Sorghum is not toxic to cattle, all types of sorghum is raised and used for grazing, forage, and silage for cattle. Sorghum is still grown in the United States and is processed or used for a substitute for sugar, molasses, honey, or other sweaters. This product is known as "sorghum syrup"Sorghum is toxic at a certain life stages, generally beginning at 2 years old, or after it's cut or been allowed to seed (reports vary and it varies between types too) and it will kill cattle that are allowed to graze it, never mind chooks.
Sorghum can contain lethal levels of prussic acid (better known as cyanide). As well as cyanide, sorghum can have elevated levels of nitrates. Drought, reproduction, attack (whether by insects or human mechanisms) etc can all inspire elevated levels of toxins in the average plant.
Best wishes.
http://www.southernliving.com/food/how-to/sorghum-syrup
Grain sorghum is an annual sweet grain. It is not a perennial. There are however some types of sorghum that are advertised as being bird resistant. These types of sorghum have a high acid or tannin content. This type of sorghum will give you a bad stomach ache if you eat too much of it. Sorghum grain is still widely used in Africa and also eaten or drunk as whole grain or beer still containing the grain.
When a load of grain sorghum reaches the elevator it is tested to see if it is the bird resistant variety before it is off loaded. The reason I know this is because over 60 years ago I rode around with my dad for two days in a two & a half ton truck loaded with bird resistant grain sorghum (sleeping in the cab) before pop found an elevator that would buy it. All I can say about this experience is "Thank goodness for 25¢ gas and Moon Pies."

I am unsure if the poster meant to say that grain sorghum is a perennial instead of an annual or that grain sorghum stored for two years will go bad, (which is true with sorghum contaminated with the Aflatoxin yeast spores associated with drought) or if he meant to say that grain harvested from second year or perennial sorghum plants is toxic.
Long stored grain sorghum like all stored grain is dangerous mostly because sorghum grain is today mostly grown as a drought resistant crop (at least in the USA) and because of this resistance to drought grain sorghum is highly prone to being infected with the afore mentioned Aflatoxin spores.