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The objective of the in situ digestion method is to measure the dry matter disappearance of the food subjected to the effect of ruminal environment. The food was placed in bags that were incubated in the rumen of cattle through a permanent cannula, located in its dorsal sac at different sampling times. The experimental studies with this technique has been developed to evaluate different types of food, feeding conditions and types of diets, even, not only for dry diets, but also with fresh materials (Ørskov and MacDonal 1979, Boetto et al. 2013, Lentz et al. 2013 and De La Cruz et al.2013). The hard molasses blocks (HMB) correspond to a particular form of physically presenting a ruminal protein activator (RPA) and are defined as cylindrical slow-release products, whose specific weight is between 1.2 and 1.4 kg.L-1 and resistance to compression between 8 to 12 kgf.cm2. They function as an accelerating formula of ruminal digestion processes and have been designed to provide nutrients to populations of ruminal cellulolytic microorganisms of cattle. Its formulation includes a combination of protein meals, urea, minerals and molasses. These HMB are insalivated and swallowed whole or in large pieces by animals and their nutrients are dissolve slowly and synchronously throughout the day (D'Ascanio 2014). The objective of this study was to determine the disappearance (D) and the disappearance speed (DS) of whole HMB of the rumen of fistulated cows fed with low nutritive quality hay.
The study was conducted in the field of the Estación E xpe ri m e nt a l Agrope c ua ri a de l INTA Ba l c a rse (33°40'13 "South, 65 °23'25" West, 130 m o.s.l ), near Balcarce city, Buenos Aires Province, Argentina . For the study, the deferred autumn growth of the Panicum maximun cv. gatton (Gp) grass was used, from 2012 to 2013, from a rural property located on the National Route No.81, km 1320, Ibarreta town of (Formosa Province), cultivated in the autumn of 2012. During the winter of 2013, bales between 18 and 20 kg of average weight were made and kept in a warehouse until the moment of their evaluation. For the study of the D and the in situ DS of dry matter of the whole HMB, with nutritional quality of CP (40.6); NDF (32.5), ADF (12.1) and ashes (13.2). Three Aberdeen Angus cows were used, cannulated in rumen with 403 kg of average live weight, housed in open individual pens of 18 m2 each, with concrete floor. The animals when entering to the test were dewormed with Ivermectin, identified with an ear tag and individually weighed on a mechanical scale to perform the fit of forage intake and of a dose of HMB fitted to 2 g.kg-1of live weight. Previous to sampling, the animals were initially accustomed to the diet for 30 d. In the last four days, the voluntary intake of forage was measured and the daily supply was fitted above 10 % with respect to the real intake, previously estimated at 1.8% of live weight. The HMB was daily supplied at 8:00 am and the animals completely intake it.
For the development of the experiment, tubular plastic mesh bags, with hole size of 3 mm were used. An amount of 100 grams of whole HMB were weighed in duplicate for each sampling time and placed in 5 x 20 cm bags. The bags were closed and introduced into the rumen's ventral sac by means of a 50 cm long nylon rope with a handlebar at its end for their fixing. Subsequently, the bags were extracted in duplicate at 30, 60, 120, 240 and 480 minutes of incubation. All the bags extracted from the rumen and two bags not incubated (hour 0), were immediately manually rinsed with water, to remove the loose material separated from the block. The rinsed bags were placed in stainless steel trays and dried in an oven with forced air circulation at 60 °C to constant weight. Then they were transferred to a desiccator for 30 minutes and weighed. The difference between the initial weight of the sample placed in the bags and the weight of the residue corresponded to the material disappeared from the bag for each incubation time. The particle size was determined to the remnant through a screened mesh. Analysis of variance was carried out according to a completely randomized design with 3 repetitions and the test was applied to compare Duncan's means for P <0.05. For the interpretation of the results, the statistical program INFOSTAT, version 2012 (Di Rienzo et al. 2012) was used.
Table 1 shows the D and DS values of the HMB between the different sampling times with significant differences between them (P <0.001). The soluble fraction of the whole blocks at rinsing was 2.57 %. The average D of the HMB was 9.5 %.h-1 and was fitted to a quadratic polynomial regression model, where as the incubation time increased the D value of the whole blocks increased until reaching a maximum value of 76.4 % at 480 min. (8 h). The DS of the average HMB resulted in 213 mg.min-1, which responded to the same polynomial, in which as the incubation time was increased the DS was reduced, being marked its deceleration from 225 min. of incubation, probably due to the progressive accumulation of indigestible DM in NDF and ADF form, which could form aggregates of particles of size between 4 and 7 mm or more, much higher than the holes of 3 mm plastic mesh opening of the bags that were used.
Table 1 Average values of D and DS of hole HMB between sampling times
a, b, c, d, eDifferent letters indicate significant differences
At the end of the evaluated period there was still a remnant of 23.4 % of blocks inside the plastic mesh bags, with the aforementioned particle size, which could escape to the rumen degradation, when passing through the reticulo-omasal orifice. Figure 1 shows the fit of the quadratic model to the values obtained from the D of the HMB, indicated by its corresponding second-order polynomial regression equation, and R2 of 95 %. For the DS of the HMB, which corresponds to the same type of equation, with R2 of 67 %.
It is concluded that the HMB achieve a slow release of their components over time, finding material in the rumen of the cannulated cows still at 480 min. of incubation.
