Utilization of low quality roughage by goats and sheep

Abstract

A sequential series of studies was conducted to investigate the utilization of low quality roughage by goats and sheep. It has been suggested that goats were able to eat more of, and possibly better digest, low quality roughages than sheep. Two aspects that might have contributed to the higher DOMI in goats were higher digestion rate in the rumen and higher protein: energy ratio of absorbed products of digestion.

The first two experiments were preliminary experiments to establish whether there was a difference in level of intake between the two species on low quality feeds. Rumen ammonia and inorganic sulphur concentrations were measured to aid interpretation of any differences established. Goats consumed more than the sheep, whether the DOMI was expressed in relation to metabolic body weight (W⁰•⁷⁵) or simply body weight (W) by 1.40 or 1.69 times, respectively. Higher rumen concentrations of ammonia nitrogen (82 vs 65 mg NH₃ N/1) and inorganic sulphur (7.3 vs 4.3 mg S/1) were measured in goats than in sheep.

The third experiment investigated whether the higher rumen ammonia concentration in goats was implicated in their higher intake through better rumen microbial activity. A range of rumen ammonia concentrations was established in the two species by supplementation of a basal diet with urea. There was little effect of rumen ammonia concentration on rumen microbial activity as indicated by the %DM apparently degraded in nylon bags suspended in the rumen. However, when digestion rate was related to the %CP in the diets, sheep had slightly better rumen microbial activity than goats. In this context, therefore, rumen ammonia concentration did not appear to be a good parameter for assessing nitrogen adequacy for rumen microbes in individual animals.

A fourth experiment was designed to clarify the above finding, by using a more robust experimental design, involving ad libitum feeding of both species and pair feeding of goats to the intake of sheep. This confirmed that the rumen microbial activity in sheep, as measured by digestion rate, tended to be better than in goats even at lower rumen ammonia concentration, and thus suggested that the higher rumen ammonia concentration generally observed in goats when feeding on low quality roughage was unlikely to be the cause of higher OM intake in goats. Furthermore, the higher rumen ammonia level in the goats was not due to the level of intake, as restricted goats had a similar rumen ammonia concentration to goats fed ad libitum.

Three further experiments (5, 6, 7) examined whether goats had a higher duodenal protein supply than sheep, particularly in relation to energy supply. This was measured as g duodenal NAN/kg DOMI and also as P/E (g intestinally digestible protein/MJ digestible energy). In Experiment 5, both species had similar %DM digestion in the reticulorumen (19) and similar flow of NAN mg/kg W⁰•⁷⁵/d at the duodenum (421) at similar DM intakes. Sheep tended to have higher duodenal NAN g/kg DOMI and P/E values than goats (15.2 vs 12.4 g digestible protein/MJ DE). A small number of animals was used (n = 5) and high variability in values of parameters within species was apparent. Volume of rumen digesta (l/kg W) determined at slaughter was, however, consistently higher in goats than in sheep (0.19 vs 0.17).

In Experiment 6, the responses of DOMI/ W⁰•⁷⁵/d in both species to urea and sulphur supplementation were investigated. Urea (0.65%) and sulphur (0.03%) supplementation (% of feed DM offered) tended to increase the intake of both species but not significantly (8.5 vs 7.0 g DOMI/kg W⁰•⁷⁵). The apparent digestibility of DM in the rumen of both species also tended to increase slightly with urea and sulphur supplementation (24.2 vs 20.0%). Flow of duodenal NAN mg/kg W⁰•⁷⁵/d did not vary as a result of supplementation or between animal species (mean 352). While there was no difference between the two species in the g duodenal NAN/kg DOMI or P/E values, supplemented animals tended to have lower values for these parameters than control animals (13.2 vs 15.2 g digestible protein/MJ DE).

A feature of intake on poor quality forages is its irregularity from day to day and between animals and this was especially so during Experiments 5 and 6. Accordingly, there was large variation in the extent and site of digestion. Experiment 7 was designed to examine these parameters further using a different low quality forage (fescue straw) known to promote a higher intake. Goats had higher DOMI/W than sheep. In this case, the flow of NAN through the duodenum of goats was higher than in sheep (952 vs 765 mg NAN/kg W⁰•⁷⁵/d). However, when expressed as g duodenal NAN/kg DOMI, sheep tended to have a higher value (48 vs 42). Thus the higher NAN flow through the duodenum of goats was mainly due to their greater intake of food. This again suggested that the higher DOMI/W in goats was not due to higher g duodenal NAN/kg DOMI or P/E values. The apparent digestibility of DM in the rumen of goats tended to be higher than that of sheep (36 vs 30 %). This may have been associated with the goats' longer apparent retention time for DM in the rumen (28 vs 22 h) and significantly longer retention time of a liquid phase marker in the rumen (13 vs 10 h) than sheep. The higher rumen digestion may have made only a minor contribution to the goats' higher intake. The higher intake of goats than sheep was accommodated by a significantly much larger rumen volume (0.23 vs 0.14 l/kg W) leading to a larger rumen fill of DM (25.8 vs 15.7 g/kg W).

It was concluded that goats do not have a higher rumen digestion rate or a higher duodenal NAN/DOMI ratio but rather accommodate higher intake of feed by a higher level of rumen fill. It was further suggested that the drive for a higher DOMI/W by goats on sub maintenance/maintenance quality forages may be the result of their higher maintenance energy requirement.