Six experiments were conducted to investigate methods of improving nutrient uptake from autumn-grown rhodes grass (Chloris gayana) by lactating dairy cows in the subtropics.
In the first experiment, the growth, nutrient content and in vitro dry matter (DM) digestibility of standover and regrowth Callide rhodes grass (Chloris gayana cv. Callide) were compared during summer and autumn. Pasture was stood-over from 28 December 1994 to 19 April 1995. Regrowth pasture was cut every six weeks during this period. Average metabolisable energy (ME) content of regrowth leaf and stem were 9.0 and 8.5 megajoules (MJ).kg DM-1 respectively. Average ME content of standover (SO) leaf and stem were 8.5 and 7.8 MJ. kg DM-1 respectively. The crude protein (CP) and ME contents of the regrowth (RG) herbage were generally superior to SO herbage, and cell wall constituent concentrations of RG herbage were consistently lower than SO herbage. All rumen degradable protein (RDP):ME ratios of SO herbage fell below 7.8g RDP.MJ-1 (ARC 1984; SCA 1990), while this occurred on only four occasions for RG herbage.
The responses to RDP and molasses supplementation were examined in two experiments. In the first experiment (Experiment 2), four ruminally-fistulated, multiparous Holstein Friesian cows in early lactation were offered mixed diets based on autumn-grown Callide rhodes grass containing 6.9, 8.2, 9.5 and 10.8 g RDP per MJ of ME, ina4x4 latin square design. Positive linear relationships were found between ratios of RDP: ME and daily intakes of organic matter (OM) and CP, and milk yield. A positive linear relationship was found between the ratio of RDP: ME and protein yield. Significant negative linear relationships were found between OM digestibility, neutral detergent fibre (NDF) digestibility, the ratio of molar proportions of (acetate + butyrate): propionate and RDP: ME. Positive linear relationships were found between the ratio of RDP: ME and rumen concentrations of NH3, total volatile fatty acids (VFAs), and molar proportions of valerate, isovalerate and isobutyrate. RDP: ME had no effect on rumen digestion kinetics.
Molasses supplementation was examined in the second experiment (Experiment 3). Three of the cows in mid-lactation from the previous experiment were given diets based on autumn-grown Callide rhodes grass containing no molasses, 125 g molasses, kg DM-1, 250 g molasses, kg DM-1 or 250 g molasses plus 0.02 g monensin.kg DM-1 in a 4x4 incomplete latin square design. Increasing dietary molasses content linearly increased diet intake, but had no effect on milk production or diet digestibility. Molasses significantly decreased the concentration of total VFAs and raised the molar proportion of propionate. The molar proportion of rumen butyrate was significantly reduced by monensin. Negative linear relationships were found between dietary molasses content and rumen retention time, total mean retention time and rumen available DM content of Callide rhodes grass hay. Feeding molasses with autumn-grown Callide rhodes grass to cows in mid-lactation can increase intakes but can have a negative effect on the rumen availability of this herbage. Molasses can be included up to 25% of DM intake in mixed diets based on Callide rhodes grass without affecting milk production for Holstein Friesian cows in mid-lactation.
Although some positive responses were obtained to RDP and molasses supplements, the low energy content of autumn-grown Callide rhodes grass still remained a primary constraint to milk yield. Three experiments were conducted to evaluate the effects of treating autumn-grown Callide rhodes grass with alkalis and oxidants on its nutritive value. Ina 3 X 5 factorial experiment (Experiment 4), the effects of water (250, 500 and 750 g kg DM-1) and urea (0, 20, 40, 60 and 80 g urea, kg DM-1) were examined. Significant interactions between water and urea (P<0.05) were found for concentrations of nitrogen, NDF and 48 hour in sacco disappearance of DM (INSDMD), OM (INSOMD), and NDF. Significant negative relationships were also found between urea treatment levels and concentrations of acid detergent fibre (ADF), acid detergent lignin (ADL) and hemicellulose (linear), and acid detergent insoluble nitrogen content, and the 48 hour in sacco disappearance of ADF (quadratic). Positive linear relationships were found between water treatment level and concentrations of ADF and ADL. In Experiment 5, three alkalis (Ca(OH)2, NaOH, CaO) and two oxidants (NaOCl andH2O2) were applied to autumn-grown Callide rhodes grass levels of 0, 20, 40, 60 or 80 g kg.DM-1 NaOH, Ca(OH)2, and CaO lowered cell wall content and improved in sacco disappearances of DM, OM, NDF and ADF while NaOCl lowered cell wall content but had no effect on in sacco disappearances. H2O2 had no effect on either cell wall content or in sacco disappearances when applied alone, and gave no additional responses (over those obtained from NaOH) when applied with NaOH as alkaline hydrogen peroxide. In Experiment 6, haylage made from 100 day old regrowth of Pioneer rhodes grass (Chloris gayana cv. Pioneer) was treated with NaOH, CaO, a microbial inoculant (MI) or a combination of NaOH and MI, under field conditions. CaO had no effect on nutrient value, or OM or DM in sacco digestibility. NaOH significantly improved INSDMD. A significant, positive interaction between NaOH and MI on INSOMD was found. The treated haylage was fed to twelve Holstein Friesian heifers in a randomised complete block design to examine effects of NaOH and MI in a 2 x 2 factorial experiment. NaOH decreased NDF, hemicellulose, cellulose and ammonia contents of the haylage and increased its pH NaOH also increased digestible organic matter intake (DOMI). MI raised haylage pH and also increased the number of combined prehension and chewing bites required to consume haylage ADF.
It is concluded that low ME and RDP content and poor pasture growth are major limitations to nutrient uptake from rhodes grass by dairy cows during autumn in northeastern Australia. Neither supplementation with RDP nor molasses substantially improved the nutrient uptake of autumn-grown Callide rhodes grass by lactating Holstein Friesian cows. However RDP supplementation improved milk production when fed at above current recommendations. Treatment of autumn-grown Pioneer rhodes grass with NaOH improved its nutrient content and the DOMI of non-lactating heifers and may offer a practical way of improving the nutrient uptake of lactating cows offered this herbage.