Introduction

In tropical regions, sugar cane forage is frequently utilized by farmers in ruminant feed¹. Due to its low cost of dry matter produced and harvest concur with the poor availability of forage, although daily cut is a common practice may not be economical because of the labor implied². The ensiling of sugar cane is a forage conservation method that can increase NDF digestibility³ and soluble carbohydrates^2,4,5. Some positive effects on digestion and productive performance encourage its research, for example, crude protein of ensiled sugar⁶ and feed efficiency was higher in dairy cows fed with inoculated sugar cane silage respect fresh sugar cane⁷. However, its TDN value is low and the N content of sugar cane silage is insufficient to maintain N equilibrium due to its poor N content⁸.

The shrub commonly called “gold button” or “wild sunflower” (Tithonia diversifolia) contains between 14 and 28 % of crude protein, and has high ruminal degradability of dry matter and low phenol and tannin content⁹. Crude protein of Tithonia diversifolia is similar to alfalfa but the former is higher in rusticity. Moreover, the price of alfalfa is high because of its low availability in these regions. As a cheap source of protein, Tithonia diversifolia may be utilized up to a 30 % level of inclusion without negative effect^10,11 on productive performance and digestibility.

Energy and N supplements increase the efficiency of ruminal microorganisms in forage fed ruminants¹². Nevertheless, nitrogen digestibility may be altered by the energy source because of the different site of the digestion¹³. Rice polishing is a byproduct with similar metabolizable energy and greater protein values respect high used grains such as sorghum and corn¹⁴ because of its elevated protein and lipid content¹⁵ . Positive N-balance has been observed with diets that include 25 to 50 % of rice polishings in the concentrate mixture of lactating goats¹⁶ and cows¹⁷. Also, rice polishing inclusion in diets based on sugarcane has improved the productive performance of cattle^18,19, but there is no evidence about the influence of rice polishing supplementation in diets based on sugar cane silage. The objective of this study was to evaluate the effect of the replacement of alfalfa with Tithonia diversifolia in sugar cane silage based lamb diets with and without rice polishings complementation.

Material and methods

Animal management procedures were conducted within the guidelines of locally- approved techniques for animal use and care NOM-051-ZOO-1995; technical specifications for the care and use of laboratory animals.

This experiment was conducted at the Unidad Académica de Medicina Veterinaria y Zootecnia of the Universidad Autónoma de Nayarit located in Compostela, Nayarit, Mexico (21° 17´46´´N and 104° 54´ W).

The T. diversifolia was harvested at the university 60 d after the last cut. The forage was then chopped to obtain a particle size of between 2 and 3 cm and sun dried for 72 h and turned every 24 h. The sugar cane silage was prepared utilizing whole sugar cane plants harvested (24 mo after sowing) at a farm with clay soil near the experiment location. Sugar cane (SC) plants were chopped to obtain a particle size of between 2 and 6 cm. Three (3) percent of homemade inoculum (10 % of sugar cane molasses, 0.5 % of urea, 5 % of poultry waste, 1 % of yogurt and 83.5 % of water) ^(20,21, 1 % of urea, 0.1 % of ammonium sulfate and 0.25 % of diammonium phosphate (on wet basis) were added to SC by spraying it on each layer of sugar cane during ensiling. Sugar cane forage was compacted with a tractor at each 30 cm layer, with the compacted SC forage then covered with polyethylene and 15 cm of soil during 45 d.

Results

Discussion

The ADF content of T. diversifolia in this study is greater than the values reported by different authors¹⁰, who utilized T. diversifolia leaves only, while the CP content concurred with others^10,28. The nutrient content of T. diversifolia depends on age and plant part²⁹ while the higher ash content of T. diversifolia compared to alfalfa may be explained by its elevated concentration of minerals³⁰ such as Ca, P, and Mg³¹. Higher NDF and ADF content in the non-RP diets reflect the higher fiber fraction concentration in T. diversifolia and alfalfa compared to RP, while EE content increased with the addition of RP. Concurs with the higher ether extracts in rice polish based concentrate compared to concentrate with wheat bran observed¹⁷. The crude protein content of sugar cane silage (13.36 %) was higher than that without the addition of inoculums⁸. Silage pH value reflected good fermentation⁸ and was around to those reported in pure sugar cane and sugar cane plus 0.5 and 1 % of formic acid³².

The protein solubility (40 %) of T. diversifolia fodder³³ is one limitation for the use of high levels of this forage in lamb diets²⁸, a limitation that may be avoided by including rapidly fermentable carbohydrates³⁴. There is a synergy between protein and sugar supplementation because the microbial population is improved to a greater extent when protein is supplied in addition to sugar rather than when they are supplied separately³⁵.

Rice polishings facilitated the digestion of the TD diet due to the increased energy caused by the increment in ether extract and highly fermentable carbohydrates in the rice polishings. Moreover, the lower NDF content in the diets containing RP led to greater digestibility due to the negative relationship between NDF and digestibility. While complementation have increased the total diet digestibility of organic matter, the fiber-based energy have presented greater digestibility levels than a grain based supplement³⁶. In order to maintain an acceptable intake and digestion of low quality forages requires the synchronization of the degradable protein and carbohydrate supply from the energy supplement¹².

Similarly, dry matter digestibility values between 60 and 66 %, and values of between 64 and 68 % for organic matter digestibility when T. diversifolia leaves were supplemented at levels of 10 and 40 % have been observed³⁷. In contrast, with the lower in vivo digestibility (41 to 53 %) in diets with 20, 35 and 50 of T. diversifolia combined with Taiwan grass and concentrate²⁸. The utilization of different diet composition and techniques for digestibility determination may explain the variable results observed in different trials. The Taiwan grass²⁸ contained a similar percentage of NDF to the levels for sugar cane silage recorded in this study. However, the protein in sugar cane silage is twice that of Taiwan grass.

There is a close relationship between in vitro digestibility and rumen undegradable protein digestibility (r²=0.090) and a moderate relationship between rumen undegradable protein and CP (r²= 0.48 and 0.42)³⁸.

Similar N retention as a percentage of N intake to diets with greater digestibility (80 %) caused by a greater amount of concentrate in the diet have been mentioned³⁹. In dairy cattle, NDF content is inversely related to N digestibility⁴⁰. Although the NDF content of T. diversifolia is greater than alfalfa, N retention as a percentage of absorbed N is 30 % greater in TD compared to AA diet, which can be attributed to the higher essential amino acid content of T. diversifolia⁴¹, compared to alfalfa⁴² which could be mobilized for tissue deposition or the synchrony between the degradation of energy and the protein contained in the ingredients of the diet. Similarly, greater concentration of serum protein have been found when animals, fed with threshed sorghum top, were supplemented with plant foliage⁴³.

The high amino acid content in T. diversifolia enhances its N availability⁹, which concurs with the fact that protein supplements with sufficient essential amino acid content, such as fish meal, have recorded high digestion and absorption levels⁴⁴. Rumen undegradable protein digestibility may not be similar for all forages. Improved N retention is desirable since it has a favorable impact on both the environment and the production economy because of the reduction of N loss through urine and feces. Based on the present results, supplementation with forage with high quality protein may improve N retention with rice polishings complementation. On the other hand, greater N retention with lower levels of T. diversifolia in concentrate had been attributed to the anti-nutritive components of this forage⁹. Nevertheless, no difference in N retention was observed when with alfalfa diets were compared to those with T. diversifolia diets in this study.

Dry matter intake was improved by RP supplementation in both alfalfa and TD diets. Similarly, higher total dry matter intakes when concentrate was added to the whole sugar cane plant diets of goats and lambs (11 kg live weight) compared to those without concentrate were observed; however, they did report live weight losses⁴⁵. Also, average daily gains (26.1 g/d) similar to those recorded in this study have been found in goats fed diets based on Panicum maximum plus 30 % of T. diversifolia and concentrate¹¹.

The DMI observed coincides with that of diets with dry matter digestibility between 64 and 68 %⁹. Although the in vitro digestibility of the DM and OM in TD+RP was the highest, the ADG was similar (P>0.05) to that in AA+RP in response to the same dry matter intake. Intake is positively related to production⁴⁶ and depends on the ability of the feed to provide the nutrients needed⁹.

The RP diets contained a lower NDF percentage due to the fact that the rice polishings contained around one third of the NDF of alfalfa or T. diversifolia. The NDF content of all the experimental diets was higher than the 25 % which promotes digestibility, intake and weight gain^47,48. The elevated levels of forage in the diet may limit energy intake and ADG⁴⁹. Nevertheless, the similar performance of animals fed AA and TD encourages the further study of the effect of T. diversifolia supplementation in diets with higher energy and rumen undegradable protein content and lower fiber content on the performance of young animals.

The superior N retention and DMI of lambs supplemented with RP was reflected in increased ADG. Rice polishings improved ADG, since its inclusion decreases fiber fraction percentage and increases the energy content of diets. The energy supplements influence animal performance and forage utilization, potentially increasing the opportunities for nutrient synchrony in the diet¹².

The substitution of alfalfa with T. diversifolia reduced the feed cost with and without rice polishings complementation, 133 % ($1.02 vs $2.38 MXN) and 49 % ($1.97 vs $2.93 MXN), respectively. The sugar cane silage-based diet supplemented with T. diversifolia and complemented with rice polishing showed the lowest cost per kg of weight gain.

Conclusions and implications

Alfalfa can be replaced by Tithonia diversifolia in fiber rich diets with no effect on dry matter intake, average daily gain and feed conversion. The inclusion of rice polishings in sugar cane silage based diets improves digestibility, N retention and productive performance because it increases energy and decreases the fiber content.