Strategies to enhance efficiency of microbial protein production in cattle consuming tropical forages

Microbial crude protein (MCP) is the main source of protein supplied to ruminants. Several studies report that the efficiency of MCP production (eMCP) from tropical forages is low due to low rumen degradable protein (RDP) content. The studies reported in this thesis determined the eMCP from tropical forages varying in crude protein (CP) and dry matter digestibility (DMD), examined strategies to increase eMCP and the possible practical methods of application under extensive cattle production systems. This was done through four metabolism experiments, one rate of digestion experiment and a preference test experiment. In Experiment 1, the eMCP of 3 tropical forages (speargrass (Heteropogon contortus), Mitchell grass (Astrebla spp) and pangola grass (Digitaria eriantha)), and temperate ryegrass (Lolium multiflorum cultivar aristocrat) was measured. All tropical forages were associated with low eMCP (102 - 110 g MCP/kg digestible organic matter (DOM)) due to low CP and RDP content. The RDP content of the tropical forages ranged from 43 - 103 g RDP/kg DOM compared to 214 g RDP/kg DOM in ryegrass. Retention time of Cr-EDTA and Ytterbium in the rumen ranged from 13 - 34 and 29 - 59 h for tropical forages respectively, compared to 10.2 and 13.2 h for ryegrass. In Experiment 2, the rate of digestion of speargrass, Mitchell grass and ryegrass varied when incubated in the rumen of steers consuming speargrass hay (1.8% CP). Addition of a variety of nitrogen supplements; urea-ammonium sulphate (US), US plus branch chain amino acids (leucine, isoleucine and valine) plus phenylalanine (USAA), casein, cottonseed meal (CSM), yeast and Chlorella (Chlorella pyrenoidea) algae to supply approximately 170 g RDP/kg DOM improved the rate of digestion of all three substrates. However, there was no difference in rate of digestion of the three substrates between the different nitrogen supplements. In Experiments 3 and 4, steers fed medium quality pangola grass hay (9.9% CP and 61% DMD) or low quality Mitchell grass hay (4.5% CP and 41% DMD) were supplemented with US, casein and USAA. US was provided at 130, 170, 210 and 250 g RDP/kg DOM. The USAA treatment was provided at a RDP/DOM of 170 with US and then amino acids (USAA) were provided in the same ratio and level as the amino acids in the casein treatment at 130, 170, 250 and 300 g RDP/kg DOM. Increased RDP intake from all supplements increased the rumen ammonia-N concentration for both hays. The molar percentage of branch chain fatty acids (BCFA) increased 3 - 4 fold (6.6% of total VFA) above control with increasing intake of USAA or casein supplements for both hays. However, eMCP did not increase in response to increasing RDP intake from either supplement for both hays. The RDP supplementation also had no effect on intake, rate of digestion and retention time of Cr-EDTA in the rumen. In Experiment 5, steers fed Mitchell grass hay (3.8% CP and 44% DMD) were supplemented with US and Spirulina (Spirulina platensis) algae. The US was supplied to provide 90, 130, 170 and 210 g RDP/kg DOM and the Spirulina supplied to provide 90, 130, 170 and 290 g RDP/kg DOM. Increased RDP intake from US and Spirulina was associated with linear and quadratic increases in eMCP respectively. The eMCP at the highest level of US (93 g MCP/kg DOM) was below the minimum feeding standards value of 130 g MCP/kg DOM, whilst the highest level of Spirulina reached the higher end of the feeding standards (166 g MCP/kg DOM). Increasing RDP intake resulted in increased hay DM intake and total DOM intake, which were both higher for the Spirulina than the US supplement (24.5 vs.18.5 g DM/kg W/d and 11.9 vs. 7.6 g DOM/kg W/d). The retention time of Cr-EDTA and lignin in the rumen decreased with increasing RDP intake and were both shorter with Spirulina compared with US supplementation (15 vs. 21 h for Cr-EDTA; 45 vs. 56 h for lignin). In addition, Spirulina increased the BCFA concentration to 4.2% of total VFA. In Experiment 6, four different concentrations of Spirulina solution were offered to steers fed restricted amounts (14 g DM/kg W/d) of pangola hay (3.4% CP). Drinking water containing Spirulina was highly preferred and had no deleterious effect on water or hay intake. Steers strongly preferred Spirulina solution to water, when offered both solutions. Approximately 20.5% of imbibed water by-passed the rumen, which was not influenced by the Spirulina concentration in the drinking water. It may be concluded that RDP is the primary factor affecting eMCP. High eMCP was associated with low retention time of water in the rumen and high intake. Once RDP is not limiting eMCP, a source of RDP rich in true protein, nucleic acids, fatty acids, vitamins and minerals, such as Spirulina algae, may be required for cattle fed low quality tropical hay to achieve the maximum eMCP reported in the feeding standards