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Proceedings: Kikuyu Technology Day
Nutritive Value of Kikuyu
Energy and Mineral Content of Kikuyu
KwaZulu-Natal Department of Agricultureby TJ Dugmore
Introduction
Kikuyu was first planted on Cedara in c.1915. Pioneering research work was conducted in the 1930's, investigating the potential of kikuyu for dairying. By 1940 approximately 100 acres (c. 40 ha) was established to kikuyu on Cedara.
Mineral content
The chemical composition of kikuyu, recorded in the 1930's at Cedara was:
CP - 23.7 % Lime - 0.39 % (0.27 % Ca) P2O5 - 0.86 % (0.37 % P) Potash - 4.6 %
The high potash content of kikuyu was noted by A J Taylor, a chemist at Cedara, involved in the earlier work on kikuyu. However the earlier workers did not comment upon the adverse Ca:P ratio in kikuyu. The typical chemical composition of kikuyu at Cedara, for old, well established pastures, is presented in Table 1.
Table 1. Approximate chemical composition of kikuyu on Cedara.
|
Mineral |
% DM |
Chemical component |
% DM |
| Ca P K Mg Na Ca:P K/(Ca+Mg)* |
0.24 0.33 0.36 0.30 0.03 0.76 2.90 |
CP ADF NDF NSC# EE NPN NO3N |
20 35 65 5 2.8 0.78 0.70 |
* derived on
an equivalent basis
# non-structural
carbohydrate
The inverse ratio of Ca:P necessitates the supplementation of feedlime, above that derived from dicalcium phosphate, to the diet/mineral supplement. The analysis of blood sampled from cows on Cedara in 1970 revealed a low calcium content at the end of summer. These low blood calcium levels coincided with low conception rates for these cows. Supplemental calcium, in the form of feedlime, was found to improve conception rates. For beef cattle on kikuyu, a feedlime and salt lick appears to be sufficient on kikuyu grazing.
A K/(Ca+Mg) ratio of less than 2.2 (on an ionic equivalent basis) in the diet is recommended. Higher ratios are associated with milk fever. The following equation can be used to derive this ratio:
K = (% K x 25.6)
(Ca + Mg) (% Ca x 49.9)+(% Mg x 82.3)
The lack of sodium in kikuyu, a natrophobic plant which accumulates its sodium in the roots and not the leaves, requires correction in the diet. A Na:K ratio exceeding 15:1 in the salivary fluid is desirable. Low levels of sodium are implicated with bloat, due to the buffering effect of sodium in the rumen. Dairy cattle at Cedara have exhibited signs of a salt deficiency, namely the licking of urine. Consequently the salt content of the mineral supplements for use on kikuyu has been increased by 10% at Cedara.
The magnesium in fresh herbage is relatively unavailable to livestock, and potassium and nitrogen are antagonistic to magnesium uptake in the plant. Typically a magnesium shortage causes grass tetany. However, grass tetany is not commonly seen on kikuyu. Dairy cows at Cedara have responded to magnesium supplementation through improved fertility, i.e. fewer services to conception and shorter inter-calving periods.
Crude protein, energy and digestibility
The crude protein content of old established kikuyu pastures at Cedara, fertilized with 210 kg N/ha split into three dressings over the season, range from 16 to 24% of the dry matter, with a CP degrabability of 0.75. The structural carbohydrate (fibre) fractions comprise approximately 35% of the dry matter for acid detergent fibre and 65% for neutral detergent fibre. The non-structural carbohydrate levels in kikuyu are very low, ranging from 3 to 8% of the DM. Good responses are therefore expected from supplementation with starch or other forms of highly available carbohydrate in the rumen. Kikuyu has relatively high ether extract (fat/oil) contents for a roughage at 2.8% of the DM. The non-protein nitrogen content of kikuyu ranges from 20 to 30 % of the crude protein fraction, not unusually high for a high protein forage.
The metabolizable energy content (MJ ME/kg DM) of kikuyu on Cedara has been determined, by in vivo digestion trials, to be 9.3 MJ in the spring, 9.0 MJ in summer and 8.8 MJ in the autumn. On Cedara, these in vivo digestion trials have shown that high nitrogen levels in kikuyu have a negative impact on intake and digestibility, especially with very young material. The variation in digestible organic matter (DOM) and CP content of kikuyu over the growing season is reflected in Figure 1. The influence of nitrogen on dry matter intake (by steers) and digestibility (by sheep) to animals fed fresh kikuyu herbage is explained by the following regression equations.
Organic matter digestibility (%) = 71.6 - 0.62 (CP%); r = - 0.546
Dry matter intake (% of body weight) = 2.794 - 1.092 (NPN, % DM); r = - 0.781
Selection studies on kikuyu, using oesophageal fistulated steers, indicated that there was selection against high N (> 15% CP) in the herbage.
Figure 1. Seasonal variation in DOM and CP content of kikuyu.
Conclusion
Kikuyu grazing has its limitations in terms of mineral composition which need to be addressed in any feeding programme utilizing kikuyu. However, these can easily be addressed at no great cost to the producer. In terms of digestibility, kikuyu has its limitations and has been shown to be at the lower end of the scale for improved tropical species.
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