Perennial ryegrass is an intensive, temperate pasture grass that responds well to applied fertiliser.
The purpose of this project was to study the effects of fertiliser on the productivity and quality of
perennial ryegrass in KwaZulu-Natal and how this impacts on animal intake. It was hypothesised
that over-application of fertiliser to a perennial ryegrass pasture would negatively affect the quality
of the herbage for grazing by dairy cattle and that intake would be affected. Thus the project
aimed to assess the effects of applied fertiliser on yield, quality and intake of an established
perennial ryegrass pasture.
The trial consisted of a set of six separate experiments. Each experiment focused on the
interaction between two of the major nutrient elements nitrogen (N), phosphorus (P), potassium
(K) and sulphur (S). The experiments (NxP, NxK, NxS, PxK, PxS and KxS) were managed
separately to avoid possible transfer of nutrients during grazing, which would result in the
contamination of treatments. Each factor had three levels (low, medium and high), giving a total of
nine treatments per experiment. Each of the experiments was replicated three times in a
randomised block design.
Increased fertiliser N application rates increased perennial ryegrass yield with a pattern of
diminishing return, where split applications above 40 kg N ha-1 produced smaller increases in yield
when compared with the response at lower applications of N. Applied P, K and S did not affect
yield, suggesting that even the lowest application levels were sufficient to not limit production.
Nitrogen application affected apparent intake, but it is suggested that this is due to the yield effect
rather than a direct effect of N on apparent intake. The application of P, K and S did not affect
apparent intake.
Results from this study showed that the quality of perennial ryegrass herbage, especially in terms
of feed value to dairy cows, can be significantly affected by applied fertiliser. The extent of the
response was affected by sampling date (i.e. time of year) and this must be taken into account
when planning a fertiliser management strategy. This is particularly so with respect to N fertiliser
recommendations.
Crude protein (CP) content of herbage increased with increasing levels of applied N and the
extent of the response was influenced by season. P, K and S did not affect CP concentration in
herbage, except in the PxK experiment where increased levels of K lowered herbage CP. Applied
N considerably increased the concentration of non-protein nitrogen (NPN) in perennial ryegrass
herbage. P and S did not affect NPN levels, whereas applied K decreased NPN content in the
iv
NxK and PxK experiments. Non-structural carbohydrate (NSC) content of herbage was decreased
by applied N but was unaffected by applications of P, K and S. Neither neutral detergent fibre
(NDF) nor acid detergent fibre (ADF) was affected by applied fertiliser. In this study herbage P
declined and herbage Ca increased with increasing levels of applied N. The addition of fertiliser K
resulted in lower herbage Ca values. There was no herbage S response to applied fertiliser in this
study.
Classification and regression tree (CART) analysis identified the primary determinant of apparent
intake in experiments containing N as a factor as the amount of material available to be grazed
and that NSC, NPN and ADF are also determinants of apparent intake. Cows do not regulate diet
choice within the short-term time frame of a meal. Thus intake is determined by short-term needs
rather than by meeting long-term nutrient requirements. Fibre creates physical fill within the
rumen, thus restricting intake. High NPN content is associated with high nitrate values. The
reduction in intake of herbage with high nitrate content may be due to reduced palatability or to a
negative feedback system limiting the intake of nitrate and ammonium. Increased NSC content is
associated with increased intake, possibly through the effect of sugar on herbage palatability.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/10827 |
| Date | January 2010 |
| Creators | Findlay, Nicola Jean. |
| Contributors | Du Toit, Justin Christopher Oakes., Kirkman, Kevin Peter. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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