Rafael Ramos-Santana

Short-term tillage practices on soil organic matter pools in a tropical Ultisol

In tropical regions, pasture establishment involves tillage operations. Adoption of conservation tillage practices could result in lower costs and in improved soil quality by decreasing soil organic carbon (SOC) losses. This study investigated the effects of 3 tillage practices on the establishment of Brachiaria decumbens and on the total SOC and soil organic nitrogen (SON) content and its fractions in an Ultisol from the humid mountain zone of Puerto Rico that was previously under pasture. The treatments evaluated were no-tillage, minimum tillage, and conventional tillage (CT). At 120 days after planting (DAP), plant cover and density was improved in the CT treatment compared with the other treatments. At 180 DAP, there were no significant differences in the SOC, SON, aggregate size distribution, distribution of C within aggregate size classes, and labile C physical fractions among tillage treatments. Approximately 60% of the total SOC associated with aggregates was found within macroaggregates. About an equal proportion of the particulate organic matter (POM) was associated within aggregates and nonaggregate-protected free light fraction, and these were not affected by tillage management. Lower amounts of C mineralised after disruption of macroaggregates containing POM with high C/N ratio was probably due to immobilisation of the more labile protected C (iPOM). Labile forms of C were greater in macroaggregates than in microaggregates, yet comprised a lower proportion of total SOC, suggesting that macroaggregates have a greater proportion of C physically protected from microbial attack. The results indicate that there are no short-term changes in the tendency of the soil to lose C and N as a result of tillage practices for the establishment of pastures in this soil.

Agronomic comparison of six bermudagrasses from southern United States with five tropical grasses in central Puerto Rico.

Abstract The dry matter yield (DMY), in vitro organic matter digestibility (IVOMD) and crude protein (CP) content of eleven grasses were studied during the years 1992 to 1994 under mob grazing conditions. Offered and consumed dry forage do not differ significantly (P<0.05) among the highest yielding accessions Cynodon nlenfuensis PRPI 2341, Cynodon plectostachium PRPI 11487, and Cynodon dactylon cv. Tifton 85. However, the commercial Cynodon nlenfuensis PRPI 2341 was consistently the highest yielding accession both years. Among the other grasses, Digitaria eriantha showed the lowest levels of refused dry forage, thus presenting the best forage acceptability by grazing animals. Digitaria eriantha also showed the highest levels of IVOMD in both the short and the long day seasons. It is concluded that none of the evaluated Cynodon dactylon accessions from southern United Sates, nor the digitaria and brachiaria species adapted better than the commercial accession Cynodon nlenfuensis PRPI 2341. Warm‐season perennial grasses are grown extensively in the southeastern United States. Among the grasses, one of the most widely grown is from the species Cynodon dactylon, commonly named bermudagrass (Eichhorn, 1984). The choice of a bermudagrass cultivar for establishment of pastures is an important decision for cattlemen seeking high weight gains of stocker steers throughout the warm grazing season of the southern United States (Greene et al., 1989). Selection of improved bermudagrass hybrids has dramatically increased forage yield over that of common bermudagrass. Improved forage quality has also resulted in improved performance of grazing animals fed harvested hybrid bermudagrass forage (Gates et al., 1989). Reports on average daily weight gains (ADG) of yearling steers grazing different bermudagrass hybrids varied between 0.68 to 0.79 kg animal‐1 day‐1 during the summer season of southern Louisiana (Faw et al., 1986). Hill et al. (1993) in Tifton, GA, found yearling ADG of 0.67 and 0.65 kg animal‐1 day‐1 for the recent bermudagrass releases Tifton 85 and Tifton 78, respectively. Forage quality dry matter yield and grazing performance support the hypothesis that Tifton 85 will likely become an important hay and grazing forage for the southern United States. In Puerto Rico, Cynodon nlenfuensis PRPI 2341 (stargrass) is at present the grass cultivar most frequently grown on dairy farms (Ramos‐Santana and Randel, 1996). Yearly ADG of 0.59 kg animal‐1 day‐1 has been reported for young steers grazing stargrass in the humid tropical zone of Puerto Rico (Vicente‐Chandler et al., 1983). At Ona, FL, lower ADG of 0.54 kg has also been obtained with stargrass (Mislevy, 1989). The objective of this paper is to compare the agronomic performance (dry matter yield and quality) of six of the most outstanding bermudagrass hybrids from the southern United States, with five tropical grasses from the cynodon, brachiaria, and digitaria genera under the tropical conditions of central Puerto Rico.