Quality Traits in Forage Sorghum Harvested at Early Head Emergence and at Physiological Maturity

Information about the extent of variation in quality traits among plants sampled at the same developmental stage but on different dates would be useful to forage researchers. The primary purpose of this study was to obtain such information for five forage sorghum [Sorghum bicolor (L.) Moench] cultivars. Percent dry matter, crude protein, and in vitro dry matter disappearance of leaf, stem, and whole plant samples were determined for field-grown samples harvested on three different days during early head emergence (EHE) and once during physiological maturity (PM) for each cultivar. Orthogonal comparisons between EHE and PM sampling stages and among the· EHE samples were made. Results of the 2-year study indicated that statistically significant differences existed among EHE samples and between EHE and PM samples. However, differences among EHE samples were generally much smaller than those between EHE and PM samples and may be of little importance for some applications. Additional index words: Protein, Digestibility, Dry matter, IVDMD, Sorghum bicolor. DIFFERENCES in quality traits of forage sorghum [Sorghum bicolor (L.) Moench 1 sampled at various stages of development or under different management regimes are well documented (1, 3, 5, 6, 9). Much less information is available, however, on possible differences among plants of the same genotype and within the same field plot that reach a given developmental stage at different ages in terms cf O:3.ys after planting. It would be useful for forage sorghum researcht, to know whether the quality traits of a plant that reaches a specific developmental stage on a certain day provide an accurate indication of the traits of other plants in the plot as they reach this stage several days later. An easily identified stage in the development of forage sorghum is the end of the boot stage which is signaled by the beginning of head emergence. This study was designed to determine the extent of variation in percentages of dry matter, crude protein, and in vitro dry matter disappearance occurring among forage sorghum plants that reached early head emergence at different times after planting. Plants also were sampled at physiological maturity for comparison with the samples taken at head emergence. Materials and Methods Five forage sorghum cultivars ('Atlas', 'Brawley', 'Coleman', 'Early Hegari-Rox', and 'White Collier') were grown in 1976 and 1977 in a randomized complete block design with three replications at the University of Nebraska Agronomy Farm, Lincoln, Nebr. 1 Contribution from the USDA-ARS, and the Dep. of Agronomy, Nebraska Agric. Exp. Stn., Lincoln. Published as Paper No. 6861, Journal Series, Nebraska Agric. Exp. Stn. Received 21 Apr. 1982. 2 Former graduate research assistant, Univ. of Nebraska, now assistant professor of agronomy, Auburn Univ.; George Holmes professor of agronomy; and supervisory research geneticist, USDA-ARS, and professor of agronomy, Univ. of Nebraska, Lincoln, NE 68583. 3 Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the USDA or the Univ. of Nebraska, and does not imply its approval to the exclusion of other products that may also be suitable. Each plot consisted of five 6.7-m rows spaced 0.76 m apart. Planting dates were 5 May 1976 and 16 May 1977. An application of 112 kg N/ha was disked into the soil [Kennebec silt loam (Cumulic Hapludoll) 1 prior to each planting, and plantings were followed immediately by applications of a mixture of propachlor (2-chloroN-isopropylacetanilide) and atrazine [2-chloro-4-(ethylamino)-6(isopropylamino)-s-triazinel at 3.75 and 1.25 kg a.i./ha. To supplement rainfall, the plots were flood irrigated once, at about midsummer, both in 1976 and 1977. The stage of early head emergence (EHE) was defined as occurring when the very tip of the head was first visible. When at least one culm in each row of a cultivar reached this stage, sampling of that cultivar was initiated. One culm at EHE was sampled from each row of the cultivar on sampling day one (EHE 1). Two and 4 days later, another single culm that had just reached the EHE stage was harvested from each row of the cultivar (EHE 2 and EHE 3, respectively). One set of samples also was taken for each cultivar at physiological maturity (PM) as indicated by black layer formation (4). The five culms harvested from a plot at each sampling day were cut off at the surface of the soil and were separated into leaf blade, stem plus leaf sheath, and head portions. Each culm was treated individually for assay. The immature head was considered to be part of the stem in all EHE samples. The separated samples were weighed and then dried at 75 C in a forced draft oven to constant weight. Dried samples were ground in a Wiley3 mill to pass a 1mm screen, and subsamples were stored for subsequent analysis. Percent crude protein (CP) was determined for each sample as Kjeldahl N X 6.25 (7), and in vitro dry matter disappearance (IVDMD) by a modification of the two-stage technique of Tilley and Terry (8). Percent dry matter (DM) was calculated for each sample, and values for CP, IVDMD, and DM were calculated for the total plant from the values for the leaf blade, stem, and, for PM samples, head portions. Analysis of variance with orthogonal comparisons (2) was completed for each of the traits under study. Results and Discussion Mean squares for some of the sources of variation involved in this study are shown in Table 1. For many of the measured traits, statistically significant variations were associated with year interactions. Therefore, the mean squares are presented on an individual year basis. Inspection of these mean squares shows that significant differences were detected between the EHE and PM stages for all traits except total plant IVDMD in 1977, and that significant (EHE vs. PM) X cultivar interactions were found in all cases except total plant CP in 1976. Significant differences among the three EHE sampling dates were shown for most traits. However, the mean squares for variation among EHE samples were generally much smaller than those for EHE vs PM. Means for each cultivar X sample time X year X trait combination are shown in Table 2. As expected, leaves and stems of all cultivars harvested at EHE were lower in DM than those harvested at PM. The difference between EHE and PM samples was especially pronounced for total plant values, reflecting the relatively high DM percentages of heads at the PM stage (separate data for heads not shown). Decreases in forage CP and IVDMD are generally expected as plants advance in maturity. Comparison of CP values of leaves, stems, and total plants indicates that such a decrease occurred for all cultivars between EHE and PM. Similarly, leaves of all cultivars decreased in IVDMD between these two stages. However, Early Hegari-Rox was