Effect of nitrogen on production of Paspalum atratum on seasonally wet soils in north-east Thailand

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1 Tropical Grasslands (999) Volume??, Effect of nitrogen on production of Paspalum atratum on seasonally wet soils in north-east Thailand M.D. HARE, W. SURIYAJANTRATONG, P. TATSAPONG, C. KAEWKUNYA, K. WONGPICHET AND K. THUMMASAENG Faculty of Agriculture, Ubon Ratchathani University, Ubon Ratchathani, Thailand Abstract Four trials were conducted in north-east Thailand to examine effects of nitrogen fertilisers and cutting frequencies on growth of Paspalum atratum (cvv. Hi-Gane and Ubon) on infertile, low lying, seasonally wet soils. Nitrogen applied at 20 kg/ha every 30 days throughout the wet season, increased dry matter yields by nearly 90% in one trial and over 250% in a second trial. Applying higher rates (40 and 80 kg/ha N) every 30 days increased dry matter yields further but the increase in dry matter per unit of N was reduced. The yield response (kg DM/kg N) from applying nitrogen as urea in the wet season ranged from 8 (480 kg/ha N) up to 69 (20 kg/ha N). However, when nitrogen as NPK fertiliser was applied, the yield response ranged from 20 (94 kg/ha N) up to 50 (47 kg/ha N). Nitrogen rates of 80 kg/ha N every 30 days were required to maintain crude protein concentrations in the whole plant above 7%, but only when the fields were not waterlogged. Cutting every 60 days produced significantly more dry matter than cutting every 30 days but crude protein concentrations were lower. alone gave little increase in dry matter yields of P. atratum. When applied at 3 6 t/ha with a compound fertiliser (N:P:K, 5:5:5) (32 kg/ha), dry matter yields were 35 40% above yields produced when similar rates of compound fertiliser were applied without cow manure. The implications for farmers are discussed. Correspondence: M.D. Hare, Faculty of Agriculture, Ubon Ratchathani University, Warin Chamrab, Ubon Ratchathani 3490, Thailand. Michael@agri.ubu.ac.th Introduction In village pasture systems in north-east Thailand, pastures are usually grown on the poorest soils, as more fertile soils are used for growing food and cash crops. Furthermore, village farmers usually apply a maximum of 20 kg/ha N in a 6- month growing season, if they apply any fertiliser at all. Consequently, most improved pastures in north-east Thailand are nitrogen deficient. The exception would be some commercial dairy farmers in the central part of Thailand who apply more fertiliser (0 20 kg/ha N 3 5 times in the wet season) to improved pastures of guinea grass (Panicum maximum) or napier grass (Pennisetum purpureum). Recent research has shown that Paspalum atratum is well suited to waterlogged acid soils which become seasonally dry in north-east Thailand (Hare et al. 999a; 999b). These soils are mainly grey podzolic soils (Khorat soils) and low humic gley soils (Roi-et soils), and cover >40% of the area. They are largely structureless with very low nutrient-holding capacity and high bulk densities (Mitsuchi et al. 986). They are also very low in organic matter ( %) and nitrogen ( %) (Hare et al. 999a). Despite growing on poor soils, P. atratum cv. Ubon produced over 30 t/ha DM in a 6-month wet season (Hare et al. 999b) when fertilised every days with N (40 kg/ha), K (50 kg/ha), P (20 kg/ha) and S (20 kg/ha), a total of kg/ha N. Therefore, to produce over 30 t/ha DM on poor soils, P. atratum requires large amounts of nitrogen fertiliser. A 7% crude protein concentration in feed is considered a critical level (Milford and Minson 966) because below this level nitrogen needed by rumen microorganisms becomes limiting unless some other form of crude protein supplementation is given to grazing cattle (Kalmbacher et al. 997). Applying nitrogen (40 kg/ha) every 30 days to Ubon paspalum maintained crude

2 2 M.D. Hare, W. Suriyajantratong, P. Tatsapong, C. Kaewkunya, K. Wongpichet and K. Thummasaeng protein concentrations above 7% in north-east Thailand (Hare et al. 999b). In Florida, Kalmbacher et al. (997) showed that crude protein in P. atratum cv. Suerte dropped below 7% 55 days after receiving 56 kg/ha N, and recommended applying 56 kg/ha N every days to P. atratum pastures when grazed by young cattle. The objectives of the research were: to examine the production and quality of P. atratum under different levels of nitrogen in order to recommend an optimum nitrogen rate for P. atratum pastures growing on infertile waterlogged soils in north-east Thailand; and to determine if cow manure could be substituted for chemical fertiliser. Materials and methods The research was conducted over 3 years ( ) in Ubon Ratchathani province, Thailand (5 N) on the Ubon Ratchathani University farm at 3 sites in a 2 hectare paddock. Rainfall was recorded km from the trial paddock (Table ). The soil in the paddock is classified as a very sandy, low humic gley soil (Roi-et soil series) with some mixture of grey podzolic soil (Khorat soil series) and usually is waterlogged from July to early October. A soil test taken in June 995 showed that the soil was acid (ph 4.6), with low organic matter ( %), N ( %) and P (3 3 ppm, Bray no. 2 extraction method) concentrations and very low K (20 50 ppm). The site was formerly rice paddy land but prior to cultivation had supported native grasses (mainly Eremochloa spp.) for 6 years. Trial. Effect of nitrogen rates on Hi-Gane paspalum The first trial in 996 at site was conducted on a -year-old sward of P. atratum cv. Hi-Gane at the end of the wet season in 996 to study the effect of nitrogen in providing early dry season forage. The design was a randomised complete block (RCB) with 4 replications and 5 nitrogen rates (0, 20, 40, 60 and 80 kg/ha N). The trial commenced on November 5, 996, when the plots were cut at 5 cm from ground level and the nitrogen applied as urea. In addition, all plots received P (20 kg/ha), S (20 kg/ha) and K (50kg/ha). Each plot measured 4 5 m. A single dry matter cut was taken 6 weeks after applying fertiliser (December 7, 996), from six 0.25m 2 quadrats cut 5 cm from ground level in each plot. The fresh sample was weighed and a 200 g subsample from each plot was dried at 70 C for 48 hours and dry weight recorded. Trials 2 and 3. Effect of cutting and nitrogen rates on Ubon paspalum The second trial at site 2 was conducted in 997 on a 2-year-old sward of P. atratrum cv. Ubon and the third trial, in 998, was at site 3, adjacent to site 2, on a -year-old sward of Ubon paspalum. Both trials were factorial RCBs with 4 replications, 2 cutting frequencies (30 and 60 days) and 4 rates of nitrogen (0, 20, 40 and 80 kg/ha N) applied as urea every 30 days. Trial 2 commenced on May 23, 997 and Trial 3 on May 28, 998, when the swards were cut to 5 cm from ground level and urea and other fertiliser applied (P, S and K rates as in Trial ). Each plot measured 5 9 m. Nitrogen was applied every 30 days and P, S and K every 60 days. In total, the N treatments received 0, 20, 240 and 480 kg/ha N. Six 30-day interval sampling cuts, and three 60-day interval sampling cuts were taken in each trial. At each cut, four 0.25m 2 quadrats were taken from each plot, weighed and subsampled as in Trial. The dried subsamples were bulked across replicates and then 3 samples per treatment were analysed for total N, in order to calculate crude protein levels (%N 6.25). After each sampling cut, the remaining forage in the plots was cut to 5 cm above ground level before applying fertiliser. Trial 4. Effect of chemical fertiliser and cow manure on Hi-Gane paspalum In the fourth trial, 3 levels of dry cow manure (0, 3 and 6 t/ha) and 3 levels of N:P:K (5:5:5) fertiliser (0, 56 and 32 kg/ha), were applied to the site sward of Hi-Gane paspalum (3 years old), with 4 replications in a factorial RCB. Each plot measured 5 5 m. An analysis of 6 samples of the cow manure showed that it contained 2.63% N, 0.62% P and 2.87% K. The cow manure was collected from the university dairy

3 Effect of nitrogen on production of Paspalum atratum on seasonally wet soils in north-east Thailand 3 shed in May 998, dried and stored. The same cow manure was used for each application. The trial commenced on May 29, 998, when the plots were cut to 5 cm from ground level and the fertiliser treatments applied. Three 45-day interval sampling cuts were taken and the trial was terminated after the 3 rd cut on October 2, 998. After the st sampling cut, the fertiliser treatments were re-applied, but were not applied after the 2 nd cut. Total amounts of N applied were 58 and 36 kg/ha N for the 2 levels of cow manure, respectively, and 47 and 94 kg/ha N for the 2 levels of NPK. At each cut, four 0.25m 2 quadrats were sampled from each plot and analysed for dry matter and crude protein concentrations as in Trial 2. The remaining forage in the plots was cut following sampling. Data from all trials were analysed using the IRRISTAT program from The International Rice Research Institute (IRRI). Crude protein data were not statistically analysed because samples were bulked across replicates. Results Rainfall Total annual rainfall during each year of the study was similar to or slightly above the longterm average of 563 mm/year (Table ). Site received heavy rain in Trial during the first week of November 996 and plots remained waterlogged throughout most of that month but were dry by December 996. Trial 2 at site 2 was severely waterlogged from August October 997 but no waterlogging occurred in trials 3 and 4 the following year during the May October 998 experimental period. Trial. Effect of nitrogen rates on Hi-Gane paspalum Application of 40 kg/ha N and above significantly (P<0.05) increased dry matter production of Hi-Gane paspalum compared with the control, in a 6-week period at the beginning of the dry season (Table 2). Yields on all treatments where N was applied were not significantly different. Table. Rainfall at Ubon Ratchathani University during the study and the long-term mean. Month Mean rainfall, Rainfall Mean (mm) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total Table 2. Effect of nitrogen rates on dry matter production of Hi-Gane paspalum (Trial ). Nitrogen rate Dry matter b ab a a a Means followed by different letters are significantly different (P<0.05) by Duncan s Multiple Range Test. Trials 2 and 3. Effect of cutting and nitrogen rates on Ubon paspalum In Trial 2, 60-day cutting produced significantly more dry matter than 30-day cutting overall (Table 3). Dry matter yields increased progressively with level of N applied at both cutting frequencies but differences were not always significant. At 30-day cutting, maximum yield occurred at the highest level of N but yields plateaued at 40 kg/ha N with 60-day cutting. Nitrogen at 40 and 80 kg/ha generally increased CP levels (Table 4) when plots were cut in June, July and November but not in September and October when all plots were severely waterlogged. Very low crude protein levels (<5%) were found in many plots receiving either 0 or 20 kg/ha N, more particularly when cut every 60 days.

4 4 M.D. Hare, W. Suriyajantratong, P. Tatsapong, C. Kaewkunya, K. Wongpichet and K. Thummasaeng Table 3. Effect of cutting interval and nitrogen on total dry matter production of Ubon paspalum (Trial 2). N applied each 30 d 60-day cutting interval (3 cuts) mean LSD (P<0.05) nitrogen cutting interval: 4704 Total N applied over trial (0, 20, 240 and 480 kg/ha). Table 4. Effect of cutting interval and nitrogen application on crude protein concentration of Ubon paspalum (Trial 2). N applied each 30 d Crude protein concentration 60-day cutting interval (3 cuts) Jun Jul Aug Sep Oct Nov Jul Sep Nov (%) Total N applied over trial (0, 20, 240 and 480 kg/ha). In Trial 3, cutting every 60 days produced significantly more dry matter than cutting every 30 days (Table 5). Increasing N levels significantly increased dry matter yields at both cutting frequencies. Yields continued to increase (P<0.05) to 80 kg/ha N when cut every 60 days but there was no significant (P>0.05) increase above 40 kg/ha N when cut every 30 days. Crude protein concentrations in Ubon paspalum exceeded 7% when 80 kg/ha N was applied (Table 6). Lower nitrogen rates had little or no effect on crude protein concentrations. Crude protein concentrations were higher in Trial 3 than in Trial 2 because the plots in Trial 3 were not affected by waterlogging. Table 5. Effect of cutting interval and nitrogen application on dry matter production of Ubon paspalum (Trial 3). N applied each 30 d 60-day cutting interval (3 cuts) mean LSD (P<0.05) nitrogen cutting interval: 983 Total N applied over trial (0, 20, 240 and 480 kg/ha). Table 6. Effect of cutting interval and nitrogen application on crude protein concentration of Ubon paspalum (Trial 3). N applied each 30 d Total N applied over trial (0, 20, 240 and 480 kg/ha). Trial 4. Effect of chemical fertiliser and cow manure on Hi-Gane paspalum Both levels of NPK fertiliser significantly increased total dry matter production of Hi-Gane paspalum above the control yields but in the absence of cow manure there were no significant differences between the 2 rates of NPK fertiliser (Table 7). Applying cow manure without NPK fertiliser failed to significantly increase dry matter production of Hi-Gane paspalum (Table 7), but both rates of cow manure gave substantial increases in combination with the highest level of NPK (32 kg/ha) at the first and second cuts. Yields in these plots exceeded 000 kg/ha at the first cut, 3000 kg/ha at the second cut and kg/ha for the total of three cuts. The highest rate of cow manure (6.25 t/ha) also significantly increased dry matter yields in combination with the lower rate of NPK fertiliser (56 kg/ha). There were no residual effects from either cow manure or NPK fertiliser at the third cut. Yield responses Crude protein concentration 60-day cutting interval (3 cuts) Jun Jul Aug Sep Oct Nov Jul Sep Nov (%) The response in dry matter per unit of N was the greatest in Trial 4 with NPK fertiliser (Table 8). The highest response in dry matter yield from applying NPK was 2 times the highest response from applying urea. Generally, the higher the nitrogen rate, the lower was the response in dry matter per unit of N. The lowest response was from applying cow manure alone.

5 Effect of nitrogen on production of Paspalum atratum on seasonally wet soils in north-east Thailand 5 Table 7. Effect of application of cow manure and chemical fertiliser on production of Hi-Gane paspalum. Table 8. Yield responses (kg DM/kg N) from applying nitrogen to Paspalum atratum. First dry matter cut Chemical fertiliser NPK (5:5:5) LSD (P<0.05) 2584 ns; Chemical fertiliser **; Second dry matter cut Chemical fertiliser NPK (5:5:5) LSD (P<0.05) 330 *; Chemical fertiliser **; Third dry matter cut Chemical fertiliser NPK (5:5:5) LSD (P<0.05) 67 ns; Chemical fertiliser ns; Total dry matter yield Chemical fertiliser NPK (5:5:5) LSD (P<0.05) 538 **; Chemical fertiliser *; Trial Treatment N level Yield response (kg/ha N) (kg DM/kg N) Urea 42-day cutting Urea 30-day cutting day cutting Urea 30-day cutting day cutting NPK 45-day cutting NPK = = = = cut; 2 6 cuts; 3 3 cuts; 4 3 cuts. Discussion This study has shown that without N fertiliser the growth of P. atratum on low fertility, waterlogged soils is less than half the yield produced by applying kg/ha N every 30 days. Nitrogen is fundamental to plant biochemistry (Whiteman 980) and we observed a general yellowing or chlorosis of the leaves and a reduction in growth of P. atratum plants in low nitrogen treatments. Nitrogen rates as low as 20 kg/ha applied every 30 days increased P. atratum dry matter yields by 90 and 250% in Trials 2 and 3, respectively, above yields in control plots. While higher rates in Trials 2 and 3 increased dry matter yields further, the response in dry matter per unit of N applied was typically curvilinear (Humphreys 987), with increased dressings of N giving, overall, less increase in dry matter per unit of N (Table 8). The yield response of P. atratum in this study was typical of the general response of tropical grasses to of 20 to 50 kg DM/kg N (Humphreys 987) when N as urea was applied (with P, K and S as a basal dressing). When N as NPK fertiliser was applied, the yield response

6 6 M.D. Hare, W. Suriyajantratong, P. Tatsapong, C. Kaewkunya, K. Wongpichet and K. Thummasaeng (20 and 50 kg DM/kg N) was twice the highest response from applying urea, suggesting that N in NPK fertiliser was not leached so quickly out of the low organic soil. We have observed that pastures fertilised with urea need more frequent fertiliser than pastures fertilised with NPK. The choice between applying urea or NPK compound fertiliser will, for the most part, depend on economics. Urea is cheaper, and currently in Ubon Ratchathani it costs US $0.9/kg ($0.4/kg N) and NPK costs US $0.22/kg ($.46/kg N). The best NPK treatment (56 kg/ha Trial 4)) produced 02 kg DM/US $ and the best urea treatment (40 kg/ha Trial ) produced 78 kg DM/US $. However, while NPK is more expensive, consideration must be given to the observed longer pasture response to NPK and the importance to pasture of other elements in the compound fertiliser of P (5%), K (5%) and recently the addition of S (9%). Unpublished data from our project shows a significant response by signal grass (Brachiaria decumbens) and Hi-Gane paspalum to K on these lowly fertile soils. The results in trial showed that, even early in the cooler part of the dry season in the monsoon tropics, a nitrogen response is possible (Teitzel et al. 97). Minson (967) showed that applying N in autumn can delay the decline in nutritive value of tropical pasture at this time and improve intake and liveweight gains. For tropical forages, the critical dietary crude protein level is 7% (Milford and Minson 966) below which voluntary intake is depressed. In our study, nitrogen rates of 80 kg/ha N every 30 days were required to produce pasture crude protein concentrations of the whole plant above 7% but only when the fields were not waterlogged. Lower rates of kg/ha N caused rapid growth of P. atratum, producing a dilution effect so that crude protein concentrations were similar to those in plants receiving no nitrogen. To maintain critical crude protein concentrations in the animal diets, farmers must apply nitrogen at least every 30 days or apply nitrogen less frequently and feed protein meal supplements. Suitable legumes that grow well and persist on these waterlogged soils have not yet been identified (Hare et al. 999a). Amounts of supplements to be fed will depend on the type of animal product being produced and on grazing management. Grazing animals select for leaf and in our unpublished grazing trials the crude protein concentrations in P. atratum leaves have averaged 0% over the wet season and the stems 6%. These pastures had a leaf to stem ratio of 60 to 40% (fresh weight basis) and they were fertilised with 25 kg/ha N every 28 days. Therefore, animals grazing frequently fertilised pastures probably will not often experience low dietary crude protein levels as they will select for leaf, but animals fed whole plants in the yard may, as they have less choice of diet. Dairy farmers in north-east Thailand usually cut and carry forage to their yarded cows, but avoid dietary protein problems by feeding protein meal supplements to their cows to maintain milk yields. However, beef and draft animals are commonly grazed on pasture and farmers usually do not feed them supplements, so these animals may experience low dietary crude protein levels if pastures are not fertilised regularly. We have observed that animals do not readily graze P. atratum pastures that are fertilised infrequently with low nitrogen rates, even if the pastures are still leafy and young (30 40 days growth). They preferentially graze annual grass weeds before finally grazing P. atratum. This grazing behaviour may reflect different N concentrations in different components of the pasture. Advanced maturity of P. atratum (60-day growth), while producing significantly more dry matter than less mature plants (30-day growth), produced forage, on average, 2 units lower in crude protein concentration. We have observed that dairy cows are reluctant to graze mature P. atratum pastures and there has been criticism in other parts of Thailand that dairy cows find P. atratum unpalatable. These pastures may be too mature, low in nitrogen and highly fibrous. However, buffalo and native beef cattle in villages readily graze P. atratum. Farmers must decide whether better quality and possibly better animal acceptance of pasture is more important than producing large quantities of poor quality pasture. The application of cow manure to P. atratum was not effective on these lowly fertile soils, even though the rates of 3 and 6 t/ha were equivalent to 79 and 58 kg/ha N, respectively. The reasons for this remains unclear, but one hypothesis is that the nitrogen in the cow manure failed to dissolve and become available to the plants. When cow manure was applied with 32 kg/ha NPK fertiliser (5:5:5), dry matter yields of P. atratum were increased by 35 40% above yields produced by similar N:P:K rates without cow manure. The cow manure may have prevented the

7 Effect of nitrogen on production of Paspalum atratum on seasonally wet soils in north-east Thailand 7 rapid leaching of nitrogen from the low organic matter soil, making it available for a longer period during the 45-day interval between fertiliser application and cutting. We observed that P. atratum plants receiving cow manure with 32 kg/ha NPK were very green at harvest, whereas plants in other plots showed signs of yellowing. Use of cow manure on pastures in north-east Thailand is rare. It is used mainly on vegetable gardens, around fruit trees and, if abundant, on rice fields before planting. On some dairy farms, cow shed effluent is applied to pastures. While our results show minimal short-term impact on pasture DM yields, longer-term studies are needed to assess effects of the manure on soil structure and fertility. Nitrogen application to P. atratum pastures growing on soils with low N levels which are periodically waterlogged in north-east Thailand, will improve gross production, with some increase in crude protein concentration. Nitrogen can be applied strategically to improve autumn production or may be used throughout the wet season. The frequency with which pastures should be grazed will depend on the relative importance of quality or quantity of pasture produced plus the willingness of the farmer to provide protein supplements. Economics will be the ultimate determinant. Acknowledgements We thank the Thailand Research Fund (TRF) for providing financial support to this research program and the Faculty of Agriculture, Ubon Ratchathani University for research facilities. We also thank Mrs Chaisang Phaikaew for advice during the program and Mr Sukri Saipraset for technical assistance. References HARE, M.D., THUMMASAENG, K., SURIYAJANTRATONG, W., WONGPICHET, K., SAENGKHAM, M., TATSAPONG, P., KAEWKUNYA, C. and BOONCHARERN, P. (999a) Pasture grass and legume evaluation on seasonally waterlogged and seasonally dry soils in north-east Thailand. Tropical Grasslands, 33, HARE, M.D., BOONCHARERN, P., TATSAPONG, P., WONG- PICHET, K., KAEWKUNYA, C. and THUMMASAENG, K. (999b) Performance of para grass (Brachiaria mutica) and Ubon paspalum (Paspalum atratum) on seasonally wet soils in Thailand. Tropical Grasslands, 33, HUMPHREYS, L.R. (987) Tropical Pastures and Fodder Crops. 2nd Edn. Intermediate Tropical Agriculture Series. (Longman Scientific and Technical: England). KALMBACHER, R.S., PATE, F.M., MARTIN, F.G. and KRET- SCHMER, A.E.Jr (997) Supplementation of diets of weaned steers grazing Suerte Paspalum atratum. Soil and Crop Science Society of Florida Proceedings, 56, MILFORD, R. and MINSON, D.J. (966) Intake of tropical pasture species. Proceedings of the XI International Grassland Congress, Brazil, 964. pp MINSON, D.J. (967) The voluntary intake and digestibility in sheep, of chopped and pelleted Digitaria decumbens (pangola grass) following a late application of fertiliser nitrogen. British Journal of Nutrition, 2, MITSUCHI, M., WICHAIDIT, P. and JEUNGNIJNIRUND, S. (986) Outline of soils of the Northeast Plateau, Thailand. Their characteristics and constraints. Technical Paper No.. Agricultural Development Center in Northeast: Khon Kaen, Thailand. TEITZEL, J.K., MCTAGGART, A.R. and HIBBERD, M.J. (97) Pasture and cattle management in the wet tropics. Queensland Agricultural Journal, 97, WHITEMAN, P.C. (980) Tropical Pasture Science. (Oxford University Press: Oxford). (Received for publication??????; accepted??????)