Pot experiments were used to determine the effects of age, light, temperature, mineral nutrition and wilting on the HCN potential (HCNp ) of white clover (Trifolium repens L.). HCNp, adjusted for pre-expenmental HCNp, was reduced by higher light intensity (505 v. 220 mg HCN kg-1 DM), by increased temperature (408 v. 317 mg HCN kg-1 DM) and by phosphorus application (382 v. 343 mg HCN kg-1 DM). These effects were largely explained by the hypothesis that HCNp diminished as the size and vigour of the plant increased, and the use of dry matter production as a covariate on HCNp removed the significant effects of light, temperature and phosphorus. In two experiments wilting plants prior to harvest increased HCNp by 15 and 24%, and reduced dry matter yield by 9 and 13%. HCNp declined curvilinearly as the white clover aged. The conditions shown to favour high HCNp, namely, immaturity, moisture stress, low light intensity, cool temperature, and inadequate P supply, are those which occur periodically in the late autumn and early spring of temperate climates. Sheep in late pregnancy at this time, could, if white clover comprises a substantial proportion of their diet, experience thiocyanate absorption as a consequence of HCN detoxication and hence increase the risk of goitri to the lamb.
The effect of seven factors, namely genotype, plant maturity, nitrogen fertilizer, phosphorus fertilizer, water stress, light intensity and temperature, on the hydrogen cyanide potential (HCN,) of forage sorghum was studied in three pot experiments.Fivefold differences occurred between genotypes in HCN,, with a breeder's line, X45 106, selected for low HCN, having a maximum of 520 mg HCN kg-' DM (dry matter) compared with 2300 and 2450 mg kg-1 DM for cvs Zulu and Silk respectively. In X45 106, HCN, (mg HCN kg-' DM) declined curvilinearly with age d (days from sowing) (HCNp=8460-320d+ 3.1d2) and linearly in Silk (HCN, = 9020 -1 lOd), but the decline in Zulu was not statistically significant. Nitrogen (equivalent to 200 kg ha-1 of N) increased HCN, (P< 0.001), but more so in full light (100 mg kg-] compared with 1430 mg kg-') thanin 50% shade (190 mgkg-1 compared with 690 mgkg-1). In one experiment, acute water stress appeared to reduce HCN,, but this was confounded with the strong decline due to aging. In another study, acute water stress had no effect on HCN,. Neither the application of superphosphate nor change in light intensity, nor change in temperature had a direct significant effect on HCN, in these studies.Breeding and selection for low HCN, appears a promising approach to ensuring that sorghum plants will provide non-toxic forage from an early stage of growth.
Progress in improving the feeding value (FV) of forage plants has been slow despite the benefits to animal production that can result from using plants of intrinsically higher FV. The slow progress is due in part to a lack of consensus on the criteria to be used in breeding or selecting forages of high FV. This paper reports the use of the Delphi technique to obtain such a consensus from an international panel of specialists in grazing animal production.The specialists, from Europe, USA, New Zealand and Australia, ranked eleven criteria for improving the FV of grasses and legumes for liveweight gain and for wool production.In general, the primary criteria chosen were high digestibility, easy comminution, high nonstructural carbohydrate, high crude-protein and, for sheep kept primarily for wool production, a high sulphur-amino acid content. High relative palatability, high lipid-content and erect growth habit were ranked as least important. It was generally considered that mineral content and anti-quality constituents should be monitored rather than making them specific breeding objectives, although some exceptions were noted. Minor importance was attached to having 'appropriate' tannins in grasses; however, this was given a high priority in legumes. These criteria, and the desirability of reducing the extent of protein degradation in the rumen are discussed in relation to comments made by panel members.
Results are presented for the final two years of a previously described experiment.The return of dung and urine by sheep to a ryegrass/white-clover ley was controlled by suitable harnesses to give four treatments (no dung or urine, dung, urine, dung and urine) which were combined in a replicated factorial design with four levels of nitrogenous fertilizer application (0, 52, 182, 312 1b. N per acre).Applied nitrogen and urine were the dominant factors affecting botanical composition. The percentage of ryegrass increased and that of clover decreased with the progressive increases in nitrogen application. Volunteer species (mainly Poa spp.) contributed up to 20 per cent by the final year, the maximum occurring under the medium-high nitrogen treatment.Urine restricted the incursion of weed grasses. Combined with urine or the full retum of excreta, high levels of applied nitrogen increased herbage production by up to 120 per cent. There was little response to dung except at the highest nitrogen level.The yield response to applied nitrogen was almost linear. In the absence of animal retums response was poor, partly due to shortage of potash. When both excreta were withheld the light nitrogen dressing depressed the annual production compared with the control; where both excreta were retumed together with this dressing no reduction occurred in annual yield and the spring yield was improved (p <0-05).
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