Salivary amino acid concentrations in zebus (Bos indicus) and zebu hybrids (Bos indicus x Bos taurus) fed a tannin-rich diet

Many animals show adaptation to tannins in the form of tannin-binding salivary proteins (1). Among ruminants, such proteins have been demonstrated in saliva of several species (usually browsers and intermediate feeders) (2, 3, 4, 13). There is some circumstantial evidence to suggest that zebu cattle (Bos indicus) are different from temperate cattle breeds with respect to their salivary and digestive physiology. Apart from differences in susceptibility to heat and tropical disease (5), a difference in salivary anti-tannin defenses (and a resulting difference in rumen physiology) could be another reason zebu cattle are particularly suited for agricultural systems in the tropics, where available forages often contain high levels of tannins (6, 7). Although non-proline-rich proteins exist that also have affinity for tannins (1, 8), it is interesting to compare the proline content of different cattle breeds. Here, we report such a screening for a comparison of zebu cattle and zebu-HolsteinFriesian in the Jimma area located at 7°40′N and 36°50′E at 1760 masl in southwest Ethiopia. For the study eight heifers were used: four were zebu (100% Bos indicus) and four were zebu × Holstein Friesian (HF) crosses. The blood level of crossbreed heifers (Bos indicus × Bos taurus) were composed of 70% HF+30% zebu, heifer 1; 66% HF+ 34% zebu, heifer 2; 68% HF+ 32% zebu, heifer 3 and 70% HF+ 30% zebu, heifer 4. The animals were 2.5 years old with comparable body weight and similar body condition scores. The body condition score was evaluated based on 1-9 point score scale (9). They were fed on a diet that included the tannin-rich plant Albizia gummifera for 28 days. The animals were fed on a local hay mixture as a basal diet and experimental diet of leaves of A. gummifera. The diets were composed weekly to ensure that cattle would consume A.gummifera at a rate of 10% of total dry matter (DM) requirement, estimated as 2.5% of live body weight. To minimize selectivity by the animals, the A. gummifera forage was provided in the morning (8:00) whereas hay mixture was offered only later at 10:30. After 21 days, saliva samples were collected from the animals’ mouths using a sponge. When the sponge was saturated with saliva, it was squeezed manually (with the investigator wearing fresh latex gloves), allowing the collection of a minimum of 10 ml saliva into a plastic cup with screw top. The saliva was then passed through a tea sieve to remove feed particles, and stored at -43°C. When the samples were thawed for analysis, they were passed through a 0.3µm syringe filter to remove bacteria. Amino acids were determined according to Hendriks et al. (2002). From these data, the proportion of proline in the total amount of measured amino acids was calculated. Differences between genotypes were evaluated by means of a Student’s t-test. Significant differences were considered at P < 0.05.