their basal diet, but they always ate some of both foods. Lambs did not feed exclusively at the Ž
. location with a variety of foods P and E . Rather, they fed at the location nearest the shelter that
contained the macronutrient lacking in their diet. As availability of the food with the needed macronutrient declined in one location, lambs moved to the nearest location that had food with the
needed macronutrient. When food that complemented their basal diet was moved to a different location, lambs foraged in the new location. Collectively, these results show that lambs challenged
by imbalances in energy or protein selected foods and foraging locations that complemented the nutrient content of their macronutrient imbalanced basal diets. q 2000 Published by Elsevier
Science B.V. All rights reserved.
Keywords: Protein and energy; Macronutrient imbalance; Sheep; Foraging location
1. Introduction
While water, cover, predators, and social interactions all are likely to influence choice of foraging location at one time or another, differences in macronutrient content among
forages may also cause ruminants to forage in different locations even during a meal. Range and pasture lands contain a variety of plants with different nutritional profiles and
animals must locate plants of different nutritional quality to meet needs. As plants grow,
Ž .
and their sensory flavor and nutritional characteristics change, animals must adapt their choices of foods and foraging locations to avoid eating a nutrient-deficient diet. Our
objective was to determine if lambs fed macronutrient imbalanced diets selected foraging locations that met their macronutrient needs.
When given a choice among foods that vary in macronutrients, lambs ingest a diet Ž
with a constant ratio of protein to energy Egan, 1980; Provenza et al., 1996; Wang and .
Ž Provenza, 1996a,b . In so doing, they maximize growth rate Kyriazakis and Oldham,
. 1997 . Lambs maintain macronutrient balance by discriminating between digestive
Ž .
feedback signals from protein and energy Villalba and Provenza, 1999 . The proportion of protein and energy consumed and their rates of degradation in the rumen affect food
preference and animal performance by influencing the supply of nitrogen and energy to Ž
rumen microorganisms and to the host ruminant Sinclair et al., 1993; Villalba and .
Provenza, 1996, 1997a,b,c . Domestic and wild herbivores eat a variety of plants of different growth stages while
foraging on pastures and rangelands. Red deer apparently match nutrient needs to time Ž
. spent grazing, though there is debate whether protein Langvatn and Hanley, 1993 or
Ž .
Ž energy Wilmshurst and Fryxell, 1995 is more important see also Berteaux et al.,
. 1998 . The preferences of red deer change seasonally, in part due to changes in plant
Ž .
phenology Wilmshurst et al., 1995 . Perhaps in an effort to find plants that best meet their nutritional needs, while minimizing time spent searching, ruminants eat a variety of
Ž .
foods from different locations even within a meal Provenza, 1996 . Given the apparent importance of protein and energy to herbivores, and the ongoing
debate over which is more important, our objective was to determine whether lambs would forage at locations with foods that complemented the macronutrient composition
of their basal diet. We hypothesized that lambs fed a basal diet low in protein or energy would forage where foods high in protein or energy were located.
2. Materials and methods
