Forage utilization by kids and lambs

Comparative aspects of intake, digestibility, partitioning of digestion, energy requirement, energy utilization and water metabolism of kids and lambs have been made. Ad libitum intake and digestibility were determined in a series of short-term (4-6 weeks) experiments with forages ranging in N content from 6.6 to 26.8 g N/kg DM. The forage types were meadow hay, lucerne hay, prairie grass hay, ryegrass hay, ryegrass straw, cocksfoot straw and barley straw. No consistent differences between the animal species were found in the digestibility of DM, OM or NDF. However, as digestibility of the forage declined the ratio of DOMI (kids: lambs) increased indicating that with maintenance and sub maintenance forages kids were able to maintain a higher intake of digestible nutrients. The difference between the species was particularly evident below an OMD of approximately 0.6. A high and a low quality meadow hay (23.9 and 7.1 g N/kg DM, respectively) were offered during longer periods of 13-16 weeks. No differences in OMD occurred between the species on the high quality forage but with the low quality forage OMD in lambs gradually declined over the 16 week period whilst in kids OMD was maintained. This further emphasizes the ability of kids to obtain and maintain a higher input of digestible nutrients with low quality forage. Higher DOMI of forages with low N content appeared to be related to an ability of kids to maintain higher rumen NH₃-M concentrations. The quantitative partitioning of digestion in the stomachs (R), small intestine (SI) and large intestine (LI) of both species was examined in fistulated kids and lambs offered a high quality meadow hay at varying levels from maintenance to ad libitum. No differences were observed between the species in digestion of OM, NOF and NAN at any of the sites and regression equations relating nutrient flow (g/kgW⁰•⁷⁵/d) against nutrient intake were established. Approximately 0.60, 0.29 and 0.13 of the in vivo DM digestion (0.78) occurred in the R, SI and LI, respectively, in both species and approximately 0.94 and 0.06 of the in vivo NDF digestion (0.82) occurred in the R and LI of both species. There was a gain of total N (22%) and NAN (15%) in the R of both species whilst net disappearance of NAN in the SI and N in the LI was approximately 0.62 and 0.27, respectively. All these parameters varied slightly with level of feeding. The protein energy apparently absorbed in the small intestine per unit metabolizable energy intake (0.17) was low and not different between the species. A further group of kids and lambs were offered the same high quality meadow hay in a comparative slaughter experiment to compare the species for maintenance energy requirement (MEm) and efficiency of use of ME for growth (kg). There was no significant difference in MEm (0.44 vs 0.37 MJ ME/kgW⁰•⁷⁵/d in kids and lambs respectively) and in kg (0.28 ±0.067 vs 0.22 ±0.032) between the two animal species. The efficiency of use of absorbed amino acid for protein deposition was estimated by using the regression equations established for nutrient flow past the abomasum and ileum of fistulated kids and lambs to calculate absorbed amino acid in animals in the comparative slaughter trial. There was a marked difference in efficiency of use of protein for body protein deposition (0.59 ±0.097 vs 0.31 ±0.037; P0.05; kids vs lambs, respectively). The protein energy content of the energy gain was low (0.20) and not different between the species. Kids consistently consumed less water than did lambs (mean ratio of water intake - kids: lambs - was 0.70). When flow of water down the digestive tract was examined in fistulated animals consuming the high quality meadow hay, both species gained water across the R but there was a greater proportional addition of water across the R of kids compared to lambs such that water flow out of the abomasum was similar for both species. Most water absorption occurred in the small intestine of both species and there was no difference in faecal water output between the species. There was no consistent difference between the species in the route of water excretion (urine water/faecal water). It was concluded that, with high quality forages used to promote live weight gain, there was little difference between kids and lambs in the utilization of forage. With low quality maintenance or sub maintenance forages, kids appeared able to obtain and maintain higher levels of nutrient input