Diet selection and performance of cattle and horses grazing in heathlands.

As the number of horses is steadily increasing in Cantabrian less-favoured areas, mostly occupied by nutritionally poor heathland communities, the objective of this research was to study and compare diet selection and performance of crossbred horses and beef cattle (Asturiana de los Valles breed) grazing these areas during the summer. A total of 20 cows (seven lactating their calves and thirteen dry) and 20 mares (eight lactating their foals and twelve dry) were managed during 3 years on a heathland area from June to September and October. The proportion of grass (mostly Pseudarrhenatherum longifolium) and heather (Erica umbellata, E. cinerea and Calluna vulgaris) in the diet selected by cows and mares was estimated in two occasions (July and September) each year using the alkane markers. All animals were weighed at the beginning, middle and end of the summer grazing season. Mares selected heath grasses and rejected heather species more than did the cows (P < 0.001), particularly in early summer (0.85 v. 0.65 herbage proportion in July; 0.63 v. 0.55 in September). Lactating state did not affect diet composition. Mares achieved more favourable (P < 0.001) body weight (BW) changes than cows, but there was also an interaction with season (P < 0.001). From June to August, cows lost BW, being the daily changes (in percentage of initial BW) -0.15%/day in lactating and -0.05%/day in dry cows, whereas both lactating and dry mares gained BW (0.16 and 0.12%/day, respectively). From August to October, lactating cows and mares lost more BW (P < 0.01) compared with their non-lactating counterparts (-0.19 v. -0.10%/day in cows, -0.14 v. -0.05%/day in mares). With regard to the offspring, BW gains between June and August were similar in calves and foals (0.83 and 0.90%/day, respectively), but thereafter calves achieved higher (P < 0.05) BW gains than foals (0.37 v. 0.16%/day). It is concluded that the low nutritive quality of these heathlands restricts, both cattle and horse performance, even during short summer grazing season, being more penalized lactating animals, especially the cows. Although calves were able to maintain acceptable growth rates, foals showed reduced ones during the second half of the grazing season. Other plant communities with better nutritive quality should be provided to assure replenishment of body reserves in dams, and enhance offspring gains to maintain sustainable grazing systems with productive herds of cattle and horses in these heathland areas.