PERCENTAGE YIELD DIFFERENCE, AN INDEX FOR EVALUATING INTERCROPPING EFFICIENCY

Rationale: Percentage Yield Difference (PYD) has not been considered as one of the indices to interpret intercropping advantage in crop mixture Aim:To evaluate intercropping efficiency using percentage yield difference (PYD) and to compare the index with other indices Study design: The design was a 2 X 5 factorial in a randomized complete block design and replicated three times. Location: The study was carried out at the Lower Niger River Basin Development Authority, Ejiba in the Southern Guinea Savannah zone of Nigeria in 2008 and 2009 cropping seasons. Methodology: Erect and prostrate cowpeas and maize were mixed at five population ratios. Sole crops of each crop at full population were included as control treatments. Land Equivalent Ratio (LER), Area Time Equivalent Ratio (ATER), Monetary Equivalent Ratio (MER) and Percentage Yield Difference (PYD) were estimated and compared. Results:Results of this study showed that LER, ATER, and PYD values were similar for the two cultivars in the two years. LER values ranges between 1.18 – and1.27 in 2008 and 1.12 – and1.30 in 2009. MER did not follow the same trend as LER and ATER. Lower values were obtained particularly in 2009. However, the highest MER 1.35 and 1.23 in 2008 and 2009 respectively were obtained in prostrate cowpea / maize mixture at population ratio of 100:50. The PYD advantage varies between 5-33% in the two years. Conclusion: The comparable values of PYD with other indices suggest that it can be used to evaluate intercropping efficiency in crop mixture.


INTRODUCTION
Intercropping as one of thea types of mixed croppingis the practice of growing more than one crop on the same field at the same time. This practice is widely practiced among small holder and peasant farmers in the tropics [1,2]. Mutsears etal. [3] pointed out the potential advantagesof the system such as better utilizationof natural resources (solar radiation, mineral nutrients and water ), higher labour productivity and reduced risk of crop failure as compared with sole cropping. [4]Reported the superiority of the system over sole cropping in early and late maturing cowpea /maize mixture. In another study, [5] also noted that intercropping could be an eco-friendly approach at reducing weed problem through nonchemical methods.
Efficiency of intercropping over sole cropping has been assessed by various indices. Mead and Willey [6] use the concept of land equivalent ratio(LER) defined as the totalland area required under sole cropping to give the yield obtained in the intercrop mixture. Adetiloye and Ezedima [7] use the concept of land equivalent coefficient (LEC) defined as theproduct of land equivalent ratio in the intercrop. It was developed to assess the interaction and productive potential in crop mixtures. Hiebsch and Mc Collum [8] proposed the concept of area time equivalent ratio (ATER), as a modification for LER. This takes into consideration the crop dimension in the field that is, the time the crop occupies the field from planting to harvesting. Ofori and Stern [9] observed and noted that this index was appropriate in crop mixture where component crops have different maturity dates. Monetary equivalent ratio (MER) defined as the sum of the ratio of intercrop monetary returns to the highest sole crop monetary return to the entire land area occupy by all intercrops per unit time [10].
While all these indices and others relate with the yield differences that exist between the componentcrops in the mixture,percentage yield difference (PYD) as an index has not been documentedand compared to interpret intercropping advantages. This research was therefore designed to interpret/ evaluate intercropping advantages using percentage yield difference between the component intercrop and the sole crop and to compare the advantage with other indices.

MATERIALS AND MEHTODS
The result of fField trial involving erect and prostrate cowpea in an inercrop with /maize were evaluated for the efficiency of the intercropping system,using the Land Equivalent Ratio Season.The percentage yield difference of the treatments were also calculated and compared with the LER, ATER and MER values. The treatment consist of five population ratios of cowpea and maizecombined in a 2 x 5 factorial and arranged in a randomized complete block in three replicates. Sole crop of cowpea and maizeat full population were included as control treatments. Phosphorous was applied as single super phosphate (18%P 2 O 5 ) at the rate of 60kg/ha ofP 2 O 5, ectare two days before planting. Urea was split applied in two split to the maize, at 3 and 6 weeks after planting at the rate of 120kgN/haectare.Pendimethalinherbicide was applied immediately after planting and cylothrin was sprayed at the rate of 40ml/20 litres2ml/L of water using knapsack sprayer atthe onset of flowering till harvesting of cowpea to control insect pests.Cobs from ten maize plants at the inner rows were harvested, while all the pods in each plot of cowpea were also harvested. Yield data collected were statisticallyanalysed using [12] models and treatment means compared at P = .05 using Duncan's Multiple Range Test. PYD value is inversely proportional to yield advantage that is, the lower the value the higher the efficiency of the system and vice versa.

DISCUSSION
The compared values of LER and ATER in all the parameter follow similar trend to the obtainedpercentage yield difference. This indicates that the yield reduction of component crop A was compensated by increased yield of component crop B. This yield differencewhen expressed in percentage will give the intercropping efficiency for the intercrop in land and in time dimensions. The efficiencyof the mixture when evaluated withMER did not follow similar trendswith percentageyield difference values. The obtained values (MER) were lower than the percentage yield difference in all the parameters. This contrastsuggests that the percentage yield difference cannot be used asan index to interpret intercropping efficiency when the objective of theend user is monetaryadvantage. This assumption is also valid for LER and ATER. This corroborated with the earlier findingsof [8,12],who observed that the efficiency of intercropping might be misleadingwhen LER alone is used, particularly when monetary gain is the primary objective of the end user.
These comparablesimilar values of PYD with LER and ATER make it a valid index for interpreting intercropping advantage in crop mixture. This simple and reliable method is adaptable for a wide range of crop mixtures.