Agronomic Productivity and In Vitro Fermentation Kinetics of Maize-Sorghum Replacement Intercropping

Technical Summary: Agronomic Productivity and In Vitro Fermentation Kinetics of Maize-Sorghum Replacement Intercropping

Problem Statement
Climate change and increasing water scarcity in arid and semi-arid regions threaten agricultural sustainability, particularly for ruminant forage production. While forage maize (Zea mays) offers superior nutritional quality for high-producing dairy cattle, it is water-intensive and susceptible to yield penalties under drought stress. Conversely, forage sorghum (Sorghum bicolor) exhibits remarkable drought resilience and water-use efficiency but often presents suboptimal nutritional characteristics, such as lower crude protein (CP) and higher neutral detergent fiber (NDF), when grown as a sole crop. Although intercropping is recognized as an ecologically sound strategy to enhance resource utilization, significant knowledge gaps remain regarding how specific replacement row patterns (spatial arrangements) influence morpho-physiological traits, biomass partitioning, and the subsequent cascading effects on forage chemical composition and ruminal fermentation kinetics under authentic, large-scale field conditions.

Methodology
This study employed a large-scale, un-replicated strip trial conducted across two distinct agricultural regions in Golestan Province, Iran (Gorgan and Ali-abad), during the summer growing season. The experimental design compared three cropping patterns:

1. Sole Maize (1M): Pure stand of forage maize.
2. Intercropping 1M:1S: One row of maize alternated with one row of forage sorghum.
3. Intercropping 1M:2S: One row of maize alternated with two rows of forage sorghum.

Each treatment strip covered 2,100 m² (40 rows, 75 m length). Agronomic management followed commercial standards, including mechanical sowing, fertilization, and harvesting at the dough/hard-dough stages.

Data collection involved:

• Morpho-physiological traits: Plant height, stem diameter, tiller number, leaf count, and leaf-to-stem ratio.
• Yield components: Fresh and dry biomass partitioning (stem and leaf) and total forage yield.
• Ecological efficiency: Calculation of the Land Equivalent Ratio (LER).
• Chemical analysis: Determination of Dry Matter (DM), Ash, Crude Protein (CP), Crude Fiber (CF), and Neutral Detergent Fiber (NDF).
• In vitro fermentation: Gas production kinetics were assessed using the Menke and Steingass (1988) protocol with rumen fluid from fistulated sheep. Parameters included gas production at 24h and 48h, kinetic fractions ($a$, $b$, $c$), in vitro dry matter digestibility (IVDMD), metabolizable energy (ME), and total short-chain fatty acids (SCFA).

Statistical analysis utilized a Completely Randomized Design (CRD) with 30 random sub-samples per treatment, employing ANOVA with Duncan's and Tukey's post-hoc tests.

Key Results

• Morphology and Biomass: The 1M:2S pattern significantly altered canopy architecture, producing the highest number of tillers (2.85) and leaves (19.60), though it resulted in the shortest plant height (131.7 cm) and smallest stem diameter (9.56 mm) due to interspecific competition. Sole maize maintained the largest stem diameter (17.47 mm) and highest leaf-to-stem ratio (0.47).
• Yield and Efficiency: The 1M:2S treatment achieved the highest total dry matter yield (7.77 ton/ha), representing an 18.55% increase over sole maize (6.54 ton/ha). This pattern yielded a Land Equivalent Ratio (LER) of 1.18, indicating an 18% biological advantage over monoculture. The 1M:1S pattern also outperformed sole maize with an LER of 1.12.
• Nutritional Composition: The 1M:2S pattern resulted in a substantial 49% increase in Crude Protein (9.75%) compared to sole maize (6.55%). However, this pattern also exhibited the highest NDF content (61.59%), whereas the 1M:1S pattern showed the lowest NDF (55.36%).
• Fermentation Kinetics: A distinct divergence was observed between yield/quality and digestibility. The 1M:1S pattern demonstrated superior in vitro dry matter digestibility (IVDMD = 78.8%) and metabolizable energy (10.52 MJ/kg DM). In contrast, the 1M:2S pattern, despite its higher protein and yield, showed lower IVDMD (69.2%) and ME, attributed to its higher fiber content and more lignified structure, though it exhibited the highest fractional gas production rate ($c = 0.068 h^{-1}$).

Significance and Claims
The paper claims that maize-sorghum replacement intercropping represents a viable agroecological framework for sustainable forage production in water-limited environments. The study concludes that different planting ratios allow for the tailoring of forage characteristics to specific ruminant needs:

• The 1M:2S pattern is identified as the optimal strategy for maximizing biomass yield and crude protein content, making it a high-yielding, health-promoting basal diet suitable for dry cows and replacement heifers where high NDF levels can help manage body condition and prevent metabolic disorders.
• The 1M:1S pattern is presented as offering a synchronized nutritional balance with superior digestibility and energy density, making it an ideal feed source for high-producing lactating dairy cows where energy intake is the primary bottleneck.

By integrating drought-tolerant sorghum into maize-based systems, the authors assert that producers can mitigate climate-induced yield risks while securing high-quality nutritional inputs, thereby supporting the economic viability and sustainability of dairy enterprises in arid and semi-arid regions.