Effects of Different Nitrogen Dose and Sources as Top-Dressing on Yield and Silage Quality Attributes of Silage Maize

: Effects of different nitrogen doses and sources applied as top-dressing on dry matter yield and quality of maize silage were investigated in this study. Along with 7.5 kg N da -1 application as starter at sowing in the form of 15-15-0+Zn, nitrogen doses of 0, 7.5, 15 and 22.5 kg da -1 were applied as top-dressing at 6-leaf stage of plants in the forms of ammonium nitrate, urea, DMPP blended ammonium sulphate nitrate and urea with NBPT urease inhibitor. Experiments were conducted in randomized blocks-factorial experimental design with 3 replicates in 2013 and 2014. The greatest dry matter yield were obtained from 15 and 22.5 kg N da -1 in 2013 and from 22.5 kg N da -1 in 2014. Nitrogen sources had also signifi cant effects on dry matter yield. According to the average of two years, both DMPP blended ammonium sulphate nitrate and urea with NBPT urease inhibitor increased yield respectively by 7 and 3% as compared to ammonium nitrate and urea fertilizers. Nitrogen doses signifi cantly improved the silage quality attributes. Nitrogen treatments increased silage protein ratio and decreased ADF and NDF ratios of silage samples. The greatest relative feed value was obtained from 15 kg N da -1 treatment. It was concluded based on present fi ndings that besides the nitrogen doses, nitrogen sources also signifi cantly improved yield.


INTRODUCTION
Maize is one of the most signifi cant roughage sources of animals. It is used either in fresh forms or as silage in animal feeding. Maize is quite available for machine-culture, has high unit area yields, can be cultured as a second crop in several regions, has high digestibility levels, provides quality and palatable source of silage and is ensiled without any needs for additives. Therefore, it is among the mostly preferred feed source both in Turkey and throughout the world (Celebi et al. 2010, Ileri et al. 2018. Maize is a C4-plant, thus has quite high dry matter productions. Maize uptakes much more nutrients from the soil than the other plants. Therefore, proper and well-balanced fertilization should be practiced over maize fields to get desired yield levels. Nitrogen is among the most important nutrients infl uencing maize yield and feed quality (Carpıcı et al. 2017, Fallah & Neisani 2017. Maize plants get into a rapid growth phase at 6-leaf stage and thus signifi cant increases are observed in water and nitrogen uses in this stage of growth (Rozas et al. 1999, Girma et al. 2011. Rapid nitrogen uptake is generally observed between V10-V14 stages (Mueller et al. 2017). Depending on environmental factors and genotypes, maize plants get about 65-70% of total nitrogen content at vegetative growth stage and get the rest from the soils after tassel formation (Gallais & Coque 2005, Mueller & Vyn 2016. Thus, sufficient nitrogen levels should be available in soils throughout the vegetative growth stages until tassel formation. However, nitrogen efficiency in maize culture is at quite low levels (<50%) (Gagnon et al. 2012). Majority of supplied nitrogen is lost through leaching, denitrification, volatilization, immobilization and soil erosion.
Special fertilizers are produced by the industry to reduce or prevent such losses. Stabilized fertilizers are among the most important ones of these special fertilizers (Trenkel 2010). N-(n-butyl) thiophosphoric triamide (NBPT) treated urea is one type of stabilized fertilizers. NBPT urease inhibitors control urease enzyme activity transforming urea into ammonium. They reduce hydrolysis of the urea, prevent abrupt increases in pH around the fertilizer granules and ultimately decrease NH 3 volatilization losses (Rozas et al. 1999). 3,4-dimethylpyrazole phosphate (DMPP) treated fertilizer is another type of stabilized fertilizers. DMPP nitrification inhibitor has quite low solubility in water and does not have any phytotoxic impacts on plants (Zerulla et al. 2001). This nitrification inhibitor even at small quantities may inhibit nitrification for about 6-8 weeks based on soil temperature and bacteriastatic impacts on Nitrosomanos bacteria (Zerulla et al. 2001, Villar & Guillaumes 2010. So, DMPP nitrification inhibitor is commonly used in nitrogenous fertilizers. Different researchers indicated that DMPP inhibitor significantly diminished nitrogen losses through denitrification and nitrate leaching (Wu et al. 2007, Liu et al. 2013, Martinez-Alcantara et al. 2013.
Various researches have been conducted about the effects of different nitrogen sources and stabilized fertilizers on maize yield and quality (Gagnon et al. 2012, Motavalli et al. 2013, Abalos et al. 2014, Halvorson & Bartolo 2014, Mota et al. 2015. However, results varied largely based on climate, soil factors and management practices. Therefore, in this study, effects of different nitrogen doses and nitrogen sources applied as top-dressing at 6-leaf stage on silage maize yield and silage quality were investigated. In this way, efficiency of increasing doses of different nitrogen sources applied at a certain growth stage with the greatest need for nitrogen was compared.

Experimental site
Experiments were conducted over the experimental fields of Agricultural Research and Implementation Center of Erciyes University-Kayseri located between 39° 48' North latitudes and 38° 73' East longitudes and at an altitude of 1053 m in the years 2013 and 2014. According to long-term averages for climate parameters of the experimental site, annual average total precipitation is 391 mm and annual average temperature is 10.6 o C. Monthly minimum, average and maximum temperatures, monthly total precipitation and monthly average relative humidity values throughout the vegetation periods of the experimental years are given in Table I. Experimental soils were sandy-loam in texture. Soils were almost neutral in pH (7.6) and unsaline (EC=0.183 dS m 1 ). Organic matter content was 0.5%, lime content was 1.27% and available phosphorus was 10.159 kg P 2 O 5 /da.

Experiment
Experiments were conducted in randomized blocks-factorial experimental design with 3 replicates. NK Atria dent corn variety of Syngenta was used as the plant material of the study. As top-dressing, 33% ammonium nitrate (AN), 46% urea (U), DMPP blended ammonium sulphate nitrate (DMPP+ASN, trade name ENTEC 26) and NBPT blended urea (NBPT+Urea, Trade name UTEC 46) fertilizers were used. Before sowing, 7.5 kg N da -1 was applied to all plots in the form of 15-15-0+Zn and were incorporated into soil with a cultivator. Sowing was performed on 09 May 2013 and 1 May 2014. About 70 cm spacing was provided between the plots and 1 m spacing was provided between the blocks. Plot lengths were arranged as 3 m. Six rows were opened with a hand marker adjusted at 70 cm. Sowing was performed as to have two seeds in every 20 cm and thinning was performed after emergence. Sprinkler irrigation was performed for homogeneous emergence. In addition to 7.5 kg da -1 starter nitrogen, 0, 7.5, 15 and 22.5 kg da -1 (N0, N7.5, N15, N22.5) nitrogen were supplied in the above-specified forms 35-40 days after sowing through scatter in between the rows as top-dressing and experimental site was irrigated with drip irrigation. Then based on plant moisture status, irrigations were performed in 10-day intervals.
Harvest was performed manually at 50% milk-dough stage of the kernels on 28 August 2013 and 27 August 2014. Two rows from each side and 50 cm sections from the top and bottom of each plot were omitted as to consider side effects and 10 plants were harvested from the remaining sections of each plots. Harvested plants were weighed to get green herbage yield of the plots. Two plants were selected from each plot as to represent the plot; they were freshly weighed; separated into stem, leaf and ear; wilted for a while; dried at 65 °C until a constant weight and weighed again. Leaf, stem and ear ratio of dry herbage were determined. Green herbage yields were multiplied by dry matter ratios to get dry matter yields. Fresh plants of each plot were chopped into 2-3 cm pieces and ensiled into 3 kg vacuum bags. Silages were kept at room temperature for 60 days. They were opened, emptied into appropriate containers, 150 g samples were taken from each silage bag and samples were dried at 65 °C until a constant weight to get dry matter ratios. Another 40 g samples from each silage bag were shaken in 360 ml distilled water, filtered through and pH of filtrates was measured with a pH meter. With the aid of dry matter ratios and pH values of silage samples, Flieg points were calculated by using the equations provided in Oten et al. (2016) derived from Kılıç (1986). Dried silage samples were ground to pass through 1 mm sieve. Nitrogen content of ground samples was  (1978).

Statistical analysis
Experimental data were subjected to variance analysis with "SPSS for Windows" software in accordance with randomized blocks-factorial experimental design. Significant means were identified with F-test and means were grouped with the aid of Duncan's multiple range test. Initial variance analysis was performed on combined years, and then variance analysis was performed separately for each year since the years were found to be significant in initial analysis. Present dry matter yields varied with the nitrogen doses. The greatest dry matter yields were obtained from N15 and N22.5 treatments (2270 and 2216 kg da -1 ) in 2013 and from N22.5 treatment (2469 kg da -1 ) in 2014 (Table II). Dhital & Raun (2016) indicated that yield and nitrogen respond of maize might vary in the same location from year to year and such variations were mostly attributed to unexpected changes in environmental conditions.

RESULTS AND DISCUSSION
As compared to the classical urea, NBPT+urea and DMPP+ASN fertilizers increased dry matter yields respectively by 1.6 and 4.3% in the first year and by 5 and 2.5% in the second year; as compared to ammonium nitrate, NBPT+urea and DMPP+ASN fertilizers increased dry matter yields respectively by 8.78 and 11.67% in the first year and by 6.44 and 3.84% in the second year. In a previous study carried out with maize, Pasda et al. (2001) reported that DMPP+ASN treatments increased kernel yield by 2.6% as compared to ASN. Gagnon et al. (2012) reported that urea, polymer-coated urea and DMPP inhibitor urea effected kernel yields differently in different years. Abalos et al. (2014) carried out a metaanalysis and reported that inhibitor fertilizers (DMPP, DCD and NBPT) increased yields by about 7.5%, fertilizer efficiency varied based on environmental and management factors and was greater in coarse-textured soil and irrigated conditions. Pasda et al. (2001) indicated that yield increase achieved with nitrification inhibitor was not resulted solely from prevention of nitrogen losses, but also from ammonium nutrition. Huffman (1989) indicated that NH 4 + and NO 3 forms of nitrogen could be used in maize culture, the plant response to combined ammonium and nitrate nutrition was better than the plant response to nitrate alone and in this sense, nitrification inhibitors were significant in prolonging ammonium nutrition durations. When the classical fertilizers were compared within themselves, Urea showed significantly higher yield over AN in 2013. The difference between Urea and AN was not significant in 2014. Present findings comply with the results of Biswas & Ma (2016) who reported an inconsistent effect of N source on yield and N use efficiency indices of maize.
With regard to dry matter yields, fertilizer source x dose interaction was found to be significant (P<0.05) in the first year and insignificant in the second year. The greatest dry matter yield was obtained from N15 dose of DMPP+ASN fertilizer and the lowest dry matter yield was obtained from N7.5 dose of AN fertilizer (Figure 1). While stabilized fertilizers had greater yields at high N doses, urea fertilizers had high yield at N7.5 treatment. Regression equations revealed that in 2013, the maximum yield in AN treatments (2083 kg da -1 ) was obtained from 18.33 kg N da -1 dose, in urea treatments (2261 kg da -1 ) from 17.50 kg N da -1 , in DMPP+ASN treatments (2419 kg da -1 ) from 19.86 kg N da -1 and in NBPT+urea (2336 kg da -1 ) from 19.56 kg N da -1 dose. In 2014, the maximum yield in AN treatments (2428 kg da -1 ) was obtained from 25.21 kg N da -1 dose, in urea treatments (2386 kg da -1 ) from 18.01 kg N da -1 , in DMPP-ASN treatments (2513 kg da -1 ) from 23.25 kg N da -1 and urease inhibitor treatments (2566 kg da -1 ) from 20.48 kg N da -1 dose ( Figure 2). Leaf ratio of the years 2013 and 2014 was respectively measured as 16.28 and 17.59%. The greatest stem ratios were observed in N0 treatments (47.32 and 36.20%) and nitrogen treatments reduced stem ratios. The greatest ear ratio was observed in DMPP treatment (45.93%) in the first year and the differences in ear ratios of nitrogen sources were not significant in the second year. The lowest ear ratio was observed in N0 treatments of both years and nitrogen treatments increased ear ratios as compared to the control. In silage maize culture, majority of herbage yield and nutritional values come from the ears (Coors et al. 1997). Therefore, high ear ratios are desired at harvest. In present study, stem ratios decreased and ear ratios increased with nitrogen treatments. Celebi et al. (2010) reported the greatest stem ratio and lowest ear ratio for the control plots without any nitrogen treatments. Akar et al. (2014) indicated that maize plants exhibited a normal growth with sufficient nitrogen levels and such levels speeded up ear kernel formation, increased ear ratio and thus reduced stem ratio, but plants were forced to bloom earlier, had shorter growth periods and thus quite less number of kernels formed at the end of ears under deficit nitrogen conditions.
The greatest protein ratios in both vegetation periods were observed in N22.5 treatments (6.98 and 8.66%) and the lowest ratios were observed in N0 treatments. Protein ratios increased with increasing nitrogen doses (Table III) increasing nitrogen doses or increasing nitrogen content of plant tissues. RFV is commonly used to estimate feed intake and energy values and it is a compounded index of ADF and NDF. Feed RFV values are classified under 6 categories and the feeds with RFV>151 are classified as prime class feeds (Rohweder et al. 1978). Nitrogen doses significantly influenced relative feed values and the greatest RFV was obtained from N15 treatments of both years (170.86, 155.94).
Silage pH values varied between 3.84 -3.97% and both the treatments and the years did not have any significant effects on pH values. Flieg point is used as an easy way of assessment for silage quality attributes. Flieg point is calculated based on pH and dry matter ratio and all factors effecting pH and dry matter ratio thus directly influence Flieg points (Oten et al. 2016). As compared to the control, nitrogen treatments increased Flieg point of silage samples.

CONCLUSIONS
Considering the entire findings together, it was concluded that DMPP+ASN and NBPT+Urea fertilizers applied as top-dressing increased dry matter yields as compared to classical fertilizers, but fertilizer sources did not have any significant effects on investigated silage quality parameters. As compared to the control, stem ratios decreased and ear ratios increased with increasing nitrogen doses. Nitrogen treatments increased dry matter yield, silage protein ratios and decreased ADF and NDF ratios of silage samples. It was finally concluded based on present findings for yields and quality attributes that 7.5 kg N da -1 as starter fertilizer at sowing and 15-22.5 kg N da -1 as top-dressing at 6-leaf stage of plants significantly improved yield and quality and stabilized fertilizers as the source of nitrogen significantly improved yields.