Bird-resistant sorghum research


Bird damage to sorghum crops is so severe in some parts of the world
that only bird-resistant (BR) varieties can survive until harvest.
Unfortunately, the polyphenolics (tannins) which produce astringency,
 and thereby repellency, also reduce the palatability, digestibility,
 and nutritional quality of any foods containing them. The lower
 marketability of these varieties has, in turn, resulted in a worldwide
 trend to abandon all tannin-containing sorghums. This situation is
 especially serious for farmers in regions that have severe bird damage
 problems and cannot otherwise protect their crops or grow alternatives
 such as wheat, corn, or rice. The objectives of our research efforts
 are to develop a better understanding of bird resistant sorghums
 through definitive laboratory analyses of their chemical, biochemical,
 and physiological characteristics and how these correlate with bird
 repellency. Several reports in the literature indicate that tannin
 content does not necessarily reflect the bird resistance potential or
 biochemical properties of a sorghum variety. We evaluated a number
 of BR sorghums and the results generally confirm this observation.
 Chemical tests: The properties of tannins have been more extensively
 studied in some fruits (e.g., bananas, peaches, plums) than in
 sorghum. For example, immature bananas are astringent but they lose
 this characteristic at maturity. The tannin molecules "grow" (poly-
 merize) during the maturation process and lose their astringency
 primarily because of size. Three chemical tests, vanillin (V),
 Folin-Denis (FD), and leucoanthocyanin (LA) are indicators of this
 tannin development process. Ratios of these values are indicators of
molecular weight differences. There is an increase of V/LA values
and a decrease of V/FD values as maturation progresses. In all cases,
reactivity is lower in ripe fruit than in green or immature fruit. We
conducted these same tests on eight BR sorghum varieties in four dif-
ferent stages of maturity. The results are given in Table 14.

Table 14. Mean polyphenol values (%) for extracts from eight BR
 sorghum varieties in four stages of maturity.


Stage of Vanillin Folin-Denis Leucoanthocyanin
maturity (V) (FD) (LA) % V/FD % V/LA

Milk 0.15 0.47 0.30 31.9 50.8

Light dough 0.28 0.62 0.66 45.2 42.4

Firm dough 0.37 0.70 0.74 52.8 50.0

Mature 0.15 0.40 0.47 37.5 31.9