Highest yield performance is obtained in the syn-1 generation

The highest yield performance is obtained in the syn-1 generation, hybrid vigor declining with subsequent generations. It is generally estimated that a synthetic forage cultivar of cross-fertilized diploid or polyploidy species will experience a maximum yield decline of 10–12% from syn-1 to syn-2 generation, as previously stated. The yield decline is less in subsequent generations. Sewall Wright proposed a formula to predict the F2 yield of a group of inbred lines as follows:

where F2 is the expected performance of the F2, F1 is the mean F1 hybrid performance from combinations of inbred lines, P is the average performance of inbred lines, and n is the number of inbred lines. That is, one can increase F2 yield by increasing the average F1 yield, increasing yield of parental lines, or increasing the number of lines used to create the synthetic. This formula assumes that the species has diploid reproduction and that the parents are inbred. Hence, even though shown to be accurate for maize, it is not applicable to polyploid species and those that are obligate outcrossers.

The formula may also be written as follows:

Studies involving inbred lines and diploid species have indicated that as the number of parental lines increase the F1 performance is increased. Parental lines with high combining ability will have high F1 performance. In practice, it is a difficult task to find a large number of parents with very high combining ability. Furthermore, predicting yield performance of synthetic cultivars of cross-fertilized diploid and polyploidy forage species is more complicated than is described by their relationship in the equation. Given a set of n inbred lines, the total number of synthetics, N, of size ranging from 2 to n is given by:

As inbred lines increase, the number of possible synthetics increases rapidly, making it impractical to synthesize and evaluate all the possible synthetic cultivars. The theoretical optimum number of parents to include in a synthetic is believed to be about 4–6. However, many breeders favoring yield stability over yield ability tend to use large numbers of parents ranging from about 10–100 or more. Large numbers are especially advantageous when selecting for traits with low heritability.

Synthetics of autotetraploid species  are known to experience severe and widespread decline in vigor between syn-1, and syn-2, which has been partly attributed to a reduction in triallelic and tetrallelic loci. Higher numbers of tetrallelic loci have been shown to be associated with higher agronomic performance of alfalfa. The number of selfed generations is limited to one. Selfed seed from selected S0 plants are intermated to produce the synthetic population. The rationale is that S0 plants with high combining ability would contain many favorable genes and gene combinations. Selecting specific individuals from the segregating population to self could jeopardize these desirable combinations.

Additive gene action is considered more important than dominance genotypic variance for optimum performance of synthetic cultivars. In autotetraploids in which intralocus and allelic interactions occur, high performing synthetic cultivars should include parents that have a high capacity to transfer their desirable performance to their offspring. Such high additive gene action coupled with a high capacity for intralocus or allelic interactions will likely result in higherperforming synthetics.

Synthetic cultivars exploit the benefits of both heterozygosity and heterosis. J.W. Dudley demonstrated that yield was a function of heterozygosis by observing that, in alfalfa crosses, the F1 yields reduced as generations advanced. Further, he

observed that allele distribution among parents used in a cross impacted heterozygosity. For example, a cross of duplex _ nulliplex always had higher degree of heterozygosity than say a cross of simplex _ simplex regardless of the clonal generation used to make the cross. Natural selection changes the genotypic composition of synthetics. The effect can have significant consequences when the cultivar is developed in one environment and used for production in a distinctly different environment. There can be noticeable shifts in physiological adaptation as

well as morphological traits. For example, growing alfalfa seed in the western states

for use in the Midwest, the cultivars may lose some degree of winter hardiness, a trait desired in the production region of the Midwest. A way to reduce this adverse impact is to grow seed crop in the west using foundation seed from the Midwest.