Patterns of diet mixing by sheep offered foods varying in nutrients and plant secondary compounds

Abstract

Ruminants foraging on landscapes choose among plants that differ in nutritive value and kinds and amounts of plant secondary compounds (PSCs). We hypothesized that food availability and the sequence of food ingestion influence intake of PSCs and nutritious foods by sheep because PSCs increase need for nutrients. We fed two nutritious foods – alfalfa and alfalfa–barley mixture (50:50) – and three isoenergetic (DE: 2.9Mcal/kg) and isonitrogenous (CP: 126g/kg) foods that contained oxalates, tannins, or terpenes. The experiment consisted of an 8-d conditioning period and four trials in which we tested our hypotheses. In the conditioning period, the three PSC-containing foods were offered ad libitum but the two nutritious foods were restricted to 300g/d and the daily intake of each food was measured. In trial 1, all five foods were offered from 0800 to 1600h. In trial 2, the two nutritious foods were offered from 0800 to 1200h whereas the three PSC-containing foods were offered from 1200 to 1600h. In trial 3, the three PSC-containing foods were offered from 0800 to 1200h while the two nutritious foods were offered for the next 4h. Finally, in trial 4 all five foods were offered again from 0800 to 1600h. Each trial lasted 8 d, except the last one, which was 5 d. In the four trials, foods were offered ad libitum and the amount of each food ingested was measured daily at hourly intervals. As conditioning progressed, consumption of all three PSC-containing foods increased, and the pattern of eating changed from oxalate>tannin>terpene to tannins>oxalates>terpenes. During trial 1, lambs ate alfalfa–barley>alfalfa=tannins>oxalates=terpenes. In trial 2, sheep ate 1490g/d of nutritious foods in the morning and 863g/d of PSC-containing foods in the afternoon for a total of 2353g/d. In trial 3, when the sequence was reversed, sheep ate 1021g/d of PSC-containing foods in the morning and 1564g/d of the nutritious foods in the afternoon for a total of 2585g/d. By trial 4, the pattern of diet mixing had changed such that sheep ate alfalfa–barley>tannins>alfalfa=terpenes>oxalates. In summary, sheep ate more PSC, and more nutritious food, when PSCs were offered in the morning as opposed to the afternoon. Limiting the availability of nutritious alternatives encouraged animals to learn to use different kinds of PSCs and enhanced diet breadth. Beyond the effect of diet restriction, lambs learned the benefits of mixing food with tannins, terpenes, and oxalates and continued to do so even when they subsequently had ad libitum access to nutritious foods.