The effect of harvesting time (morning vs. afternoon) on the chemical composition and nutritional value of sun-drying alfalfa, clover and barley forages

Introduction[1] Forages are grown mainly for feeding livestock, especially in dairy cows, because adequate roughage is needed in diets to provide good rumen function. However, as more roughage is fed, the energy density of the diet is reduced. So, the production of high quality forage is very important for dairy producers. High quality forage has direct effects on animal production efficiency, including weight gain, milk production, and reproductive success. Producing and conserving of high quality forage is a challenge because several factors can be affected forage quality including plant species, soil fertility, maturity at harvest, and harvesting (mowing, field curing, baling or chopping) and storage methods and other factors (weeds, insects and diseases). Fiber and energy contents are the most important in forage quality measures. As the fiber level increases, the energy content generally decreases. Therefore, improving forage quality can be achieved by managing forage carbohydrate content. Carbohydrates are the primary source for ruminants and contribute 60 to 70% of the net energy used for milk production and are classified as structural and non-structural. As usual, structural carbohydrates defined as neutral detergent fiber (cellulose, hemicellulose, lignin and portion of the pectin) and non-structural carbohydrates consist of the sugars, starches and pectin. Non-structural carbohydrates are a highly digestible energy source and together with degraded protein, are needed by the rumen for microbial growth and digestion. Plants accumulate sugars during the day via photosynthesis, but incur a net loss at night via dark respiration. This diurnal cycling reflects the concentration of total nonstructural carbohydrates in forages. The aim of this study was to evaluate the effect of morning versus afternoon cutting time on chemical composition, gas production parameters and digestibility of sun-drying alfalfa, clover and barley forages. Materials and Methods About 5-7 cm above the soil stage, alfalfa, clover and barley were harvested in two times, at 06:00 AM and 18:00 PM. Whole Alfalfa and clover plants were harvested at the first bud stage of development and whole barley plant at the medium dough stage of maturity used. Their nutritive value was evaluated through the determination of chemical compositions and in vitro gas production techniques. Samples were tested in an in vitro gas production method (96 h incubation) and batch rumen culture system (24 h incubation). Rumen fluid was collected before the morning feed from three fistulated Dalagh male sheep (45 ± 1.5 kg live weight fed on a forage diet at a concentration of 40:60). In vitro gas production was measured in triplicate and for each replicate; a sample of 200 mg DM was used. The bottles were then filled with 30 ml of incubation medium that consisted of 10 ml of rumen fluid plus 20 ml of buffer solution and placed in a water bath at 39 °C. Gas production was recorded at 2, 4, 8, 16, 24, 48, 72 and 96 h. Total gas values corrected for blank incubation and gas values expressed in ml g-1 of DM. The asymptotic gas production system (A) and rate of gas production (c), organic matter digestibility (OMD), metabolizable energy (ME) and short chain fatty acids (SCFA). A medium similar to one developed for gas production was used for batch rumen culture system to measure pH, and NH3-N and in vitro digestibility. The pH of the media was measured after 24 h incubation. After 24 h incubation, the contents of each glass bottle were empty, strained through four layers of cheesecloth and then 10 ml of strained rumen fluid was acidified by 10 ml of 0.2 N HCl for determination of NH3-N using the distillation method. Finally, all contents remaining in the bottles were filtered through nylon bags, oven dried at 60 °C for 48 h and analyzed for IVDMD and IVOMD. Results and Discussion Results showed that cutting time affected chemical composition of alfalfa and clover forages significantly (p < 0.05), but had not effect on barley forage. Afternoon cutting forages had lower content of NDF and ADF and higher levels of starch and WSC compared to morning cutting forages. There were no significant differences between afternoon and morning cutting forages on gas production parameters (P > 0.05). However, Afternoon cut forages had higher gas production potential than morning cutting forages. Results showed that harvest time had significant effect on DMD (66 vs 59) and OMD (64 vs 58.5) of barley forage. Although, time harvesting had no effect on MCP, EMCP and PF (P>0.05), but Afternoon cut forages had higher MCP, EMCP and PF than morning cutting forages. Conclusion Generally, it was concluded that with considered all factors that affected quality and nutritive value of forages, delaying forage harvest until late afternoon could result in improve nutritive value of forage.