Leaf and stem dry matter digestibility and ruminal undegradable proteins of alfalfa cultivars

Abstract

Increased dry matter digestibility and ruminal undegradable protein (RUP) concentration are traits highly ranked to improve the nutritive value of forage legumes. Whole-plant, leaf, and stem samples of 27 alfalfa cultivars grown in eastern Canada were analysed for total nitrogen (TN), in vitro RUP, and in vitro dry matter digestibility (IVDMD). The objectives were to determine if significant variation exists among these cultivars, and to establish the relationship between nutritive value parameters of leaves and stems, and those of whole plants. Cultivars were seeded in triplicate on 2 consecutive years and evaluated during the 2 subsequent production years with two harvests per year. There were significant differences among cultivars in IVDMD of stems and plants in spring growth and summer regrowth. Cultivar differences in leaf IVDMD were significant only in spring growth, but this variation was negligible in practical terms. Cultivars with high forage IVDMD also had high stem IVDMD and this relationship was stronger in summer regrowth. Alfalfa plant IVDMD is a function of stem digestibility and the leaf weight ratio (LWR). Cultivar differences were significant for plant TN in spring growth and summer regrowth, and for stem TN in spring growth only. Leaf (270 to 306 g kg-1 TN) and stem (242 to 287 g kg-1 TN) RUP concentrations varied among cultivars in spring growth. Cultivars with high stem RUP also had high leaf RUP concentration but differences in plant RUP were not always attributed to leaf and stem RUP. The RUP concentration was on average 10% higher in leaves than in stems in spring growth, and this difference reached 15% in summer regrowth. The LWR varied among cultivars in spring growth and summer regrowth, and it was positively related to plant RUP in spring growth. Improved whole-plant RUP concentration is unlikely to be achieved by simply selecting for high RUP in leaves or stems; LWR has a principal role in determining alfalfa plant RUP concentration. Key words: Dry matter digestibility, ruminal undegradable proteins, alfalfa, genetic variation, stems, leaves