VEGETATIVE DEVELOPMENT AND GROWING DEGREE-DAYS OF TROPICAL AND WINTER FORAGES

ABSTRACT One of the limitations to producing irrigated forages is setting suitable biometric parameters to establish the entrance of animals to grazing areas. Such parameters can be measured or even estimated. Estimates are advantageous for being practical and able to be used for grazing optimization. This study aimed to evaluate the correlation among growing degree-days (GDD), canopy height (CH), leaf area index (LAI), and number of cycle days (NCD) of Guinea grass (Panicum maximum cv. Mombaça) and Bermuda grass (Cynodon spp. Tifton 85) single cropped for one year and overseeded with black oat and ryegrass in the autumn-winter season. The study was carried out from February 2016 to February 2017, and forage crops were sprinkle irrigated. LAI and CH were measured twice a week, at intervals of 3 and 4 days. These parameters were correlated to GDD accumulation at each cutting cycle (CC). All the correlations showed high coefficients in linear fits. For spring-summer cycles, Guinea grass in exclusive cultivation showed the highest averages for CH and LAI, being 102.3 cm and 5.93, respectively. For autumn- winter cycles, when this grass was overseeded, it showed an LAI value similar to that when single cropped, which was of about 5.6. Yet single Bermuda grass presented lower values of LAI (one unit lower) and CH (16.4 cm lower) in the autumn-winter season when compared to that overseeded. The spring-summer cycles of Guinea grass had a duration of 24 days, after which no increase in leaf production was registered.摘要 灌溉饲草生产的核心限制因素之一，是确定适配的生物参数以规范牲畜进入放牧区域的流程。此类参数可通过实测或估算获取，其中估算方法操作简便，可用于优化放牧管理方案。 本研究旨在评估两类饲草的相关指标相关性，涉及生长度日（GDD）、冠层高度（CH）、叶面积指数（LAI）与循环天数（NCD）：供试饲草为姆巴卡萨大黍（Panicum maximum cv. Mombaça）与狗牙根（Cynodon spp. Tifton 85），分别设置单作一年、以及秋冬季节套播黑燕麦与黑麦草的处理。 本试验于2016年2月至2017年2月开展，供试饲草均采用喷灌方式供水。研究每周两次测定叶面积指数与冠层高度，测定间隔分别为3天与4天，并将上述参数与各刈割周期（CC）内的生长度日累积量进行相关性分析。 所有相关性分析的线性拟合系数均较高。在春夏刈割周期中，单作大黍的冠层高度与叶面积指数平均值最高，分别为102.3 cm与5.93。而在秋冬套播周期中，该品种大黍的叶面积指数与单作时相近，约为5.6。单作狗牙根在秋冬季节的叶面积指数较套播组低1个单位，冠层高度则较套播组低16.4 cm。大黍的春夏刈割周期时长为24天，该周期结束后未检测到叶片产量的进一步提升。