The desirable characteristics of high-yielding varieties (HYVs), which became the breeding objectives of IRRI rice breeders (IRRI 1968), include the following:
- nitrogen responsiveness
- 110–140-day maturity
- early vegetative vigor
- high tillering ability
- short stature (about 100 cm)
- erect leaves
- sturdy stems (resist lodging, non-photoperiod-sensitive)
- resistance to common pests and diseases
Indica and Japonica Crosses
The first successful cross in IRRI is that between Peta (indica) and Dee-geo-woo-gen (japonica), producing IR8, popularly known as the miracle rice. The traditional indica varieties grown in tropical Asia, such as Peta, are generally tall; grow vigorously; are leafy; have profuse tillering; and are late-maturing, photoperiod-sensitive, and lodging-susceptible. On the other hand, japonica varieties grown in subtropical countries like Japan, Australia, and California are early-maturing, short, high-yielding, and low-tillering; lack vegetative vigor; have no grain dormancy; are susceptible to pests and diseases; have poor eating quality (for Asian), and are non-photoperiod-sensitive (Beachell 1970). It is the recombination of these different characteristics that scientists are continually working on in order to come out with desirable traits and plant types suited for tropical Asia. The other major problem areas being investigated are:
- better agronomic characteristics
- improved grain quality
- higher protein content
- drought tolerance
- adverse soil tolerance
- adverse temperature tolerance
- adaptability to higher elevation
Recommended Varieties
The four HYVs that have resistance to blast, blight, tungro, and grassy stunt are IR28, IR36, IR56, and IR58. Varieties that have wider resistance to insect pests and to most biotypes of brown planthopper are IR36, IR42, and IR56. IR20 is considered the first improved variety that is resistant to stem borer and green leafhopper. Other Asian countries have their own breeding programs. Using IRRI varieties as parent materials, these countries have developed their own local hybrids, varieties, and selections that oftentimes yielded more than the parent materials.
There are at least about 40,000 varieties of rice worldwide. Most recent developments in the late nineties are the hybrids, which are crosses from two or more parent varieties. China’s hybrids yielded from 3.5 t/ha to 6.2 t/ha, which can feed more than one billion people in that country. The Philippine Rice Research Institute (PhilRice), IRRI, and China collaborated in developing hybrid seeds with yields better than standard varieties. In the Philippines, PSBRc 72H, NSICRc 114H, NSICRc 111H, all known as Mestizo lines, produced an average yield of 6 t/ha. Other commercially produced hybrids from the private sector yielded as high as 12.95 t/ha. These are SL-8H, Bigante, Magilas 500, and Rizalina 28.
Filipino farmers are also growing aromatic varieties, all noted for their aroma, pleasant taste, texture, and fast cooking quality. The low yield of 1–3 t/ha is offset by the higher selling price, which can be as profitable as the standard commercial HYVs. Aromatic rice grown in higher elevations are Tinawon, Onongan, Innawi, Batnol, and Ayyuhip. Those grown in lower elevations up to sea level are Azucena, Binicol, Milagrosa, Milfor 6, Mimis, Jasmin, and Sinampablo, among many others. Some countries have crossed aromatic varieties such as Jasmine with HYVs and these are sold commercially in other countries.
Disease and Insect Resistance of Varieties
The performance of IRRI varieties against the most prevalent insects and diseases in the Philippines and cooperating Asian countries is significant (IRRI 1983). The International Rice Testing Program (now the International Network for Genetic Evaluation of Rice [INGER] at IRRI makes available to rice scientists worldwide pest- and disease-resistant lines for field trials in their own country.