No abstract

Estimates of genetic variances and derived statistics of pertinent traits are essential for efficient plant breeding programs. For clonal sugarcane (Saccharum spp.) populations in Louisiana, such estimates (and unconfoanded estimates of genotype by environment [GE] and genotype by crop [GC] variances) were lacking. The objectives of this study were to estimate broad‐sense genetic and GE variance components for a clonal sugarcane population representative of initial stages of replicated testing and to determine the relative importance of years, locations, and crops. Thirty‐seven genotypes were planted in 1983 and replanted in 1984 in replicated tests at five locations. Data from two 3‐yr. crop cycles were used. Genetic advance (GA) indicated considerable improvement potential in sucrose yield, cane yield, and stalk number and weight. Genotypic variance was generally secondary to error variance in determining phenotypic variance; GE variances were tertiary to genotypic and error variances. Within a crop, genotype by location (GL) variances tended to be larger than genotype by year (GY). Estimates of potential of plant cane sucrose yields over years and locations implied testing across locations could substitute for years, effectively reducing the time to identify elite clones. Analysis across crops showed GC, GL and GYL interaction variances were usually larger than GY. Estimates of GA showed no difference in potential gain from replicating across years vs. crops. For several traits, the most potential for improvement is in older crop performance, and selection is best practiced with regard to crop.

The inheritance of ratooning ability and the relationship of traits among crops in sugarcane (Saccharum spp. hyb.) has not been well examined. Ratooning ability (RA) was defined as the second ratoon (SR) crop yield percent of the plant cane yield. A replicated 4-yr test at four locations of 37 genotypes was studied for two three-crop cycles. Broad-sense single-plot heritabilities for RA were low (H _< 17%), while the genetic coefficient of variation of RA was largest for sucrose yield and cane yield (GCV = 14.5%), and smallest for stalk diameter (GCV = 1.5%). Cane and sucrose yield RA demonstrated the largest potential for gain, while stalk weight, stalk diameter, and stalk length showed the least. Except for sucrose and cane yield and stalk number, other traits were highly correlated between plant cane and SR crops (r >_ 0.78). Stalk number in the younger crop was the only trait significantly correlated to ratoon crop cane yield (r = 0.56), suggesting that selection for stalk number in the younger crops would enhance older crop yields. The results indicate that SR crop yields could be predicted by first ratoon crop yields. However, the best improvement of SR yields would be realized by selection in the SR. p~ LANTING OPERATIONS and seed (stalks for vegetative propagation) costs constitute the largest input of sugarcane production (Salassi and Giesler, 1995). Inadequate ratoon crop yields limit the economic production of sugarcane in semi-tropical regions such as Louisiana, where ratoon crop yields typically decrease with age (Johnson et al., 1993; Ricaud and Arceneaux, 1986; Shrivastava et al., 1992). The reasons for this decline are complex, but primarily relate to diseases, insects, weed competition, management practices, and winter kill (Shrivastava et al., 1992). Additionally, genotypes can vary substantially in their ratoon crop yields (Chapman, 1988; Chapman et al., 1992; Ricaud and Arceneaux, 1986; Tripathi et al., 1982). Ratooning ability can be enhanced by indirect selection for disease or insect resistance, or by direct selection of genotypes with high ratoon crop yields. Traits such as high stalk number, bud viability, vigorous root formation, high biomass accumulation, and high light use efficiency have been suggested as being indicative of better ratooning cultivars (Sundara, 1989; Ferraris et al., 1993). The importance of maintaining stalk weight in older crops has also been noted (Chapman, 1988; Chapman et al., 1992). Ratooning ability can be defined in either absolute or relative terms. In absolute terms, a good ratooning cultivar is one that produces high ratoon crop yields or several profitable ratoon crops. Relative to other cultivars, a

Genetic correlations are commonly considered in the development of selection strategies. Previously reported correlations between sugarcane (Sacdmrum spp.) yield components may be of questionable value because they were obtained either from small populations or from a restricted range of environments. The effect of crop age on trait relationships has not been reported. Thistady's objective was to examine crop age effects on the geneticorrelations among sugarcane traits using path coefficient analysis. A population was planted in 1983 and 1984 at five locations in Louisiana. Harvest data were collected for three crops. Results indicated that cane yield (TCH) was the most important determinant of sucrose yield (SPH) and became increasingly important in determining the SPH in older ratoon crops (rsph,tchh = 0.91). Stalk number (SNO) was the primary determinant of TCH and became more important in determining TCH in older ratoon crops (rtch,sno = 0.77). Stalk diameter (DIA) was more important than stalk length (LEN) and stalk density (DEN) in affecting stalk weight (WT) regardless of crop age (rwt,dla = 0.70 vs. rwt,len = 0.52 and rwtden = 0.33). Brix and juice parity were highly correlated with sucrose content and with each other, and were not affected by crop age. The results indicated that selection for SPH should emphasize TCH with concentration on SNO, particularly in ratoon crops. The results also indicated that development of selection strategies of sugarcane should consider crop age in weighing the importance of yield components that affect SPH.

Sugarcane (Saccharum spp.) fiber, the dry, water‐insoluble component of the stalk, is an important quality component because of its inverse relationship to juice extraction and milling efficiency. Sugarcane cultivars in temperate regions are enhanced with S. spontaneum L. germplasm to provide increased vigor and cold tolerance. Unfortunately, S. spontaneum L. clones typically have low recoverable sucrose and high fiber content. Our objective was to estimate genetic and genotype × environment (G × E) interaction variances for sugarcane fiber content, not previously reported for a temperate sugarcane population. Twenty‐two clones from the first replicated testing stage of the Louisiana Sugarcane Variety Development Program (LSVDP) were studied in a first and second ratoon crop at three locations. Genotype × crop (G × C), genotype × location (G × L), and genotype × crop × location (G × C × L) variances were much smaller than genetic and error variances. Heritability of fiber on a single‐plot basis was 0.71 and 0.91 when heritability was based on three locations, two replicates, and one crop. From path analysis, the direct effect of fiber content on recoverable sucrose was −0.203, indicating a weak inverse relationship between these two traits. Fiber content was significantly correlated with stalk diameter, rg = −0.585, indicating that indirect selection for larger stalk diameter should decrease fiber content. Direct selection for optimum fiber content by evaluating clones in a single crop and/or location would be effective. Evaluation at different locations and during different crops, however, would offset the relatively large error variance, thereby increasing the statistical power to determine differences among clones for fiber.

tural methods, plant resistance potentially provides the most economic and environmentally effective means of The sugarcane borer [Diatraea saccharalis (Frabricius)] causes control available to the production industry (Hensley, significant damage to sugarcane (Saccharum spp.), rendering cultivar resistance important. Researchers assess borer-induced damage using 1981). Hence, cultivar characterization and selection for up to five different measures: percentage bored internodes, percentage resistance plays an important part of the evaluation exited internodes, pupation success, moth production, and a damage process. rating. The inheritance of the different damage measures and the best Cultivars express resistance via complex contribuapproach to integrate the different variables into a simplified damagetions and interactions of several components (Kyle and resistance assessment has not been well studied. Furthermore, the Hensley, 1970; Coburn and Hensley, 1972; Martin et al., relationships of the damage traits to sugar production have not been 1975; White and Hensley, 1987; Sosa, 1988; Bessin et comparatively assessed. We planted a replicated, two-location test of al., 1990; White, 1993). Thus, all of the recognized mech-28 clones typical of the selection stage screened for borer resistance anisms of resistance (antibiosis, antizenosis, and tolerin the Louisiana sugarcane breeding programs. We recorded the five measures together with sucrose production and its components. Using ance) may be expressed in cane's resistance to the sugarappropriate variance components, the heritability, expected response cane borer (Painter, 1951). to selection, and genetic correlations among the traits were used to Sugarcane is a clonally propagated crop with two construct selection indices of all combinations of the five damage breeding programs in Louisiana; one is conducted by traits studied. We used the regression coefficients of the damage traits the Louisiana Agricultural Experiment Station (LAES) on sucrose production as economic weights. The indices indicated near St. Gabriel, LA, and the other is conducted by that percentage bored internodes was the most effective single trait the USDA-ARS-SRRC Sugarcane Research Unit in to reduce sugarcane borer damage. If data collection costs were consid-Houma, LA. Experimental clones are routinely screened ered, then the subjectively assessed damage rating was the most expeby each program starting ≈6 yr after initial clonal plantditious of the traits examined. High correlation values among several of the traits lead to the observation that inclusion of more than the ing (White, 1993;Reagan et al., 1999). Clones are evalubored internode, exited internodes, and the damage rating in an index ated in similar fashion among the programs, but methwere unnecessary.

A quick, accurate method to determine the potential of a sugarcane (Saccharum spp.) cross to produce elite progeny is needed for maximizing genetic gain. Development of a practical cross appraisal method was initiated by evaluating 1,800 progeny from 15 crosses among 23 parents at two intrarow plant spacings (41 cm and 82 cm). Plant spacing was examined for its affect on stool weight variability. The goals were to identify the most reliable and/or easily obtained cross appraisal statistic and to determine the earliest breeding program stage and crop to collect these statistics. Three tests, on plant cane (PC) and first ratoon (FR) single stool seedlings and clonal plant cane plots, were conducted. Four statistics, the family mean, the estimated elite proportion (PROB), the observed elite proportion, and the best linear unbiased predictor (BLUP) were estimated and examined for each cross. These statistics were strongly correlated within each test (0.69≤r≤1.00). Family worth estimates based on single stool data were moderately correlated (ca. range 0.5≤r≤0.7) to the family worth estimates based on clonal plots.The research suggested that the potential of a cross to produce elite progeny for a trait could be accurately predicted by the cross mean of that trait. Data for the mean were the most easily obtained and, hence, would be the most practical family appraisal statistic to use in a breeding program. Correlations of statistics among the PC and FR seedlings and the clonal plots showed that the PC estimates of Brix, stalk weight, and its components, stalk length and stalk diameter, could be used for cross appraisal. Genotypic selection by the Louisiana Sugarcane Variety Development Program (LSVDP) occurs among the FR seedlings. FR stalk number and PC Brix and stalk weight data could be used to perform family selection prior to the currently practiced individual plant selection. The benefits of family selection to the LSVDP were demonstrated by the expected genetic gains for two selection scenarios. The gains were consistently larger for an initial 50% family selection and subsequent 20% individual selection than they were for simple individual selection at a 10% selection intensity. Our research also suggests that the use of a wider intrarow spacing may improve the ability to discern among seedlings due to its enhancement of stool weight variability.

tance and biological control each contribute 25%, and chemical control the remaining 40%. This pest manage-The sugarcane borer [Diatraea saccharalis (Fabricius)] is an imment program has provided effective and stable control portant insect pest of sugarcane grown in the Americas. Environmenof the sugarcane borer for approximately 30 yr. Pest tal and economic concerns are driving these sugarcane industries to consider alternatives to insecticides for controlling damaging infesta-management programs that rely on insecticides as a tions of the borer. Breeding for resistance is a viable option; however, principal control tactic, however, are under increasing little is known of the inheritance of sugarcane borer resistance. The economic and environmental pressures to reduce the inheritance of sugarcane borer resistance in sugarcane (Saccharum IPM's dependency on insecticides. spp. L.) was investigated in a field study conducted in 1990, 1992, and Plant resistance may provide the additional control 1993. We measured resistance by both plant damage response ratings needed to supplant insecticides in the IPM program. and mean percent internodes damaged. Seedling progeny (F 1 plants Studies on plant resistance to the sugarcane borer have generated from seed) from 21 to 27 crosses were evaluated each year. been published (Mathes and Ingram, 1944; Long et al., These progeny originated from a mating design with females nested 1978). These studies have included research to deterwithin males. Parental genotypes were randomly selected for borer mine mechanisms of sugarcane borer resistance and resistance, but were elite cultivars adapted to Louisiana. Data were collected from progeny infested with artificially introduced sugarcane methods to select for resistance (Kyle and Hensley, borers. Narrow-sense heritability on a single-plot basis (36 plants 1970; White and Hensley, 1987). Little is known of the measured per plot) for damage ratings (h 2 ϭ 0.73) and for percent inheritance of sugarcane borer resistance and breeding damaged internodes (h 2 ϭ 0.76) were high and of comparable magnimethods required to increase resistance in clonal poputude. For both traits, we detected neither dominance nor additive ϫ lations. Viator and Henderson (1971) found borer resisyear interaction; however, dominance ϫ year interaction variance tance to be quantitative in nature, but provided no meaexisted. The potential for genetic advance (GA) from direct selection sures of genetic variation, heritability, or potential gain against percent damaged internodes (GA ϭ 33.9% of mean bored from selection. internode) was higher than that from direct selection for lower damageSugarcane is a clonally propagated, out-crossing, perating (13.5% of mean rating). The much greater resources needed rennial crop that growers routinely harvest once per to effect selection for percent bored internodes (approx. 24 times that for rating) suggested direct selection for damage rating may be more year for about 3 yr before replanting. Ratoon crops efficient. Because the t...