The use of standing dead biomass, during the winter that was produced by warm-season grasses in the previous growing season by pregnant beef cows may be an alternative to grazing systems in the semi-arid Pampean Region of Argentina. This study, conducted over 2 years, 1990 and 1991, compared the winter forage quality produced during the previous growing season for 4 warm-season grasses; switchgrass (Panicurn virgutum L. cv. Pathfinder), kleingrass (Panicurn colorutum L.), tetrachne (Tetruchne dregei New) and weeping lovegrass (Erugrostis curvula (Schrad), Nees cv. Tanganyka). Five harvests of the summer growth started after the first frost, and were spaced evenly throughout the winter period. Changes in the standing crop of dry matter were measured and subsamples of forage were divided into leaf and stem fractions. Forage quality analyses included: crude protein (CP), in vitro dry matter digestibility @VDMD), effective rumen degradability (ED), neutral (NDF) and acid (ADF) detergent fiber and lignin. Tetruclzne dregei produced forage with a higher leafxtem ratio and of generally higher quality, than the other species, although the differences were not always significant. Its CP content was marginally below the maintenance requirements of cows. Dry matter yield of tetrachne was lower than that of weeping lovegrass, but differences were only significant in 1990. Kleingrass generally was high in quality and dry matter yield, although it was the lowest in percentage of leaves of the 4 species evaluated. Switchgrass was the least productive; the nutritive value of its forage was low, comparable or lower than that of weeping lovegrass. The first harvest date was higher in nutritive value. Although the nutritive value of leaves and stems were not compared statistically, the leaves tended to be higher than the stems. Tetruchne dregei, the best of the species evaluated in this study, is a very promising warm-season grass, which could provide nutritious forage for winter grazing systems in the semiarid Pampean Region of Argentina.

The effects of nitrogen (N) fertilization of a rye pasture (Secale cereale L.) on the chemical composition of forage and its utilization in vivo and in situ were investigated in winter (W) and early spring (S). Half of the paddock to be assayed was spread with 93 kg urea-N ha )1 (treatment F), and the other half was not fertilized (treatment NF). Each experiment lasted for 17 days; apparent dry matter digestibility in vivo, voluntary dry matter intake (DMI) and digestible DMI were measured on nine rams per treatment. Two rumen cannulated animals per treatment were under the same feeding regime, and used to collect samples of rumen contents. The concentrations of ammonia (NH 3 ) and short-chain fatty acids and pH were measured in these samples. The offered forage was also incubated in the rumen of three Holstein-Friesian steers to estimate its degradation parameters in situ. Fertilization with N increased total N and decreased dry matter (DM) and non-structural carbohydrate concentrations in both periods. DMI was reduced by fertilization. The rumen concentrations of NH 3 were higher in F than in NF in both periods. In both periods, the effective DM degradability of forage was higher in F than in NF. N fertilization affects chemical composition, voluntary intake and rumen digestion of forage from rye pasture.

is a perennial warm-season grass that begins its growing season later in the spring than does weeping lovegrass (Eragrostis curvula (Schrad.) Nees), which suggests that combining weeping lovegrass with kleingrass in a grazing sequence could increase livestock production. The objectives of this study were to determine the seasonal and inter-annual variation of the patterns of the accumulation of dry matter (DM) and the percentage of crude protein (CP) in P. coloratum, cv. Verde, in the semiarid Pampean region of Argentina. This study was performed during four growing seasons in a 3.0 ha pasture of kleingrass that was sown in the spring of 1996. The forage samples were gathered at biweekly intervals and were separated into leaf blade, stems (including sheaths and inflorescence) and senescent material. Finally, patterns of dry matter accumulation and the percentage of crude protein were evaluated in relationship to the thermal time accumulated from early October to late February. In comparison with the other growing seasons evaluated, the third growing season had less (P ≤ 0.01) accumulation of dry matter and a higher (P ≤ 0.01) proportion of green leaf blade and CP. Dry matter accumulation increased with increases in the thermal time. The CP percentage of the total biomass and of the plant parts declined as thermal time increased, and this coincided with a reduction in the proportion of green leaves and the CP percentage in each plant part. Management decisions depend on an accurate assessment of the change in sward structure and nutritive value during the growing season. The results of this study will allow selection of the appropriate grazing time and the management of sequential grazing of warm-season grasses.

The spatial heterogeneity in the structure and the productivity of the vegetation was examined in a tall fescue (Festuca arundinacea Schreb.) pasture rotationally grazed at four stocking densities in the Pampean region of Argentina. The examined pasture was grazed at the stocking densities of 3.6, 4.6, 5.6 and 6.6 animals ha -1 with a two-paddock 14-day rotational grazing system. Spatial distribution of plant height was examined as well as the percentages of short patch area (heavily utilized patches) or tall patch area (areas ungrazed or lightly defoliated). In addition, biomass, growth rate and relative growth rate were assessed for both short and tall patches. Grazing generated patchiness in vegetation structure and growth at all stocking densities. Increased stocking density caused an increase in the percentage of the short patch area in the paddocks. Short patches had relatively less live biomass than tall ones, but their relative growth rate was 31% higher than that of tall patches (0.021 ± 0.007 vs 0.016 ± 0.005 g DM g DM -1 day -1 ). The increase in stocking density enlarged the proportion of short patch areas with higher relative growth rate. The relative growth rate (average between short and tall patches) of the two highest stocking densities was 61.7% higher than that of the low stocking density treatments (0.023 ± 0.006 vs 0.014 ± 0.004 g DM g DM -1 day -1 ). Although the growth rate of the short patches did not exceed the value of the tall patches, the high value of relative growth rate appeared to indicate a higher photosynthetic capacity of the short patches. Moreover, live biomass did not decrease during the experimental period even in the short patch areas showing that, in the particular conditions of our study, overgrazing did not occur at the range of the stocking density examined.

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