In vitro study of the forage quality of the potential psyllid-resistant Leucaena hybrid

2008 
A major limitation to leucaena (L. leucocephala) productivity is the psyllid insect (Heteropsylla cubana). This insect causes yield losses by feeding on the phloem of young shoots and leaves, thus limiting the rate of new growth. Work is in progress to establish a high yielding psyllid-resistant hybrid of Leucaena using resistance genes from L. pallida; however, the forage quality of L. pallida is lower than that of the commercial cultivars. The aim of this research was to assess the in vitro forage quality of psyllid-resistant breeding lines. Lines from the breeding program were established in a stratified row column design. Agronomically elite plants were selected including 19 progeny of backcrosses of F1 and F3 hybrids (BC1 & BC3), 21 outcrosses (OC) and 10 trees of each of the commercial L. leucocephala ssp. glabrata cvv. Tarramba (T) and Cunningham (C). The youngest fully expanded leaves were sampled in February 2007, freeze-dried and prepared for analysis of crude protein content (CP), in vitro dry matter digestibility (IVDMD) and condensed tannin content (CT). Extractable (ECT) and bound (BCT) tannins were both assessed and analysed as well as the total tannin content of the plants. Crude protein was determined using a LECO CNS analyzer, IVDMD using the pepsin-cellulase method (McLeod & Minson 1978) and condensed tannins were extracted using a proanthocyanidin (butanol/HCl) assay and determined using a scanning spectrometer (Dalzell & Kervan 1998). Results for CT were measured against a L. pallida standard and therefore results are relative, rather than actual, values. Oneway ANOVA and LSDs were used to compare means for each forage quality parameter. The breeding line BC1 was not significantly different (P>0.05) in CP content to Tarramba and Cunningham, however, BC3 and OC were significantly lower (P 0.05) in IVDMD to Tarramba. Plant lines BC1, BC3 and Cunningham were found to be significantly higher (P<0.05) than Tarramba. All lines were found to be significantly lower (P<0.05) in IVDMD compared to Cunningham. The OC lines had a significantly higher condensed tannin content (ECT + BCT) than the remaining lines (20.0% DM compared to less than 15.8% DM for other lines). The comparison of means showed that BC1, BC3, OC and Tarramba were significantly higher (P<0.05) than Cunningham in CT content, with BC1 and BC3 being similar to Tarramba with means ranging from 15.1 to 15.8% DM.
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