed the concept of leaf area index (LAI). Brougham [10,11,12,13,14,15,16] demonstrated the importance of LAI for understanding the relationship between canopy light interception (LI) and herbage accumulation and the interaction between defoliation frequency and severity.
Brougham  fitted a sigmoid curve to the variation in herbage mass during regrowth of perennial ryegrass (Lolium perenne L.), white clover (Trifolium repens L.) and red clover (Trifolium pratense L.) mixed swards (variation in herbage mass with time), as well as describing the asymptotic relationship between LAI and LI [12,17]. The author demonstrated that plant growth was a function of canopy light interception and LAI, and that the rate of herbage dry matter accumulation reached a maximum constant value when there was enough foliage to intercept almost all the incident light. In general, three distinct phases may be identified in a regrowth curve. The first, soon after defoliation, is characterized by an exponential increase in herbage mass with associated increase in rate of herbage accumulation. This phase is highly influenced by plant organic reserves, climatic and edaphic conditions, and residual leaf area after cutting or grazing . The second is characterized by constant rates of herbage accumulation (linear increase in sward herbage mass). During this phase, intra and inter specific competition between plants become increasingly intense, particularly when the sward is close to maximum canopy light interception. During the third phase there is a reduction in herbage accumulation rate, the consequence of a proportionally larger increase in leaf senescence relative to leaf growth caused by leaves having reached their leaf lifespan and severe shading at the bottom of the sward .
These studies provided the basis for developing grazing management strategies based on the concepts of LAI and TNC accumulation and mobilization [19,20,21], despite the difficulties of measuring them. During the 1960s, Smith  carried out a series of experiments with alfalfa in the USA with the objective of demonstrating the importance of plant organic reserves and LAI for adequately managing pastures subjected to intermittent defoliation. As a result, the residual LAI and the organic reserve concentration remained as important considerations to be taken into account in planning and idealizing grazing management practices and strategies. Alcock  proposed three simple concepts for explaining plant responses to defoliation: (1) total availability and reutilization of organic reserves, (2) root growth, and (3) leaf area development and canopy light interception. The role of organic reserves had already been recognized as important for a long time , but with no applied results, since no grazing management strategy was produced based on it, except for harvesting alfalfa. The same happened with root growth. On the other hand, the LAI and LI concepts were used and studied in a series of experiments whose results helped to establish a strong knowledge basis for understanding the process of herbage accumulation of forage plants subjected to defoliation (cutting or grazing) regimes. Brown & Blaser  considered the use of LAI to define management practices an oversimplification of the process, arguing that on tall swards there were usually few leaves close to ground level and that lenient grazing would be necessary to ensure enough residual LAI to allow maximum canopy light interception. The authors also argued that on tall swards the photosynthetic efficiency of the leaves positioned at the base of the sward would be lower than that of leaves positioned at the top, resulting in low harvest efficiency and high risk of reduction in tiller population density.
In Brazil, during the first Symposium on Pasture Management, in 1973, it was recognized that there was a complex interplay among LAI, tillering, and TNC reserves in the regrowth of forage plants, and that the knowledge of those relationships in temperate species should be valid for tropical forages as well [26,27]. Jacques , however, argued that “despite its importance, the LAI was not enough to plan and sustain adequate management practices”, and highlighted the distinct interests and approaches of the predominant “research schools” at that time (North American and British).
Several of the existing grazing management guidelines have been based on the argument that pasture species should be used under intermittent defoliation regimes such as rotational stocking, generating a series of successive regrowth cycles (sigmoid curves), in order to better use the growth characteristics of forage plants. Under these circumstances, defoliations (grazings or harvests) should happen at the end of the linear growth phase (phase 2) so that maximum average rate of herbage dry matter accumulation could be obtained. Herbage nutritive value under those conditions, however, was usually low and it could be valuable to interrupt regrowth at an earlier stage with the objective of harvesting better quality herbage . Under continuous stocking, in cases when pastures are maintained at a steady state condition characterized by constant levels of sward LAI, height, or herbage mass, the idea of maintaining LAI to ensure 95% LI would not be valid, since leaf senescence is proportional to leaf growth, resulting in zero or even negative values of net herbage accumulation . In this case, the recommendation would be to manage swards at lower heights and lower LAI relative to intermittent defoliation regimes as a means of ensuring higher rates of herbage accumulation and harvest efficiency of the produced herbage .
From the 1960s onwards the “North-American” school started to be influenced by Gerald Mott’s work, with the introduction of the put and take stocking method for adjusting stocking rate in grazing experiments . The method consisted of using fixed groups of animals, called testers, that would represent the experimental unit for measuring animal responses, and another group that would act as stocking rate regulators, which were added to or removed from paddocks as a means of adjusting defoliation intensity based on an arbitrary criterion, originally suggested by Mott as grazing pressure. Grazing pressure was defined as the relationship between animal live weight and herbage mass per unit area [31,32]. By derivation, Mott  defined another term, carrying capacity, or the maximum stocking rate that would achieve a target level of animal performance in a specified grazing system. The put-and-take stocking method was more intensively used for research purposes with little use in practical situations, the consequence of the difficulties of its implementation in farm conditions. These concepts were used for many years by several researchers, but, according to Maraschin , without fully understanding the underlying principles involved. According to the same author, for efficient use of the herbage produced, stocking rate should always be defined in relation to the carrying capacity of pastures, a condition associated with the optimum grazing pressure. The goal under this approach would be to find an equilibrium between animal performance and animal production per unit area as a means of generating higher economical returns of pasture utilization, but without taking into account aspects of plant ecophysiology and the ecology of grassland ecosystems.
The “North American” school had a strong influence on the research with forage plants and pastures in Brazil for a long time. As a consequence, the emphasis was on describing the growth curve of pastures after cutting or grazing, their seasonality of herbage production, and the morphological and chemical composition of the produced herbage, with no particular attention to dynamic aspects related to plant population and competition for light normally associated with plant recovery after defoliation. Stocking rate and grazing pressure were considered key features and started to be used as control variables (treatments) in grazing experiments. This is corroborated by the large number of conferences on the topic at the Symposium on Pasture Management at ESALQ in Brazil [34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51] and research abstracts published in the Annual Meeting Proceedings of the Brazilian Society of Animal Science . After some time, grazing pressure evolved to the concept of herbage allowance (the amount of herbage on offer per animal), and evidences [53,54] showed that individual herbage intake was maximized when herbage allowance corresponded to three–four times the daily requirement of dry matter . In tropical pastures, after successive grazings, this could result in excessively high levels of herbage on offer characterized by large quantities of dead material and stems, low nutritive value of the herbage, and reduced intake by the grazing animals [56,57,58]. Mott , aware of that limitation, warned that the conversion of the primary production in animal product in tropical pastures would be considerably different from that in temperate pastures. As a consequence, he suggested that management practices should aim to provide the maximum amount of live tissues with high digestibility to animals, particularly leaves, as a means of increasing intake and performance. This idea led to the concept of green dry matter allowance , which evolved to leaf dry matter allowance. These practices were effective in providing a good fit between herbage allowance and animal performance data (as originally reported by Mott ) in relation to the traditional asymptotic curve , but magnified the problem of excessive herbage mass with high proportions of stems and dead material, with negative implications on herbage and animal production.
Although the conflict between the necessary sward conditions for generating high rates of herbage accumulation and those for achieving maximum intake and performance was evident in the literature since the early 1970s [56,57,58], harvest efficiency was frequently compromised as a means of favoring high levels of animal performance , resulting in low productivity and large dry matter losses to senescence, death, and decay . Better understanding and more sound grazing management practices started to emerge during the 1980s when the results from more detailed studies evaluating the dynamics of plant population and growth became available . Bircham & Hodgson  were the first to describe the dynamics of the herbage accumulation in continuously stocked perennial ryegrass swards, and showed that net herbage accumulation was the result of the balance between two concomitant and antagonistic processes—growth and senescence. These respond differently to agronomic and management practices. Consequently, evaluation of herbage accumulation only, i.e., without taking into account the independent processes of growth and senescence, could result in imperfect understanding of patterns of plant and pasture response to defoliation . The results also demonstrated that the herbage accumulation process could be adjusted through manipulation of sward structural characteristics such as LAI, height, and herbage mass, allowing for the development of grazing management targets that could be used to guide and control the grazing process in farm conditions. In this context of strict control of sward structure, the need for multi-year experiments became relatively less important when compared, for example, with experimental protocols that used stocking rate or grazing pressure as control variables, since the factors causing the treatment x year interactions were the same as those causing the treatment x environment and/or the treatment x season of the year interactions.
According to Hodgson , effective understanding of how plants and animals respond to variations in sward conditions, and consequently to management, could only be achieved in grazing experiments with rigid control of sward structural characteristics at a given state (continuous stocking management with variable stocking rate) or following a pre-specified pattern of variation (pre and post-grazing conditions of an intermittent stocking management). Korte et al. , studied perennial ryegrass subjected to cutting regimes characterized by two frequencies and two severities of defoliation, and planned their experimental treatments based on the findings of Brougham during the 1950s, using the 95% LI as the reference condition for initiating defoliation. The authors concluded that during the vegetative growth stage the 95% LI criterion could be used to define the best moment for initiating defoliation and, relative to longer defoliation intervals, would result in greater herbage production with higher proportion of leaves and lower proportion of dead material. That would indicate the ideal harvest point during regrowth (determinant of cutting and/or grazing interval), a condition that would be associated with the end of the linear phase of the sigmoidal growth curve described by Brougham . These findings indicated convergence of the available knowledge and corroborated the central role of LAI as a determinant of plant responses to grazing, highlighting the need to study and understand aspects related to sward structure, light use, and the balance between growth and senescence as a means of planning and defining efficient grazing management strategies . Chapman & Lemaire  reinforced the importance of LAI as a determinant of forage plant responses, and demonstrated that it was the result of the combined expression of the morphogenetic and structural characteristics of plants in a given environment. The paper established a reference point because it integrated the understanding from morphogenetic and ecophysiological studies from experimentation with forage plants, providing the necessary knowledge base for understanding the ecological and functional responses of plants and animals in grazing systems .
In Brazil, the first papers with information on the morphogenesis and ecophysiology of tropical grasses were published by Pinto et al. [70,71]. They reported results on leaf and stem accumulation, average tiller weight, proportion of vegetative and reproductive tillers, and rates of leaf appearance and leaf and stem elongation for andropogon (Andropogongayanus Kunth), guinea (Panicum maximum Jacq.), and setaria (Setariasphacelata Schumach.) grasses subjected to two levels of nitrogen fertilization. Gomide , in a review paper, showed results from several experiments on plant morphogenesis carried out in Brazil until that date. He indicated that the stabilization of the number of leaves per tiller and of tillers per plant could be a possible indicator for orienting and controlling the grazing management of tropical forage plants. On the other hand, Lemaire  argued that the morphogenesis of the main tropical forage species would have to be thoroughly studied in order to provide the necessary conditions for understanding plant responses to changes in management and environment. That introduced a change in research paradigm and to how the experimentation with forage plants would be conducted in the country from then onwards. In this new context, sustainability became an important feature, emphasizing pasture stability and productivity as the main goals to be achieved by the idealized management practices, highlighting the importance of rationalization and integration of the existing knowledge and results .
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