I would start with population of country - assume x% eat poultry - normal frequency of consumption (eg 1kg/week, 1kg/month, 1kg/6 month)- change of frequency in consumption (for eg if normally 50% people ate it 1x per week, I'd assume only 30% ate in once/week now given less disposable income, and calculate this 20% decline in consumption) - sum the incremental decline in kg/year consumed - multiply by price $/kg..this would give incremental decline in personal consumption market
then I'd assume x% of personal consumption is business consumption (restaurants)..justify it with some personal example like 1 out of 10 meals I eat is from restaurants. I'd also assume same frequency of consumption trends applied in restaurants just to keep things simple.
the total of the two segments would give the incremental decline. Would love to hear other suggestions and approaches.