Soil-Plant-Animal Relationships in Molybdenum- and Copper-Fertilized Forages

Ishizaki, Stanley M.
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Molybdenum and Cu are essential micronutrients for both plants and animals. Traditionally, previous studies throughout the world have involved the fertilization of acid, highly weathered, infertile soils. Many experiments have been concerned with either Mo or Cu, and those that included both were inconclusive or contradictory. Earlier investigations were concerned with soilplant or plant-animal relationships, and there have been none as an integrated soil-plant-animal system. This experiment, therefore, was conducted primarily to study the effects of Mo and Cu fertilization on forages grown in a productive agricultural soil, and to assess the relationships between soil, plants, and animals. In essence, the fate of applied Mo and Cu was followed in the soil, through the plant, and finally through animals. In order to accomplish these goals, pangolagrass (Digitaria decumbens Stent.) and intortum (Desmodium intortum (Mill.) Urb.) were fertilized with four rates of Mo and three rates of Cu. Soil and plant samples were collected and analyzed at regular intervals, and intortum was cured and fed to animals. Animal response consisted of nutrient digestibilities, blood Mo and Cu accumulation, and excretory Mo and Cu. Plant analyses included ash, crude protein (CP), ether extract (EE), neutral detergent fiber (NDF), acid detergent fiber (ADF), lignin (L), cellulose (C), and gross energy (GE). Plant minerals included P, K, Ca, Mg, Na, Mn, Fe, Cu, Zn, and Mo. Forage production was measured in terms of dry matter (DM) and CP yields. Statistical analyses were designed to determine the effects of Mo and Cu applications on soil, plant, and animal variables, and to study the relationships between soil, plant, and animal responses. The extractable soil Mo concentrations of both pangolagrass and intortum plots increased linearly with increasing Mo applications, and the coefficients of determination were 0.93 for both. The extractable soil Cu concentration of pangolagrass plots followed a curvilinear relationship, with a Mo x Cu interaction, while that of intortum was linear. Coefficients of determination were 0.73 for pangolagrass and 0.75 for intortum. Pangolagrass Mo concentration increased curvilinearly with Mo application (r2 = 0.32) with a Mo x Cu interaction. The plant Mo concentrations were adequate for plant growth, and below threshold levels of molybdenosis in animals. The Cu:Mo ratio at the higher levels of Mo fertilization were less than 2:1, and potentially molybdenotic. Pangolagrass Cu concentration was not influenced by Cu application but followed a curvilinear response pattern, and was adequate for plant growth. However, these amounts were considered marginal for animals, and a Cu supplement was suggested for animals maintained solely on this forage. A Mo x Cu interaction at the highest rate of Mo decreased pangolagrass Cu, and was attributed to the formation of insoluble cupric molybdate in the soil. Pangolagrass DM yield was extremely low and decreased slightly with Cu application. The CP, NDF, L, and GE concentrations followed inconsistent response patterns. The low yields and erratic patterns of response may have been due to the severe drought which prevailed during the entirety of the experiment. Plant analyses indicated that pangolagrass grown under the conditions of this experiment required no Mo or Cu fertilization. Intortum Mo increased curvilinearly with Mo application, and the coefficient of determination was 0.57. A slight Cu:Mo antagonism at the highest rate of Mo application was attributed to the ability of intortum to extract more Mo than Cu from the soil, and may have been related to the lack of water due to a drought. Intortum Cu concentration increased with Cu application, but the trend was not consistent. Intortum Mo and Cu concentrations were considered adequate for plant and animal nutrition; however, the Cu:Mo ratio at the highest Mo rate was less than 2:1, suggesting that Cu supplementation may be necessary for animals maintained solely on this forage. Intortum DM yield did not respond to Mo or Cu application, but GE concentration increased with Mo application and followed inconsistent trends. The severe drought was probably a contributing factor in these inconsistencies and reduced yields. It was generally concluded that intortum produced under these conditions did not require Mo or Cu fertilization for proper plant and animal nutrition. It appears that the plants were able to extract sufficient amounts of these nutrients from the soil. Digestibilities of OM, CP, NDF, ADF, L, C, and GE of intortum not influenced by Mo or Cu application. Molybdenum application increased blood Ca and Mo accumulation, but had no effect on other minerals. The blood Mo concentration of all treatment levels was below toxic levels. Molybdenum concentration in excreta increased with Mo application. When excreted Mo was expressed as percent of dietary intake, the inconsistent response patterns were attributed to the formation of insoluble Mo-Cu-S complexes which resisted absorption. When soil-plant relationships were examined, the regression equation Pm = 0.01 + 0.59Sm was developed to estimate pangolagrass Mo (Pm) from extractable soil Mo (Sm). In the case of intortum Mo (Im), the equation was Im = -0.97 + 0.97Sm. The coefficients of determination were 0.67 and 0.93 for pangolagrass and intortum, respectively. Analyses of plant-animal relationships justified intortum Mo (Im) as a "predictor" of fecal Mo (Fm) by the equation Fm = -0.13 + l.55Fm (r2 = 0.97). Although there were significant relationships between other soil-plant, soilanimal, and plant-animal variables, the coefficients of determination were extremely low, and precluded their use in regression equations. The relationships between animal Mo response variables and intortum Mo were not as close when expressed as percent of Mo ingested, and was attributed to physiological Mo-Cu-S interactions. It was proposed that the absorption, retention, and excretion of Mo and Cu were influenced by the formation of Mo-Cu-S complexes in the rumen. Insoluble cupric molybdate and copper thiomolybdate resist absorption through intestinal membranes, and are absorbed and excreted as units. The reabsorption of Mo and Cu can also be decreased by the formation of cupric molybdate at the membranes of kidney tubules. Consistent soil-plant-animal relationships for Mo were evident for applied Mo, extractable soil Mo, intortum Mo, and excreted Mo. Significant relationships were noted for: (1) extractable soil Mo with intortum Mo concentration (r2 = 0.93); (2) extractable soil Mo with fecal, urinary, and fecal + urinary Mo concentrations (r2 = 0.96, 0.72, and 0.96, respectively); (3) intortum Mo with fecal, urinary, and fecal + urinary Mo concentrations (r2 = 0.97, 0.72, and 0.97, repectively); and (4) applied Mo with fecal, urinary, and fecal+ urinary Mo (r2 = 0.98, 0.87, and 0.98, respectively).
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