Influence of Temperature Change at Bud on Composition of Alfalfa at First Flower

Alfalfa (Medicago sativa L., cv. ‘Vernal’) plants were grown to bud in three temperature regimes, H (33/24 C, day/night), W (27/18 C), and C (21/12 C), and then part of the plants in each chamber were switched to the other regimes and grown to first flower. When plants were retained in the same regime to first flower, days to bud and first flower, dry weight of plant parts, total nonstructural carbohydrate (TNC), and in vitro digestible dry matter (IVDDM) percentages in plant parts increased with a decrease in temperature, whereas concentrations of P, Zn, and Mn in the herbage decreased with a decrease in temperature. Concentrations of N, K, and Fe were highest and similar in the herbage retained in the H and W regimes and lowest in the C regime. However, concentrations of Ca, Mg, Ba, Sr, and B in the herbage were highest in plants retained in the H regime, and lowest but similar in plants retained in the W and C regimes. A change to a cooler temperature between bud and first flower generally delayed flowering, increased dry weights of plant parts and the percentages of TNC and IVDDM in these parts, and decreased concentrations of minerals in the herbage. Change to a warmer temperature had the opposite effect. Concentrations of TNC, IVDDM, and minerals in the herbage at first flower were influenced most by temperature between bud and first flower, but root TNC percentage at first flower was influenced both by temperature prior to bud and by temperature from bud to first flower. Percentages of TNC and IVDDM in the herbage at first flower were highly correlated (r=0.93). The most dramatic changes in TNC percentages in response to temperature change occurred in leaflets. Changing temperature from H to C at bud increased the TNC percentage in leaflets by 121% as compared with plants held in the H regime. In contrast, changing from C to H at bud decreased TNC percentage in leaflets by 62% as compared with plants held in the C regime. From a practical viewpoint, alfalfa herbage harvested after a cool period of weather should have a greater energy feeding value and silage fermentation capacity than herbage harvested after a warm period.