Effect of plant density on yield and quality of forage maize
--L. Bertoia, R. Burak and A. Nivio

During 1990/91, 91/92 and 92/93 field trials were carried out in the boroughs of San Vicente and Cañuelas, Buenos Aires Province to evaluate the effect of 3 plant densities (60, 80 and 100 thousand pl/ha) on the yield of dry matter from ear (EDMY) and stover (SDMY), on plant height (ALT), and on the percentage of the morphological composition of the plant (% of ears or harvest index (HI)). Among 16 hybrids per trial for this work, information from 5 hybrids that were repeated for 3 years was taken into account: Cargill Record 160, Cargill Semiden 5, Funk's Tronador, Morgan 506 and Pioneer 3456.

The planting was on typical argiudol soil. The design used was random blocks with 4 replications and a factorial arrangement of 5 hybrids x 3 densities x 3 environments. ANOVA was carried out using a mixed model and the Tukey test (5%) for multiple comparisons. The obtained values demonstrate that the increase in density produced a significant increase in the total DM production, giving means of 19270, 20830 and 22710 kg/ha for 60, 80 and 100 thousand pl/ha respectively. The analysis of the results permitted the adjustment to a first grade lineal response of the TDMY to a plant density within the evaluated interval. The trait EDMY showed significant differences between the values of 60 and 100 thousand pl/ha (10070, 9335 and 8703 for 100, 80 and 60 thousand pl/ha). In the analysis of HI (EDMY/TDMY) significant differences due to the effect of plant density were not recorded: 0.4438, 0.4424 and 0.4362 for 60, 80 and 1000 pl/ha respectively. Comparing hybrids, significant differences were seen only between Cargill R160 (47.86%) vs. Cargill SD5 (43.57%), Pioneer 3452 (42.36%) and Morgan 506 (41.76%); for Funk's Tronador (44.81%) significant differences with the rest of the participants were not shown. Effect of density upon the height of the plants was not observed (range: 2.37 to 2.35 m).

In analysing the environmental effect significant differences appear for ALT and HI. The interaction hybrids x environment was significant for the traits EDMY, SDMY and TDMY, demanding an analysis of these variables per environment.

The results obtained indicate that all the hybrids produced an increase of TDMY with greater densities without modifying the morphological composition of the plant. The characteristics of the environment produced a differential response of the evaluated materials.

In agreement with these results, the plant density used in the area of influence (60 thousand pl/ha), reaching approximately 50 thousand plants per ha at harvest is lower than the adequate amount for hybrids commonly used in silage production.

In conjunction with the analysis of the effect of the plant density upon the DMY undertaken in the first part, its effects upon 3 very important parameters for establishing the quality of maize destined for silage were also assessed in a lab: concentration of soluble carbohydrates,% protein and digestibility of the DM from ears and stover in the same 5 commercial hybrids.

Soluble carbohydrates--This is considered fundamental to reach adequate conservation of the pierced material quickly. The concentration of soluble carbohydrates of DM coming from the whole plant was not affected by the three densities evaluated (19.8%, 19.23% and 19.07% for 60, 80 and 100 thousand pl/ha respectively). Significant differences were found for the five analysed hybrids (range: 19.73% to 21.12%). Also differences were produced between the evaluated environments (range: 18.02% to 20.99%). The analysis of the plant components separately showed that in the case of ears and stover the density did not affect concentration of soluble carbohydrates (range: 26.06% to 26.89% for ears and 13.54% to 13.63% for stover). It was found that in both cases the interaction of hybrids and environment was significant.

Protein--The concentration of proteins in whole plants did not show differences between hybrids or densities. On the other hand, modifications due to environmental effects were produced. The interactions of hybrids x enviroments, hybrids x densities and densities x environments were not significant. On analyzing the components of the plant significant differences were found only for environmental effect on stover (range: 4.40% to 5.57%) and on ears (range: 6.52% to 7.28%). In the later there were also differences between hybrids (range: 6.68% to 7.25%).

Digestibility--The digestibility of the ear was affected by the density. Significant differences were found between 100 thousand pl/ha (78.77%) vs. 80 thousand pl/ha (81.20%) and 60 thousand pl/ha (80.82%). In the stover fraction significant differences were not found in the percentage of digestibility due to density. In both components the interaction of hybrids x environment was significant. The digestibility of the DM of the whole plant was not affected by the increase in density (66.44%, 67.87% and 68.06% for 100, 80 and 60 thousand pl/ha respectively).

The yield in digestible DM of the whole plant is the product of the total DM yield by the digestibility. The statistical analysis of this trait showed significant differences among the five hybrids evaluated (rank: 14,080 to 15,630 kg/ha); on the other hand, the density of 100,000 pl/ha produced a yield of 16,010 kg/ha of digestible DM, a value which significantly surpasses those attained by the density of 80 thousand pl/ha (14,650 kg/ha) and 60 thousand pl/ha (13,870 kg/ha). In accordance with these results and those obtained in the first part of this work, the greater densities employed produced significant increases in digestible DM, without varying the concentration of soluble carbohydrates and proteins of the different fractions which make up the plant. 

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