Do alfalfa grazing and lactation length affect the digestive tract fill of light lambs?

This experiment tested the hypothesis that alfalfa-raised light lambs suckling their mothers until slaughter (22–24 kg) and having concentrate ad libitum may consume forage to achieve a similar performance to their weaned concentrate-fed counterparts. Thirty-two single Rasa Aragonesa lambs were assigned to one of four treatments in a 2 × 2 factorial design. The factors were the inclusion of forage in the diet (alfalfa grazing vs. concentrate-fed indoors) and lactation length (weaning at a target body-weight (BW) of 13 kg vs. suckling until slaughter). After slaughter, the digestive contents of each gastrointestinal compartment were weighed individually and the pH of the rumen and caecum contents was recorded. Samples of the reticulo-rumen, caecum and colon-rectum contents were collected for neutral-detergent fibre (NDF), acid-detergent fibre (ADF) and lignin analysis.The empty BW was not affected by forage inclusion or suckling (P > 0.05). The fresh reticulo-rumen and whole digestive tract contents (liquids + solids) were greater in weaned alfalfa-fed than in suckling alfalfa-fed lambs (P < 0.05), but they were similar in concentrate-fed lambs (P > 0.05). On a dry matter (DM) basis, these digestive contents did not differ between alfalfa and concentrate-fed lambs (P > 0.05), but they were greater in weaned than in suckling lambs (P < 0.01). Weaned alfalfa-fed lambs had greater ruminal and caecal pH values than the rest of treatments (P < 0.05). The reticulo-rumen NDF content was greater in weaned alfalfa-fed than in suckling alfalfa-fed lambs (P < 0.05), while it was similar in concentrate-fed lambs (P > 0.05). The lowest reticulo-rumen ADF and lignin contents were observed in both groups of concentrate-fed lambs (P < 0.05). The caecum and colon-rectum NDF contents did not differ among treatments (P > 0.05). The caecum and colon-rectum ADF contents were greater in alfalfa-fed than in concentrate-fed lambs (P < 0.05). According to digesta composition, weaned lambs raised on alfalfa consumed more forage than suckling lambs on these swards. Lactation length rather than forage supply was a major effect on the digestive tract fill capacity of light lambs.