By Dr Rashid Ahmad, Bilal Hassan & Khawar Jabran
 

GRAIN crops such as wheat, rice, maize, sorghum, sunflower, dry beans, soybean and lentils constitute essential source of proteins and carbohydrates. Per hectare grain yield of these crops is far below international standards. Harvest index of modern cultivars of these intensively cultivated grain crops fall within the range of 0.4 to 0.6.

Low grain crop harvest index could be attributed to cultivation of non-recommended crop cultivars, unapproved seed used for sowing, late sowing, imperfect sowing methods, low plant population, poor plant protection, and proliferation of weeds, imbalanced use of fertiliser and non-availability of water for irrigation at critical crop growth stages.

Low crop harvest index is the major cause of less crop yield. Therefore, harvest index could be used as a yardstick for determining the gap between potential and actual yields. By definition, potential yield is the yield of a cultivar when grown in an ideal environment, with adequate nutrients and moisture, and stresses like pests, diseases, weeds, lodging are effectively controlled. On the other, actual yield is the maximum yield which could be obtained under given environmental conditions and with available inputs.

Harvest index of important grain crops is given in the table.
Harvest Index  Crop
0.40 - 0.55      Wheat
0.40 - 0.55      Maize
0.30 - 0.35      Sunflower
0.45 - 0.55      Dry Beans
0.45 - 0.55      Lentils
0.25 - 0.35      Soybean
0.40 - 0.55      Sorghum

What is grain crop harvest index? How it could be improved? These are the questions to be answered. Simply harvest index is calculated dividing total grain yield by total plant yield. Here total plant yield takes into account grain yield as well as vegetative parts of crop plants above the soil surface. Thus economic yield / total plant yield gives harvest index.

Improving yield of grain crops is a need of the hour to ensure steady supply of food to the rapidly increasing population. However, without improving harvest index, increase in yield could not be materialized. It is clear that directing maximum dry matter produced in the season to the harvesting portion would help to improve yield. It is important to mention that reduction in stem and leaf sheath dry matter to half current average values and a reallocation of this dry matter to the ear could raise the harvest index from about 0.5 to 0.62.

It is worth-mentioning that characteristics features of source sink relationship greatly alter the harvest index. Determining whether the economic yield of a crop is source limited or sink limited is more complicated because during the development and growth of the sink, the relationship between source and sink inevitably changes.

If photosynthesis is allowed full expression, sink limitation prevails. On the other, in the presence of any severe stress such as moisture, disease or insect/ pest etc. that may alter plant growth, then source efficiency may be the yield limiting factor. However, sink limitation may be eliminated by a genetic programme including increase in grain size, number of grains per unit area and individual grain weight. Thus source-sink relationship influences yield determinants of grain crops.

Even harvest index of existing crops has approached to upper limits, future yield gains will have to be sought by increased biomass production. Efficient crop and soil management practices could also improve grain crop harvest index.

Weeds impose stress on plant growth and impair growth as weed-crop competition for nutrients, moisture, light, space etc., gets intensified. Knowingly or unknowingly, the growers with the exception of some progressive ones don’t pay heed to weed control. In addition, weeds harbour insect /pest and pathogens. Thus role of weeds in declining crop harvest index is obvious. Importantly, creating awareness among the growers by using all available measures is crucial. Integrated weed management approach is considered best against weeds.

Insect/pests attack at different crop growth stages cause heavy toll on yield. On one hand, these biotic agents cause direct damage to plant while on the other hand reduce plant growth by decreasing irrigation and fertilisers efficiency. Appropriate crop and management techniques need to be adopted to keep insect/pests under control.

Balanced use of inputs like seed, fertilisers and moisture is essential for improving harvest index of grain crops. Sadly, the distribution of these inputs remained lopsided during the last seven years.

Though distribution of certified seed has increased from 194.3 million tons to 253.9 million tons from 1999-00 to 2005-06 but local grain crop seed is used on large scale that needs to be abandoned totally. It is because uncertified seed gives poor germination and less plant population.

Characteristics like high seed germination percentage, physical and genetic purity, vigour and viability are important to optimize crop harvest index. For this purpose, seed regulation scheme needs to be enacted in letter and spirits to ensure steady supply of quality seed. Seed and seed supplying-agents should follow international seed testing standards. The Federal Seed Certification Authority must be aligned to the seed requirements of the farming community and should control the escalating business of fake seed, declining quality of certified seeds and frequent crop failures due to poor quality seeds.
High prices, occasional shortages in the market and adulteration are the problems associated with fertilisers. Moreover, factors including leaching, fixation and evaporation are contributing to low fertilizer use efficiency. However, using appropriate fertiliser resource in balanced amount in accordance with crop type and soil fertility level would help to improve harvest index.

Adequate irrigation at critical crop growth stage is essential for crop growth and development. Non-availability of irrigation at critical growth stage significantly reduces crop harvest index. Extremely low grain crop harvest index during 2000-01 and 2001-02 was due to unprecedented drought. Importantly, land leveling, tillage, mulching and use of manures, time and sowing method, optimum plant population, weed eradication and insect and pests and diseases control are inevitable agronomic measures for improving grain crop harvest index. Late sowing is a major yield limiting factor of grain crops because short vegetative period adversely affect source-sink relationship.

Last but no the least, precision land leveling improves water use efficiency by curtailing irrigation application losses up to 50 per cent, enhances crop yield by 20 per cent, controls water logging and salinity, facilitates efficient use of agricultural machinery, ensures uniform uptake of nutrients, promotes judicious use of inputs, lowers cost of production, optimises land and water resources, establishes uniform crop stand, increase efficiency of cultural practices such as weeding, spraying and harvesting and thus improves harvest index considerably.