Browsing Journal of Range Management, Volume 43, Number 5 (September 1990) by Issue Date
Now showing items 21-23 of 23
Genetic variability for elements associated with grass tetany in Russian wildryeGrass tetany (hypomagnesemia) may be an important factor limiting productivity of animals grazing Russian wildrye [Psathyrostachys juncea (Fisch.) Nevski]. This malady is associated with relatively low concentrations in the forage of Mg and Ca, and high values for K and K/(Ca+Mg). We studied the genetic variability in a Russian wildrye breeding population for mineral elements that relate to grass tetany in ruminants. Forty-five progeny lines, estblished as spaced plants in a randomized complete block, were sampled at the pre-boot and boot stages in each of 2 years and analyzed for Mg, Ca, K, and P. Although seasonal variation was evident, K/(Ca+Mg) of the progeny lines ranged from 3.2 to 4.6, well above the 2.2 level at which a 5% incidence of grass tetany has been found in dairy cattle. With few exceptions, progenies differed for all traits evaluated. Differences among progenies were relatively consistent over harvests for all traits. A reduced tetany potential (RTP) was computed as the sum of normalized Mg and reciprocal of K/(Ca+Mg) values, providing an estimate of the grass tetany risk for individual progeny lines. The variation among progenies, and the magnitute of broad-sense heritability estimates for RTP (0.48) and K/(Ca+Mg) values (0.31), indicate that mineral ion composition of this breeding population can be altered through breeding. The high K/(Ca+Mg) values in the population suggest that it may be helpful to introduce genetic factors conditioning lower grass tetany potential from other sources. Intercharacter correlations suggest that breeding for higher levels of Mg will be accompanied by increased Ca and, to a lesser extent, increased K.
Supplementation and monensin effects on digesta kinetics. I. Cattle grazing summer rangeSixteen ruminally cannulated beef steers grazing native summer range in the Northern Great PIains were assigned to 4 treatments in a 2 X 2 factorial arrangement. Main effects were barley grain and monensin. Rolled barley (RB) containing 7.5% molasses was fed at 0 and 1.36 kg head-1 day-1. Steers received no monensin (M) or M released at 101 mg/d via a ruminal delivery device. Forage intake and digestibiIity, ruminal fermentation, and ruminal passage rate were measured during trials in: (1) June, (2) July, and (3) August. Diet samples were collected from esophogeally fistuIated steers during each trial. Dietary crude protein was greater (P<0.05) during trials 1 (15.2%)and 3 (14.3%) than in trial 2 (10.2%). In vivo organic matter (OM) digestibiiity, ruminal fluid passage rate, and fermentation variables varied by trial (P<0.01). Forage OM intake was reduced (P<0.10) by RB, but was not influenced (P>0.10) by M or the M and RB combination. In vivo OM digestibility was increased (P<0.05) by M, while RB had no effect. Particulate passage was not affected by M or RB but gastrointestinal tract fill was reduced by monensin (P<0.05). Ruminal fluid passage rate was affected by the RB X M X Trial interaction (P<0.05). Within June and July, fluid passage rate was similar among treatments and ranged from 14.0 to 11.3 %/h, respectively. During trial 3, a RB X M interaction (P<0.05) increased fluid passage rate. Ruminal ammonia-N concentration was similar among treatments. Barleey lowered (P<0.05) ruminal pH and increased (P<0.10) total volatile fatty acids. A RB X M X Trial interaction (P<0.05) was noted for molar proportions of acetate, propionate, and butyrate. Within trials, RB, M, and their combination affected (P<0.01 to P<0.10) acetate, propionate, and butyrate. We conclude that barley, monensin, and forage quality infiuence ruminal fermentation, passage rate, and intake traits of steers grazing summer range.
Supplementation and monensin effects on digesta kinetics. II. Cattle grazing winter rangeSixteen ruminally cannulated steers grazing Northern Great Plains native winter range were assigned to 4 treatments in a 2 X 2 factorial arrangement. Main effects were protein and monensin. A soybean meal-barley pellet (P; 26% crude protein) was fed at 0 and .8 kg head-1 d-1. Steers either received no monensin (M) or M was released at 101 ml/day via a ruminal delivery device. Forage intake, rmninal fermentation, in vivo organic matter (OM) digestibility, and ruminal fluid passage and particulate digesta kinetics were measured during trials in November and January. Esophageally fistulated steers were used to collect diet samples during each trial. Dietary crude protein was greater (P<0.01) in November (8.3%) than January (4.9%). Forage OM intake was not (P>0.10) influenced by either P or M. In vivo OM digestibility was increased (P<0.05) by P (60.6 vs 57.4%) and not affected (P>0.10) by M. Particulate passage rate increased (P<0.05) when P was combined with M. Ruminal fluid flow characteristics, fluid volume and pH were not affected (P>0.10) by either P or M. Ruminal ammonia-N was increased (P<0.01) by P (2.9 vs .6 mg/100 ml) and not affected (P>0.10) by M. Total ruminal volatile fatty acid concentrations, along with molar proportions of ruminal propionate and butyrate, were not affected (P>0.10) by P or M. Ruminal acetate was decreased (P<0.10) by P and not influenced (P>0.10) by M. We conclude that supplemental protein, through ruminal modifications, has beneficial effects on OM digestibility, and can thereby provide cattle grazing winter range with additional energy at a time when it is most crucial.