Both C3 and C4 grasses exhibit similar morphological responses to shade, but they differ in photosynthesis‐related responses to shade, such as forage yield and growth rate. Morphological adaptation and reduced photosynthesis under shade may affect forage quality. The objective of this research was to determine the effect of adaptation to shade on forage quality of C3 and C4 grasses. The C3 species were tall fescue (Festuca arundinacea Schreb.), reed canary grass (Phalaris arundinacea L.), and deertongue grass (Panicum clandestinum L.); the C4 species were switchgrass(Panicum virgatum Michx.) and big bluestem (Andropogon gerardii Vit.). Grasses were established near Ames, IA, and grown under shade cloths which allowed 37 and 70% of ambient sunlight (AS) to penetrate. In addition, the grasses were subjected to a nonshaded (100% AS) treatment. Leaf blades, nonlaminar herbage, and total herbage samples were obtained at approximately 21‐d increments from late spring to early summer for 2 yr. Neutral‐detergent fiber (NDF) concentration in leaf blades was 18% greater in C4 species than in C3 species. Otherwise, few clear differences were observed between C3 and C4 types. Compared with the 100% AS treatment, NDF concentration in the 37% AS treatment was reduced 2 to 3%, whereas N concentration was increased by about 31% in nonlaminar herbage. In vitro digestible dry matter was increased 3 to 5% by shade. Nitrogen content per unit leaf area was not affected by shade, suggesting that nitrogenous compounds were concentrated by thinning of leaves. Thus, stressful growth conditions that limit photosynthate, such as shade, may improve forage quality.

Several studies have compared C3 and C4 species for response to water and temperature regimes. Little comparative information exists, however, on growth responses of C3 and C4 species to irradiance regime. The objective of this study was to determine adaptive responses of C3 and C4 grasses to irradiance regime. Three C3 and two C4 perennial forage grasses were field—established near Ames, IA, and grown under 37, 70, and 100% of full sunlight by use of polypropylene shade cloths. Morphology and growth measurements were conducted three times at ⊄21‐d intervals each year for 2 yr. Net leaf CO2 exchange rate (CER) was measured for one C3, and one C4 grass. Specific leaf weight increased and leaf‐area ratio decreased with increasing irradiance similarly in all five species. Responses of herbage yield, shoot dry weight, and crop growth rate to irradiance were two to three times greater for C4 grasses than for C3 grasses. Responses of CER to irradiance were greater for the C4 grass than for the C3 grass. Morphological adaptive responses were similar for C3 and C4 grasses, but responses closely related to photosynthesis (e.g., CER, growth rate, and herbage yield) were affected more in C4 than in C3 species.

Breeding schemes to improve alfalfa (Medicago sativa L.) forage quality based on total herbage chemical composition can result in altered morphology. Additionally, cell‐wall composition of plant parts also may be altered. This study investigated the effects of divergent selection for lignin concentration in total herbage of alfalfa on concentration of cell‐wall components and digestibility of stems. Two groups of alfalfa previously derived from herbage‐based selection were studied. Group 1 consisted of six two‐parent Syn‐2 lines randomly selected from ‘Saranac‐AR’ for high (HL) or low (LL) acid‐detergent lignin (ADL) concentration in the total herbage. Group 2 had 11 HL and 13 LL lines produced from crossing pairs of parents of the Group 1 lines. Neutral‐detergent fiber (NDF) concentration in stem bases was 4 and 3% greater in HL lines than in LL lines in Group 1 and 2, respectively. Likewise, herbage NDF concentration was 8% greater in HL than in LL lines of Group 1 and 4% greater in Group 2. There were no differences between divergent lines in NDF concentration of stem tops. Lignin concentration in cell walls of Group 1 was 8 and 6% greater in LL lines than in HL lines in stem tops and stem bases, respectively. In Group 2, the LL lines had 2% greater cell‐wall lignin concentration than HL lines in both stem fractions. Total‐herbage NDF concentration correlated more closely with leaf‐to‐stem ratio of entire plants (r = − 0.96 and −0.78 for Group 1 and 2, respectively) than with stem length and maturity. Thus, herbage‐based selection for improved nutritive quality of alfalfa results in complex changes in stem cellwall component concentrations in addition to large morphological changes.

Little information is available regarding the relationships of alfalfa (Medicago sativa L.) forage yield with yield components in seed‐established stands grown under dryland conditions. Field experiments that included two cultivars and 15 seeding rates ranging from m 2.2 to 33.6 kg pure live seed ha−1 were established in 1985 at Brookings and Highmore, SD. The objective was to determine seeding rate effects on components of alfalfa forage yield and on plant morphology several years after establishment. In the seeding year, plant density increased linearly with seeding rate at both locations. Yield components were determined in 1988 and 1989. Mean alfalfa yield during 1988 and 1989 was optimum at the 13.4 kg pure live seed ha−1 rate at Brookings, but did not respond to seeding rate at Highmore. Plant density continued to be affected by seeding rate 4 yr after establishment, but greater mortality at high seeding rates caused the response to deviate from linearity. Shoots per plant were negatively correlated with plant density (r = –0.73, P = 0.01). Shoot weight did not respond to seeding rate at Brookings but declined linearly with seeding rate at Highmore. Nevertheless, path analysis suggested that shoot weight was the major influence on alfalfa yield at both locations. Leaf‐to‐stem ratio and shoot length were not affected by seeding rate. Alfalfa yield components are affected similarly by plant density in seed‐established and transplanted stands. Although yield may not be affected by plant density, shoot weight may be affected because of differences in plant competition.

Programs to improve alfalfa (Medicago sativa L.) nutritive value have used selection criteria based on total herbage cell‐wall composition. There is only limited information addressing the effect of total‐herbage based phenotypic selection on changes in dry‐matter partitioning of the plant and on the nutritive value of plant parts. The objective of this study was to determine the effects of divergent herbage selection for alfalfa cell‐wall components on plant morphology. Two groups of germplasm were observed. Group 1 consisted of six syn‐2 lines derived from two‐parent combinations, which were randomly selected from ‘Saranac‐AR’ for high (HL) or low (LL) herbage acid‐detergent lignin concentration. Group 2 had HL and 13 LL lines that were full‐sib lines produced on pairs of parents from the Group 1 synthetics. Group 1 was grown for two harvests in a greenhouse and one harvest in the field, and Group 2 was grown for two harvests in the field. Compared with HL lines, LL lines had a greater leaf‐to‐stem ratio, especially in the fraction above the sixth node from the stem base. For LL lines, this ratio in the upper plant fraction was 53, 26, and 18% greater than HL lines for Group I greenhouse, Group 1 field, and Group 2, respectively. When harvested at the same time, LL lines were less mature than HL lines, and, for Group 2, main stem length of LL lines was about 30 mm shorter than HL lines. Dry‐matter yield of HL lines was 17, 28, and 28% greater than LL lines for Group 1 greenhouse, Group 1 field, and Group 2, respectively. Most of the differences between divergent types were related to changes above the sixth node. Thus, selection for improved nutritive value in total herbage of alfalfa may result in marked changes in plant morphology.