Effects of maize streak virus disease on yield and quality of forage of different maize cultivars.

The effects of maize streak virus disease (MSVD) on yield and quality of maize forage, a need amplified by intensive management practices for maize to obtain forage for livestock, were studied in field experiments carried out between 2001 and 2004 at the Kenya Agricultural Research Institute (KARI), Muguga and at Githunguri and Kiambaa divisions of Kiambu district, Kenya. Further the effect of resistant and susceptible cultivars, plant density, fertiliser and seed arrangement in the hole on yield of grain and forage in conditions of dense planting and natural infection with MSVD were explored. Dry matter (DM t ha -1) of maize thinning, stover and grain yields were determined. Yields were reduced, especially 14 days infection post emergence. Later infection had little effect on yields. Forage and gain yields of the MSVD resistant cultivars KH 521 and PAN 67 were least affected by MSVD. Fertiliser and plant density did not significantly influence the effects of MSVD. Where appropriate, percentage incidence and progress of MSVD in the crop was measured and severity estimated. The resistance of KH 521 AND PAN 67 was always evident by lower MSVD severities compared to the susceptible hybrids, H511 and H614. The local landrace, Gikuyu, showed some tolerance. Delayed planting increased disease severity. Chemical composition crude protein (CP) and neutral detergent fibre (NDF) of maize thinning and stover were determined. The fermentation and degradation characteristics were also examined using the in vitro Reading Pressure Technique (RPT). Infection with MSVD resulted in increased CP of thinning and stover and lower NDF especially 14 days infection post emergence. These effects varied with cultivar with the susceptible H511 showing 53% increase in CP with MSVD resistant cultivar KH 521 and local land race not affected. The MSVD resistant cultivars had higher degradability values compared to the susceptible H511. There was no clear trend on the effects of time of MSVD infection and fertiliser on degradability. The effect of cultivar, plant density, fertiliser and seed arrangement in the hole; and also effect of seed recycling on forage and grain yields were studied. The open pollinated variety, SADVEB showed more MSVD resistance tan KH 521 and H614. Where certified seed was planted, H614 consistently gave the highest stover yields followed by KH 521 and SADVEB. However, H614 and KH 521 gave the same but more grain yield than SADVEB. Where seed was recycled; there was no difference in thinning yield between cultivars. H614 and KH 521 gave the same stover yield which was higher than SADVEB. SADVEB gave higher grain yield than both H614 and KH 521. Further increases in plant density increased forage but significantly reduced grain yield. Creating a more equidistance arrangement of maize plants in the planting hole increased thinning yield but did not affect MSVD incidence. Based on the results, farmers are advised to plant a resistant cultivar, plant early and all maize crops at the same time to help minimise MSVD incidences. Where farmers cannot afford certified seed and have to recycle hybrid seed they are better off planting high yielding OPVs in terms of both forage and grain since they give better annual yields. Where dense planting is practised, farmers should be advised to space seed in the hole. Thinning should be timely and consider the condition of individual maize plants in relation to surrounding plants to reduce interplant competition