Plant litter is an important nutrient pool in grassland ecosystems. Management practices affect litter quality and may affect nutrient dynamics in pastures by altering the rates of nutrient mineralization and immobilization. The effect of management intensity on litter decomposition and nutrient disappearance was evaluated in a litter bag study on continuously stocked 'Pensacola' bahiagrass (Paspalum notatum Flü gge) pastures growing on Pomona and Smyrna sands. Treatments were three management intensities: Low (40 kg N ha 21 yr 21 and 1.3 animal units [AU, one AU 5 500 kg live weight] ha 21 stocking rate [SR]), Moderate (120 kg N ha 21 yr 21 and 2.7 AU ha 21 SR), and High (360 kg N ha 21 yr 21 and 4.0 AU ha 21 SR). Litter relative decomposition rate (k) was greater for High (0.0030 g g 21 d 21 ) than Low (0.0016 g g 21 d 21 ). Litter N, acid detergent insoluble N (ADIN), and lignin concentrations were greater for High than the other intensities at the end of the 168-d incubation period because of faster decomposition of soluble compounds. Across management intensities, approximately one-half of litter N remaining at the end of the incubation period was bound to acid detergent fiber (ADF). Net N mineralization through 128 d of incubation was only 200 to 300 g kg 21 of total N. Increasing management intensity resulted in faster litter turnover and greater nutrient release, but nutrient release from litter was small and significant quantities of nutrients were immobilized even under the most intensive management.

unit of time. For the purposes of this paper and because of its simplicity, animal live weight will be used in the Stocking rate has a major effect on animal performance, but compadenominator of the relationship, but it is understood rable stocking rates may result in a wide range in performance across that this number can be converted to animal units or environments because of differences in forage mass or sward canopy characteristics. Forage allowance is a function of both forage mass forage intake units if preferred. and stocking rate and can be a powerful tool for explaining differences Forage allowance can be a very useful tool in exin animal performance. Some methods used to express forage allowplaining animal performance on pasture if allowance ance in the literature do not allow useful comparisons across grazing occurs across a relatively wide range, such as in fixed methods or among management strategies within a method. In addistocking rate experiments with multiple levels of the tion, many include a unit of time which violates the definition of treatment factor. Typically, the relationship between forage allowance as a point-in-time measure. A meaningful method daily gain and forage allowance is linear up to some of reporting forage allowance is needed that applies across a wide relatively high allowance, after which gain levels off range of pasture management treatments. This paper suggests a (McCartor and Rouquette, 1977; Sollenberger and method that does not include a unit of time, has application across Moore, 1997). The interaction of forage allowance and continuous and rotational stocking methods, and within the rotational stocking method applies to any size or number of pasture subunits.supplementation rate on milk production has been an important recent area of research in temperate, pasturebased dairy systems (Wales et al., 1999).

Nutrients cycle among pools within an ecosystem, and losses of nutrients to the environment accompany each transfer from pool to pool. Efficient recapture of nutrients by plants is critical in extensively managed grasslands if these swards are to persist. In intensively managed systems, the greatest contribution of efficient recapture of nutrients may be minimizing loss of nutrients to the environment and associated negative impacts. Regardless of management intensity, grassland management decisions should be informed by an understanding of the dynamics of nutrient cycling. A significant body of literature has emerged in recent years describing nutrient dynamics in warm‐climate grasslands. In warm climates globally, grasslands are most often low‐input production systems dominated by C4 grasses. These characteristics affect nutrient cycling, resulting in very different management challenges and opportunities than in higher input, C3–grass or legume‐dominated, grasslands. This paper will focus on warm‐climate grasslands. Within that context its objectives are (i) to describe the most prominent pools of C, N, P, and K, (ii) to discuss fluxes among nutrient pools, with emphasis on plant litter and animal excreta, iii) to describe the importance, management, and dynamics of soil organic matter, and (iv) to review the impact of grazing systems on nutrient cycling.

Little information is available directly comparing soil nutrient distribution under different defoliation managements. During 1990 (116 d) and 1991 (141 d), 'Callie' bermudagrass (Cynodon dactylon var. aridus Harlan et de Wet) pastures grazed by Holstein heifers (Bos taurus) were used to determine the effects of two rotational stocking methods and continuous stocking on lateral and vertical distribution of extractable N, P, K, and S. A hay management also was included to compare soil responses under grazing and clipping. Nutrient distribution and concentration in the Apl horizon (0-to 15-cm soil depth) did not differ among grazing methods, but N, P, and K accumulated in the third of the pastures closest to shade, water sources, and supplement feeders (lounging areas where cattle tend to congregate or rest). Similar observations were made with K in the Ap2 horizon (15-to 30-cm soil depth). Nutrient concentrations were lower or tended to be lower in the Apl horizon of the hay management than in grazed pastures because of nutrient removal in harvested herbage. Across defoliation managements, greater extractable N, P, and K concentrations were observed in the Apl horizon in 1991 than in 1990. For N and K, this was attributed to fertilizer inputs in all managements and partially to supplemental feed inputs in grazed pastures. Increases in extractable P appeared to be associated primarily with flooding of the experimental site in late 1991. This study suggests that grazing method of wellmanaged pastures may have little effect on short-term (2 yr) soil nutrient distribution, especially when grazing occurs during months when temperatures are high.

Warm‐climate grasslands are often N limited. Legume litter decomposition can contribute significantly to N input in grazing systems, but its contribution depends on litter deposition, decomposition, and chemical composition. We evaluated these responses for 2 yr in unfertilized (BG) and fertilized (BGN; 50 kg N ha−1) bahiagrass (Paspalum notatum Flügge) monocultures and in mixed swards of bahiagrass plus the legume rhizoma peanut (Arachis glabrata Benth.). Legume–grass mixture litter had greater initial N concentration (26 g N kg−1 organic matter [OM]) and lower C/N ratio (22) than BG and BGN, which did not differ from each other (18 g N kg−1 OM, C/N ratio of 31). Litter biomass relative decay rate was greater for mixtures than for bahiagrass monocultures. As a result, less biomass and N remained at the end of incubation in mixtures (62 and 76%, respectively) than in monocultures (69 and 80%, respectively). Litter deposition rate was similar across treatments, but faster decomposition and greater N concentration for legume–grass mixtures resulted in larger litter N release than in monocultures (44 and 26 kg ha−1, respectively). At the end of incubation, remaining litter biomass and remaining N decreased with increasing litter legume proportion, whereas litter N concentration and litter decay rate increased. Results indicate that legume–grass mixtures are an alternative to N fertilizer for increasing N cycling through plant litter in grasslands, and although litter deposition rates were similar across treatments, increasing legume proportion in mixtures is likely to be associated with greater litter N release.

The objective was to compare productive and metabolic responses of lactating dairy cows managed on 2 pasture-based systems using a concentrate supplement (n = 16) with those of a freestall housing system (n = 24). In a 259-d experiment, 3 multiparous Holstein cows were assigned at calving to each of 4 replicates of 2 pasture systems. For system 1, winter pastures were a mixture of rye, ryegrass, and crimson and red clover; summer pastures were pearl millet. Pasture system 2 included a rye-ryegrass mixture during winter and bermudagrass during summer. Pregraze herbage mass averaged 2.3 and 3.6 Mg/ha for winter and summer pastures, respectively; however, during August through September, pearl millet pregraze mass was reduced to about 1 Mg/ha. Daily dry matter intake by cows on pasture averaged 24.7 kg/d in winter and 19.0 kg/d in summer, of which 55% was from pasture; that of cows in confined-housing averaged 23.6 kg/d. Cows in confinement produced 19% more milk (29.8 vs. 25.1 kg/d) than those on pasture systems. Differences in concentration of milk fat, protein, or urea N were not detected among treatment groups. Grazing cows lost more body weight than confined cows (113 vs. 58 kg) and had lower concentrations of plasma glucose in the early weeks postpartum. Despite greater milk yield by cows housed in freestalls, milk income minus feed costs including that of pasture was similar for the 3 management systems. Although these pasture systems might be a viable management system in the southeastern US, extensive loss of body weight immediately postpartum for pasture-based cows are a potential concern.

‘Florigraze’ rhizoma peanut (Arachis glabrata Benth.) is a perennial legume of high forage quality adapted to warm climates, but there has been no comprehensive evaluation of its responses to grazing management. In 1988 and 1989, the effects of grazing frequency and intensity on Florigraze persistence and herbage accumulation (HA) were evaluated on a loamy, siliceous, hyperthermic Grossarenic Paleudults soil. All 12 combinations of three levels of residual dry matter after grazing (RDM, 500, 1500, and 2500 kg ha‐') and four grazing cycle lengths (GC; 7, 21, 42, and 63 d between grazings, including a 0.5‐ to 2‐d grazing period) were replicated twice. Data were analyzed by fitting multiple regression equations starting with a second order polynomial model. In 1988, rhizoma peanut HA ranged from 6130 to 10 240 kg ha− and increased linearly as GC and RDM increased. There was a GC‐by‐RDM interaction for rhizoma peanut HA in 1989, whereby at low RDM, increasing GC increased HA, but GC had less effect as RDM increased. In 1989, rhizoma peanut HA of at least 8800 kg ha− was estimated to occur with GC of 42 d or longer when RDM was 1500 kg ha− or greater. Rhizoma peanut percentage in HA was greatest with high RDM and long GC, but values of 80% or greater in the second year were estimated for RDM as low as 1300 kg ha− when GC was 63 d, or with GC as low as 7 d when RDM was above 2300 kg ha−. Lowest values were obtained with low RDM and short GC. These data indicate that unlike most tropical legumes, rhizoma peanut is productive and persistent over a relatively wide range of grazing management practices.

Relationships of forage nutritive value (NVAL) and quantity with individual animal performance on pasture have long been evaluated, but there have been few attempts to describe the specific roles or relative importance of NVAL and quantity in determining animal response. The objective of this review was to more clearly define these roles based on a comprehensive assessment of pastureland literature and the application of a meta‐analysis. It is clear that quantity and NVAL interact. When pastures present a wide range in forage mass, 60 to 90% of variation in average daily gain (ADG) can be explained by quantity, but there may be no relationship of NVAL with ADG. If pastures present only high forage mass, there may be no relationship between quantity and ADG, but NVAL may explain more than 50% of variation in ADG. The meta‐analysis and a review of studies that evaluated forages of differing NVAL across a range of forage mass provided evidence that NVAL (i) sets the upper limit for ADG, (ii) determines the slope of the regression of ADG on stocking rate (SR), and (iii) establishes the forage mass at which ADG plateaus. In contrast, forage quantity determines the proportion of potential ADG that is achieved and is the primary driver for direction of the ADG response (negative) to increasing SR. Thus, the literature supports a conclusion of interaction among forage NVAL and quantity in affecting individual animal performance on pasture, but it suggests that the roles of each are quite well defined.