TRINEXAPAC-ETHYL REDUCES PLANT HEIGHT AND SEED YIELD IN FORAGE SORGHUM

Forage sorghum presents tall plants, what makes mechanical seed harvesting a difficult task. Plant hormones can reduce plant height and facilitate the harvesting. Thus, the objective of this study was to evaluate the effect of the growth regulator trinexapac-ethyl on plant height and seed yield in forage sorghum. This research was carried out at Embrapa Maize and Sorghum, in two seasons (2017 and 2018), in a randomized complete block design, four replications and three varieties (BRS Ponta Negra, 1141574 and 0947216) under application of Trinexapac-ethyl in two stages of the plant growth (V8, V12, V8 + V12). The characteristics evaluated were: plant height, seed yield and mass of 1000 seeds. The effect of the hormone was significant in both stages of growth, with a significant reduction in plant height. The hormone applied twice at growth stages V8 + V12 had a greater effect in reducing plant height. However, the hormone reduced seed yield in the three varieties, what implies the need for further studies comparing the advantage of plant height reduction and the loss in seed yield.

The forage demand for animal feeding has significantly grown in the market, due to the increase in exports of meat and dairy products.
In this sense, the use of silage for cattle feeding during the off-season is an excellent option.
Maize and sorghum are the most used crops for silage purpose (Rodrigues et al., 2015). In Brazil, there are excellent forage sorghum cultivars, adapted to the tropical climate, with high dry matter yields, helping to meet the demand and balance of high quality silage.
Sorghum, in general, presents some advantages of use, linked to the reduced interval of feed supplying, during the seasonality of the pastures, due to the lesser rainfall regime of the period fall/winter. The first of them is the feed supplying when there is a poor production of other crops during the off-season. The other advantage is that sorghum presents a greater adaptability when compared to other crops, presenting a greater yielding efficiency under adverse conditions, such as under water stress and in different environments (Magalhães et al., 2007;Santos et al., 2013;Carvalho, 2017).
Among the physiological mechanisms of sorghum adaptation to overcome water stress, its photosynthetic efficiency and deep root system stand out (Magalhães et al., 2014).
Silage sorghum plants have strong stalks, height of 2.5 m, with a good balance in the grain/plant ratio and high dry matter yield, which guarantees high quality forage. This type of sorghum still shows good digestibility, which reflects in animal nutrition, being an excellent forage, used in cattle feedlots, and especially in dairy production (Rodrigues et al., 2015).
The expansion of livestock in feedlot regimen causes stockmen to pay attention to the need for silage storage, in order to feed the livestock with the necessary nutritional attributes throughout the year (Nardes, 2019). Thus, silage is a forage storage technique that allows for the conservation of good quality feed (Paula, 2016). The ensiling process is done by crushing the green mass of the plant, making the storage in the silo, compaction, sealing and ingestion by animal's easier, increasing palatability (Senger et al., 2005;Dunière et al., 2013).
For the production and multiplication of seeds of forage cultivars, the tall plant makes the harvesting process difficult, which only aims at cutting the panicle, where the seeds lie. As the seed approaches physiological maturity, the panicle becomes heavier; the plant stalks dry and thus, the plants become more susceptible to lodging and/or breakage (Rodrigues et al., 2015). Therefore, an alternative to improve the efficiency of mechanized harvesting of forage sorghum cultivars is the use of plant growth regulators applied to leaf area of the plants in order to reduce their height. However, according to Rademacher (2000), the use of hormones to reduce plant height must be well regulated, to avoid a loss in yield of both seed and green mass.
There is little information about the use of growth regulators in sorghum plants, mainly related to the timing of application. Therefore, the aim of this study was to evaluate the effect of plant growth regulator (trinexapaque-ethyl) on three forage sorghum cultivars, aiming to reduce plant height to help seed harvesting in production fields.

Material and Methods
The experiment was carried out in two The characteristics evaluated were: plant height, measured in cm, between the soil and the apex of the panicle; seed yield, measuring the grain mass, being corrected to 13% moisture and later extrapolated to kg ha -1 ; and mass of a thousand grains, where a thousand grains were randomly chosen and the mass determined with the aid of an analytical balance.
The data were subjected to analysis of variance. When significant, the values were compared by using Tukey's test (p ≤ 0.05). The analyses were performed with the help of the Sisvar software version 5.3 (Ferreira, 2011).

Results and Discussion
There was a significant effect for the source of variation years for the characteristics plant height and a thousand-seed mass. This effect is related to the dose of the hormone that was higher in the second year. The source of variation hormones was significant for seed yield and plant height, indicating the influence of the hormone on these characteristics. The varieties performed differently for the three evaluated characteristics ( Table 1).
The interaction years x hormones was significant for seed yield and plant height, showing that the hormone action was different between the years of evaluation, which was expected, since there was an increase in the dose of 200 g a.i. ha -1 to 250 g a.i. ha -1 from the first to the second year of planting.
On the other hand, the interaction years In Figure 1, the means of the plant heights of the four treatments studied (no-hormone, hormone applied in stages V8, V12 and in V8 + V12) are observed. It is verified that plant height was affected by the application of the hormone as soon as in V8 stage, the reduction being even more significant when applied in V8 + V12. In this case, there was a 40% reduction in plant height in the two applications (V8 + V12) of trinexapac-ethyl compared to control without application. the hormone, the application in V12 was more efficient than when applied in V8, so when it is impossible to do two applications, we must choose a later stage to perform the application In rice, the hormone trinexapac-ethyl presented a negative effect on grain yield and quality (Silva, 2009    to the use of doses greater than 150 g ha -1 of the active ingredient of the product, and that this effect is dependent on crop. Regarding the means of the three varieties, the mass of one thousand grains of forage sorghum submitted to the application of trinexapac-ethyl did not present any significant difference ( Figure   5). Similar results obtained by Alvarez et al. (2007) showed that the mass of one-thousand rice grains also resulted in a non-significant difference between treatments.
In the analysis of each variety, genotype 0947216 showed a small reduction in the mass    grasses seeking to shorten the plants and reduce lodging. In red clover, in doses of up to 0.42 kg of active ingredient per hectare, no reduction in seed germination occurred (Kirk et al., 2016).
In ryegrass, the application of trinexapacethyl at inflorescence emergence stage did not reduce seed vigor and germination, whereas in the early flowering phase it reduced seed vigor and germination by 14% and 10%, respectively (Schaeffer, 2020). It seems that the effect of the hormone on seed production and quality is dependent on the crop, dose and stage of application of the product. Studies about the effect of trinexapac-ethyl on the quality of sorghum seeds are necessary to support the results found in this work, and to help seed companies in making decision.

Conclusions
The application of the growth regulator trinexapac-ethyl significantly reduces the height of forage sorghum plants.
The hormone is most efficient when applied twice in stages V8+V12.
The hormone negatively affected seed yield, which must be carefully reviewed in order to make the correct decisions regarding the use or not of the hormone, although its effect may vary from one variety to another.